CHEN Zeyu XU Jiyao CHEN Hongbin CHEN Wen REN Rongcai HU Xiong ZHU Yajun XUE Xianghui LU Gaopeng,8 ZHANG Shaodong HUANG Kaiming TIAN Wenshou ZHANG Jiankai HU Dingzhu RAO Jian HU Yongyun XIA Yan

1(Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029)

2(National Space Science Center, Chinese Academy of Sciences, Beijing 100190)

3(School of Earth and Space Sciences, University Science and Technology of China, Hefei 230026)

4(School of Electronic Information, Wuhan University, Wuhan 430072)

5(College of Atmospheric Science, Lanzhou University, Lanzhou 730000)

6(Nanjing University of Information Science and Technology, Nanjing 210044)

7(School of Earth and Space Sciences, Peking University, Beijing 100871)

8(Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, HFIPS,Chinese Academy of Sciences, Hefei 230031)

9(University of the Chinese Academy of Sciences, Beijing 100049)

Abstract This report reviews the researches for the middle and upper atmosphere in 2020–2022 by Chinese scientists.The report consists of five parts introducing primarily the results from the aspects of the development of infrastructure,the structure and composition,the climate and modeling,the dynamics for the middle and upper atmosphere,and Coupling between Stratosphere and Troposphere,respectively.

Key words Middle and upper atmosphere,Structure and composition,Climate,Dynamics

1 Development of Infrastructure

The development of the daytime lidar at Yanqing Station was reported by Xiaet al.[1].This lidar can permit full-diurnal-cycle observation of the metal Na layer.In order to suppress the skylight background during daytime effectively with less signal losses,a dual-channel Faraday filtering unit was implemented in the lidar receiver.Based on the diurnal Na lidar system,a good number of continuous observational results that lasted more than 120 h with a good signal-to-noise ratio were obtained,demonstrating its reliability.The lidar will provide valuable observational support for investigating the rapid production and disappearance mechanisms of Na atoms.

A dual-wavelength tunable lidar system that simultaneously detects the Ca and Ca+layers has been established at Yanqing Station[2].The lidar system implements a pulsed Nd:YAG laser that simultaneously pumps two dye lasers,which reduces the hardware configuration of the lidar system.Three nights of preliminary simultaneous observations of Ca and Ca+layers are reported.

Duet al.[3]reported that a Na-K lidar was built in 2016 at s?o josé dos Compose,Brazil,by the joint work of NSSC and INPE.This system realized simultaneously observation of the potassium and sodium metal layer,and this is the first time of potassium layer detection in South America.The detection capability of potassium lidar is high,compared with the available detection results of Germany potassium layer.

A new Ni lidar has been designed and deployed at Yanqing Station[4].The Ni lidar has good stability,and makes continuous Ni measurements over an extended period,enabling the first investigation of night-time and seasonal density variation of the Ni layer.For the first time,we used the high altitude Ni lidar to determine the branching ratios of three different optical transitions from Ni in the 3d9(2D)4s3D3 state (excited at 341 nm),and showed that these branching ratios are very close to the theoretical values.

The first nighttime meteor observation by MF radar in mid-latitude China was reported by Caiet al.[5].The observation period was 12:00–22:00 UT and the observation range was 78–150 km.By using broad vertical beams,totally 94 meteor echoes were obtained with the mean height of 106.5 km and the majority of them were distributed from 97 km to 115 km.Ambipolar diffusion coefficient,angle of arrival and some relevant parameters were simultaneously analyzed using the raw data.Initial bi-hourly and nightly averaged wind profiles were calculated,which are well fitted to the wind estimations by co-located VHF meteor radar at the altitude of 100–110 km.On the other side,echoes around 140 km are successfully detected in the observation.

Banet al.[6]reported a Rayleigh scattering lidar for measuring the atmospheric density and temperature has been deployed at Zhongshan Station (69.4°S,76.4°E),Antarctica.Lidar transmitter was a frequency doubled Nd:YAG laser with about 400 mJ pulse energy and a 30 Hz repetition rate.A telescope with a 0.8 m diameter pointing to the zenith direction served as the lidar receiver.This lidar was capable of profiling the density and temperature in the Upper Stratosphere and Lower Mesosphere (USLM) region.At the vertical resolution of 300 m and the temporal resolution of 30 min,the lidar measurement uncertainties,mainly due to the photon noise,were calculated to be within 1.5% and 1 K for density and temperature,respectively.Since March 2020,this lidar has been routinely operated at Zhongshan station for exploring the atmospheric density and temperature variations and wave propagation characteristics in the polar USLM region.

Chenet al.[7]reported that a sodium Doppler lidar system with three-directional measurements of sodium density,atmospheric wind field,and temperature was established at Zhongshan (69.4°S,76.4°E),Antarctica.On 14 November 2019,a Sporadic Sodium Layer (SSL) was observed at an altitude range of 93–103 km.The temporal/spatial sodium density variations of this SSL are associated with a strong sporadic E (Es) layer at nearly the same height,which is modulated by the convective electric field.By considering the structures and the time lags of the SSL’s growth at three positions,the SSL appears to have a horizontal advection in an approximately westward direction with a velocity of the order of 80 m·s–1.This is consistent with the zonal wind velocity derived from the lidar system itself.The temporal/spatial sodium density variations strongly indicate that the formation and perturbation of SSLs are related to the evolution of ES layers due to varied electric fields and atmospheric gravity waves,while it is advected by the horizontal wind.

Tianet al.[8]presented the Beijing Mesosphere Stratosphere and Troposphere (MST) radar,established at Xianghe Observatory of Whole Atmosphere (39.75°N,116.96°E) by the Institute of Atmospheric Physics,Chinese Academy of Sciences,with the support of the Chinese Meridian Project.It was put into routine operation in late 2011.It has been verified that the horizontal wind profiles are in good agreement with the nearest radiosonde observations.The dataset of atmospheric horizontal wind speed and direction from 3 to 25 km with a vertical resolution of 600 m and a sampling interval of 30 min was provided based on the Beijing MST radar observation in 2012.This high temporal and spatial resolution horizontal wind dataset can play a unique role in atmospheric dynamics and processes in the troposphere and lower stratosphere.

Chenet al.[9]have improved the power spectral density data processing algorithms to obtain higher quality MST radar data such as horizontal wind,vertical wind,and spectral width.It has been proved to be effective and reliable after comparing the derived data with the original data,radiosonde data and ERA5 reanalysis data.

2 Structure and Composition in the Middle Atmosphere

Wuet al.[10]reported an unusual equatorial plasma bubble during the recovery phase of a geomagnetic storm.The results showed the EPB occurred after sunset and drifted westward,which is different from previous EPB.The EPBs dissipated at about one hour after sunrise and its lifetime is about three hours.Based on the simulations of the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM),they found that the rapid uplift of the ionospheric should be the main reason for triggering the EPBs.Wuet al.[11]used observational data from two all-sky imagers,GPS,Swarm satellite,and a digisonde to study a special EPB event.They found that these EPBs occurred in the region of plasma depletion structure.Then,the plasma depletion structure disappeared with time.Around sunrise,new plasma depletion appeared and might merge with those EPBs.In addition,these EPBs showed special zonal drifts within a narrow longitudinal zone.An EPB drifted eastward and other remained stationary.Based on TIEGCM simulation,they found that the different zonal drifts of these EPBs should be related to the zonal winds.Sunet al.[12]used observations by multiple ground-based instruments to investigate the physical processes accompanied by an EPB event that occurred at low latitudes over China.They provided observational evidence that an enhanced equatorward wind associated with a substorm could have re-initiated the Rayleigh-Taylor Instability (RTI) that forced several depletions to surge poleward by nearly 9° two hours before midnight,after a stay of nearly three hours at near 10° magnetic latitude.Accompanied by the growing phase of the EPB were two airglow-type blobs generated in the downwelling regions of a Large-Scale Wave-Like Structure (LSWS).They proposed a mechanism of LSWS-blob connection in which a westward polarization electric field inside the LSWS was likely to have compressed plasma downward,inducing the two airglow-type blobs in the bottom side ionosphere.During the decay phase,an enhanced poleward wind associated with a passing-by Brightness Wave (BW) was likely to have transported plasma to fill the airglow depletions,which finally evolved into brightness airglow structures.

The nighttime O2auroral emission in winter was extracted based on the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observation over 18 years[13].The horizontal structure of O2aurora zone and the vertical profile of O2auroral volume emission rate were studied.The results indicated that the O2auroral intensity,peak emission rate,and peak height were respectively in the ranges of 0.14–5.97 kR,0.97 ×102–41.01 × 102photons cm–3·s–1,and 104–112 km under the condition ofKpLevels 1–5.The peak height was negatively correlated with the auroral intensity and peak volume emission rate.The intensity and peak emission rate were positively related toKpindex but the peak height was negatively related toKpindex.The O2auroral intensity and peak volume emission rate (peak height) under solar maximum conditions were less(higher) than those under solar maximum conditions.

The SABER OH (9–7,8–6) (2.0 μm) and OH (5–3,4–2) (1.6 μm) airglow measurements are simulated by an OH airglow forward model using OH (4–2,5–2,8–5,9–6) airglow data measured by SCIAMACHY[14].The study shows that SABER “unfiltered” data are about 40% at 1.6 μm and about 20% at 2.0 μm larger than the related simulations using the SCIAMACHY data.Deviations can be reduced by about 50% when considering the SABER interference filter characteristics and the latest HITRAN OH Einstein coefficients.The rest differences may be related to model parameter uncertainties and the SABER radiometric calibration.Yuet al.[15,16]compared the observations of the sporadic E (Es) layer from global 25 ionosondes including five digisondes under the Chinese Meridian Project and the GNSS Radio Occultation (RO) satellite measurements.Their analysis indicates that there is a universal connection between theS4max index retrieved from GNSS RO measurements and the critical frequency of Es layers (f0Es).The S4 max between 90 and 130 km altitude can be used as a proxy for the intensity of the Es layer.Yuet al.[17]found a largescale winter-to-summer transport of long-lived metallic ions within Es layers driven by the lower thermospheric meridional circulation,which can account for the longstanding mystery of the seasonal variability of the Es layer.To quantitatively investigate the global distribution of metal ions and the formation mechanisms of the ionospheric Es layer,Wuet al.[18]extended the high-altitude chemistry-climate model to incorporate the full life cycle of multiple meteoric ions and atoms (Mg,Na,and Fe).The model with full ion transport significantly improves the simulation of global distribution and seasonal variations of meteoric ions.In order to help design a multistate meteor radar system,Zhonget al.[19]numerical simulated the measurement errors that affect the spatial resolution and obtain the spatial-resolution distribution in three-dimensional space for the first time.Moreover,they estimated the accuracy of retrieved horizontal wind parameters.

Xiaet al.[20]investigated the variation in stratospheric water vapor using data from observations of the Microwave Limb Sounder on the Aura satellite (MLS),ERA-Interim reanalysis (ERAI),and simulations by the Whole Atmosphere Community Climate Model (WACCM).It is found that the differences of annual-mean stratospheric water vapor among these datasets may be partly caused by the differences in vertical transports.Using budget analysis,they found that the upward transport of water vapor at 100 hPa is mainly located over the Pacific warm pool region and South America in the equatorial tropics in boreal winter and over the southeast of the South Asian high and south of North America in boreal summer.Temperature averaged over regions with upward transport is a better indicator of interannual variability of tropical mean stratospheric water vapor than the tropical mean temperature.It seems that the distributions of the seasonal cycle amplitude of lower stratospheric water vapor in the tropics can also be impacted by the vertical transport.The radiative effects of the interannual changes in water vapor in the lowermost stratosphere are underestimated by approximately 29% in both ERAI and WACCM compared to MLS,although the interannual variations of water vapor in the lowermost stratosphere are dramatically overestimated in ERAI and WACCM.The results here indicate that the radiative effect of long-term changes in water vapor in the lowermost stratosphere may be underestimated in both ERAI and WACCM simulations.

Xiaet al.[21]investigated Na layer diurnal variations from the Na lidar at Yanqing station (40.5°N,116.0°E).Considerable diurnal variation in Na density on the layer top and bottom are clearly revealed on a logarithmic scale.Intriguingly,larger height-scale nighttime extensions on the Na layer topside have been frequently observed from early summer to autumn.These observational results can provide valuable evidence for studying solar effect on the metal layer variation,atmospheric dynamical and chemical processes in the mesosphere and lower thermosphere region.

Xiaet al.[22]reported a statistical analysis of Sporadic Sodium Layers (SSLs) observed by the diurnal Na lidar at Yanqing station.SSLs occurred more frequently around 03:00–04:00 LT during nighttime and 16:00–17:00 LT during daytime,while rarely around midday.Diurnal variation of SSL occurrence is related to tidal-modulated Es height and photochemistry and ionization reactions.The possible explanations for the diurnal and seasonal variations of SSLs characteristics are discussed,and the key role of the evolution of Es layer height modulated by semidiurnal tide in the local time variation of SSL is highlighted.

Huet al.[23]found that the ozone level in the Arctic stratosphere at 100–150 hPa during 1998–2018 exhibits a decreasing trend of pre-decade–0.12 ± 0.07 ppmv from MERRA2,suggesting a continued depletion during this century.About 30% of this ozone depletion is contributed by the second leading mode of Sea Surface Temperature Anomalies (SSTAs) over the North Pacific with one month leading and therefore is dynamical in origin.The North Pacific SSTAs associated with this mode tends to result in a weakened Aleutian low,a strengthened Western Pacific pattern and a weakened Pacific-North American pattern,which impede the upward propagation of wavenumber-1 waves into the lower stratosphere.The changes in the stratospheric wave activity may result in decreased ozone in the Arctic lower stratosphere through weakening the Brewer-Dobson circulation.

Liuet al.[24]investigated the sub-seasonal relationship between the AO and stratospheric ozone over the Arctic in each boreal winter month during 1980–2017 that is independent on the El Ni?o-Southern Oscillation and Quasi-Biennial Oscillation signals using Modern-Era Retrospective Analysis for Research and Applications version 2 reanalysis data sets.Results showed that the positive AO phases correspond to two negative anomalous ozone centers located in the middle stratosphere (about 30 hPa) and upper troposphere-lower stratosphere over the Arctic in each month.There is an in-phase relationship between the AO and the Arctic ozone at 70–100 hPa in December,which is opposite to the out-of-phase relationship between these two metrics in mid-to-late winter.In December,the subtropical jet in the Pacific under the positive AO phase shifts poleward with strengthened planetary wavenumber-2 waves in the lower stratosphere.The strengthened wavenumber-2 wave flux contributes to the positive ozone anomalies at 70–100 hPa via modifying the asymmetric component of polar vortex.However,in January and February,the subtropical jet weakens during the positive AO years,along with fewer planetary wavenumber-1 waves propagating into the Arctic lower stratosphere.This weakened wavenumber-1 wave tends to result in the negative ozone anomalies via weakening the downwelling branch of Brewer-Dobson circulation and strengthening the stratospheric Arctic vortex.

Liuet al.[25]compare the relationship between the Arctic Oscillation (AO) and ozone concentration in the lower stratosphere over the Arctic during 1980–1994(P1) and 2007–2019 (P2) in January and February using reanalysis datasets.The out-of-phase relationship between the AO and ozone in the lower stratosphere is significant in January during P1 and February during P2,but it is insignificant in January during P2 and February during P1.The variable links between the AO and ozone in the lower stratosphere over the Arctic in January and February are not caused by changes in the spatial pattern of AO but are related to the anomalies in the planetary wave propagation between the troposphere and stratosphere.The upward propagation of the planetary wave in the stratosphere related to the positive phase of AO significantly weakens in January during P1 and in February during P2,which may be related to negative buoyancy frequency anomalies over the Arctic.When the AO is in the positive phase,the anomalies of planetary waves further contribute to the negative ozone anomalies via weakening the Brewer-Dobson circulation and decreasing the temperature in the lower stratosphere over the Arctic in January during P1 and in February during P2.

Xuet al.[26]studied the effects of the stationary and transient transport of ozone in the Upper Troposphere and Lower Stratosphere (UTLS) on the Ozone Valley over the Tibetan Plateau (OVTP) in summer using the daily ERA-Interim reanalysis dataset for the time period 1979–2016.They used the Lorenz circulation decomposition method to separate the stationary and transient transport of ozone into terms related to either the mean flow or eddies.The decrease in the total ozone concentration in summer is associated with the transport of ozone,which,in turn,reinforces the OVTP.The zonal(meridional) transport of ozone,which combines stationary and transient transport,strengthens (weakens) the ozone valley.The stationary zonal (meridional) transport of ozone strengthens (weakens) the ozone valley.The transient zonal (meridional) transport of ozone weakens (strengthens) the ozone valley,but this effect is weaker than that of stationary transport.The mean flow has the dominant role,especially in the stationary component.The effect of eddies on the zonal transient transport of ozone is as strong as that of the mean flow.For stationary transport,the zonal deviation of ozone transported by the zonal mean flow in the zonal (meridional)directiondominates total zonal(meridional) change of ozonewhich strengthens (weakens) the ozone valley.The transient transport of the zonal mean ozone by eddiesthe zonal deviation of ozone by the zonal mean flowand the zonal deviation of ozone by eddiesall have a strong effect on the ozone valley.By contrast,the transient transport of the zonal mean ozone by eddies in the meridional directionhas a much weaker and the smallest effect.Both the zonal deviation of ozone by the zonal mean flow and by eddies in the meridional directionhave major roles in transient meridional transport,but their roles are the opposite of each other.The contributions of stationary and transient transport to zonal transport are consistent,whereas their contributions to meridional transport are the opposite of each other.The influence of transient transport on the formation and maintenance of OVTP is not negligible.

Using Equatorial Electric Field (EEF) data observed by Swarm satellites,Liuet al.[27]investigated the main wave sources of the longitudinal structures of the EEF (LSEEF) in all seasons for the first time.They found that there existed a prominent one-peaked structure in all seasons,with a peak at 90°E–120°E and trough at 30°W–70°W,which was neglected in the past.Furthermore,they determined the main wave sources of the LSEEF in different seasons and their contributions.That is,DW2 (DE3 and DE2,DE3,DW2 and SW4)were the main wave sources of the LSEEF in spring(summer;autumn;winter) with contributions exceeding 15%.

Yanget al.[28]analyzed a Gigantic Jet (GJ) event at about 22:43:30 BJT (Beijing Time=UTC+8) on 13 August 2016 which was captured by two amateur astronomers in Shikengkong,Guangdong Province,and Jiahe County,Hunan province,respectively.Using optical images,Doppler weather radar data,lightning detection network data,magnetic field,ECMWF reanalysis,sounding data,and the infrared weather maps of MTSAT (Multi-Function Transport Satellite),they analyzed the meteorological background environment,characteristics of the parent thunderstorm,and lightning activity by using multiple data.In addition,three interesting NBEs have been found in a time window containing the GJ,and the characteristics of NBEs were also analyzed.The research shows that the GJ was associated with strong vertical development of the thunderstorm and the GJ occurred near the thunderstorm’s strong convection region (overshooting top).The negative cloud-to-ground flashes dominated during the thunderstorm evolution.Three positive Narrow Bipolar Events (NBEs) were detected within 30 s before and after the GJ.It indicates that the NBEs occurred in the upper and middle layers of the thunderstorm (at an altitude of 11–13 km) with radar reflectivity of 30–35 dBz.

Liuet al.[29]examined the optical emissions observed by the Atmosphere-Space Interactions Monitor(ASIM) on the International Space Station associated with narrow bipolar events from thunderstorm clouds penetrating into the stratosphere.The spectral measurements were obtained for such emissions related to nine negative and three positive NBEs observed by a groundbased array of receivers.It is found that both polarities NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm,suggesting that NBEs are associated with streamer breakdown.The rise times of the emissions for negative NBEs are about 10 μs,consistent with source locations at cloud tops where photons undergo little scattering by cloud particles,and for positive NBEs are about 1 ms,consistent with locations deeper in the clouds.For negative NBEs,the emission strength is almost linearly correlated with the peak current of the associated NBEs.This finding reveals the optical emissions of NBEs,which would provide a new means to measure the occurrences and strength of cloud-top discharges near the tropopause by ground-based radio signals.

Liuet al.[30]reported on two mid-latitude thunderstorms in which the outbreaks of negative NBEs produced 13 blue discharges.Using the ISUAL observations,VLF/LF sferic data from a ground-based array,infrared brightness temperatures from satellites,and the reflectivity data of an S-band radar,the meteorological conditions and charge structures of two thunderstorms were investigated.In these two thunderstorms,blue discharges always occurred in the vicinity of the coldest cloud top (195 K) and clustered within a bounded area near the convective surge,leading to the overshooting thundercloud top (reaching about 18 km) into the stratosphere.The parent thunderstorms were normally electrified with a main midlevel negative and an upper positive charge layer centered at about 15 km as inferred from source heights of NBEs.The associated negative NBEs near the tropopause indicated that they are upward positive discharges initiated between the upper positive charge layer and the negative screening layer at the cloud top.These results also suggest that blue discharges are preferred to occur in selective thunderstorms associated with strong convective surges.These intense updrafts induce a high and therefore create favorable charge structures for initiating upward positive blue discharges.

Liuet al.[31]reported that on the analyses of twomidnight thunderstorms in East China that were both characterized by outbreaks of negative NBEs.Combining with the VLF/LF radio signal measured by Jianghuai Area Sferic Array (JASA),S-band Doppler radar observation and balloon sounding data,two mid-latitude thunderstorms with outbreaks of negative NBEs at midnight in East China were analyzed.The comparison with the vertical radar profile shows that the bursts of negative NBEs occurred near thunderclouds with overshooting tops higher than 18 km.Manifestation of negative NBEs is observed with a relatively low spectrum width near thundercloud tops.The work findings suggested that the outbreak of negative NBEs in both parent thunderstorms was produced near the overshooting tops with an intense convection surge region where the significant updrafts drove the cloud top to penetrate above the tropopause.The relatively low values of spectrum width(＜5 m·s–1) around the outbreak of negative NBEs indicate that negative NBEs tend to occur under the relatively low mixing between the upper positive charge layer and screening charge layer at thundercloud tops.Further research showed that the outbreak of negative NBEs is usually associated with overshooting tops of thunderclouds.

Renet al.[32]examined the detailed development of halo/sprite events by comparing the high-speed video observation and broadband sferic measurements.Using an intensified high-speed camera,a low-light-level video camera (SpriteCam),several radio-frequency magnetic sensors,and the National Lightning Location Network(NLDN),a total of 51 sprites have been recorded over an MCS in the central United States.They selected two sprites with halo features to study.The first event was the brightest sprites observed on that night,while the second event was a dancing sprite event containing three sprite elements all following a single +CG.The lightning-induced E-field at halo and sprite altitudes is calculated to separate the static component generated from the charge displacement and the induction term generated by the movement of charge with the TL model,and they analyzed the lightning-induced E-field perturbation at the altitude of halo initiation.It is found that the E-field generated by the current pulse of charge transfer may be more important than that generated by the charge relocation for the initiation of halos.Furthermore,the traditional electrostatic field theory has been significantly supplemented.

Wanget al.[33]reported the ground-based observation of negative sprites over a tropical thunderstorm as the embryo of Hurricane Harvey (2017).Using the ground-based observations provided by an amateur photographer,Frankie Lucena,and ultralow-frequency(ULF,＜1–400 Hz)/very-low-frequency to low-frequency (VLF-LF,0.5–470 kHz) magnetic field measured in Duke Forest,they examined six red sprites produced by negative Cloud-to-Ground (CG) lightning strokes in a tropical thunderstorm that later evolved into Hurricane Harvey (2017),and analyzed these parent strokes characteristics.Most of the sprite-parent CG strokes occurred at the edge of deep convection cores (as inferred from cold cloud tops and high lightning density).It was found that tropical marine meteorological systems,such as tropical disturbances,depressions,and thunderstorms are more likely to be the main production systems of negative sprites.The frequent occurrence of 18 GJs produced by the same thunderstorm further indicates that the thundercloud charge structures of sprite-producing oceanic thunderstorms are significantly different from that of continental thunderstorms.

Wanget al.[34]reported the space-born observation of a negative sprite with an unusual signature of associated sprite current.With the observations from ISUAL and the WWLLN,they found an extremely rare case of negative sprite where the event is near the northern border of Bogotá,Colombia.Although the sprite observations associated with negative Cloud-to-Ground (CG)strokes are very rare,it contains a distinct “sprite current” feature.Since the unusual red sprite was unrelated to the parent stroke and the atypical structure of CG,we analyzed the charge transfer time.It was found that the extraordinarily long charge transfer time (5.25 ms) after the parent negative CG stroke might play a critical role in the formation of the intense sprite current and the formation condition of the unusual sprite may also be attributed to the plasma irregularities in the mesosphere.

Zhanget al.[35]reported nine TGF events related to NBEs with concurrent LF sferics and lightning location data.The LF data examined in this work were acquired respectively at multiple stations.The analyses of these TGFs found that the occurrence of NBEs was preceded by a minimum of 60 μs to 13.5 ms,and no other fast leader discharge was found within 20 ms before the TGF.Further research showed that the NBE preceding TGFs bear a harder energy spectrum with a larger proportion of high-energy photons than EIP related TGFs produced in association with the lightning leader on a statistical basis indicate.The results indicated that the NBE-related TGFs were produced by the large-scale thunderstorm E-field,supporting the relativistic feedback mechanism of TGF generation.That is,the high Efield between the two major charge layers in the thundercloud is also capable of producing TGFs,and the TGF-producing process could contribute to generating NBEs.

Zhanget al.[36]analyzed the role of chemical processes in the QBO impact on stratospheric ozone in the tropics and Northern Hemisphere during winter and early spring.During easterly QBO phases,tropical ozone concentrations decrease in the lower stratosphere and middle stratosphere but increase in the transition region between 15 and 40 hPa compared to westerly phases.Although the contributions of chemical processes to the ozone QBO signal are less than those of dynamical processes,the role of chemical processes is non-negligible.Xieet al.[37]revealed that the joint effect of El Ni?o-Southern Oscillation (ENSO) and QBO on stratospheric ozone is approximately equal to the linear superposition of their independent impacts because the phases of wave-1 and wave-2 planetary waves anomalies related to ENSO activities are broadly similar to those of QBO phases.

Wanget al.[38]found that the anomalously large Antarctic ozone hole in late austral spring is related to the weakened residual circulation and anomalous planetary wave reflection from late October to mid-November.They also showed that the anomalously large ozone loss in August is not a precondition for the anomalously large Antarctic ozone hole in late spring.

Zhanget al.[39]evaluated long-term changes in Total Column Ozone (TCO) and the ozone valley over the Tibetan Plateau (TP) from 1984 to 2100 using Coupled Model Inter comparison Project Phase 6 (CMIP6) and found that most of the CMIP6 models can capture the TP ozone valley.Further analysis revealed that coupled chemical-radiative-dynamical processes play a key role in the simulation of the TP ozone valley.Multi-model mean predicts that the TP ozone valley in summer will deepen in the future.

Xieet al.[40]pointed out that SST warming in the past 100 years has caused an increase in stratospheric water vapor.SST warming over the tropical Indian Ocean and the western Pacific has resulted in a drier stratosphere.However,tropical Atlantic Ocean warming has resulted in a significantly wetter stratosphere and is the main contributor to the increasing trend of water vapor in the past 100 years.The responses of Rossby and Kelvin waves over the Indian Ocean and western Pacific to Atlantic warming have led to a warmer tropopause temperature,resulting in more water vapor entering the stratosphere.

By deploying a UV radiometer aboard a stratospheric balloon released at Qaidam (QDM) during the Asian Summer Monsoon (ASM) period in 2019,Zhanget al.[41]provided in-situ measurement of the UV profiles from the surface to the upper troposphere and lower stratosphere over the Tibetan Plateau (TP),China,for the first time.Based on two in situ UV profiles accompanied by four ozonesonde measurements,the study exhibited detailed variations of downwelling UV and vertical ozone distributions over the TP during the ASM period.The UV differences between the surface and stratospheric balloon flight altitudes were 16.7,15.8,12.6 and 18.0 W·m–2during the four ozonesonde launches.Due to the diurnal variations in photochemical production and the stratosphere-troposphere exchange,the integrated ozone columns below 30 km ranged from 184.4 to 221.6 DU from four ozonesonde measurements.A positive correlation between UV attenuation and ozone column was exhibited under low cloud cover and clear sky conditions.

The ozone profile in the troposphere and lower stratosphere over Beijing has been observed since 2002 by ozonesondes developed by the Institute of Atmospheric Physics.As more observations are now available,Zhanget al.[42]used these data to analyze the longterm variability of ozone over Beijing during the whole period from 2002 to 2018.It is found that the ozonesondes measured increasing concentrations of ozone from 2002 to 2012 in both the troposphere and lower stratosphere.There was a sudden decrease in observed ozone between 2011 and 2012.After this decrease,the increasing trend in ozone concentrations slowed down,especially in the mid-troposphere,where the positive trend became neutral.They also used the Chemical Lagrangian Model of the Stratosphere (CLaMS) to determine the influence of the transport of ozone from the stratosphere to the troposphere on the observed ozone profiles.CLaMS simulations showed a weak increase in the contribution of stratospheric ozone before the decrease in 2011–2012 and a much more pronounced decrease after this time.Because there is no tropospheric chemistry in CLaMS,the sudden decrease simulated by CLaMS indicates that a smaller downward transport of ozone from the stratosphere after 2012 may explain a significant part of the observed decrease in ozone in the mid-troposphere and lower stratosphere.

Liet al.[43]have reported the processes of dehydration and low ozone in the tropopause layer over the Asian monsoon caused by tropical cyclones based on the Lagrangian transport calculations using ERA-Interim and ERA5 reanalysis data.Maet al.[44]investigated the mixing characteristics within the tropopause transition layer over the Asian summer monsoon region based on ozone and water vapor sounding data.

Daiet al.[45]used a Rayleigh lidar has been to study the middle atmosphere at Golmud (36.25°N,94.54°E),Qinghai,located in the northeastern part of the Tibetan Plateau.Mesospheric density profiles from 50 to 90 km were retrieved based on 205 nights of lidar observation from Aug.2013 to Oct.2015,with a total of 1616 hours of operation.They compared our lidar density measurements with SABER observations onboard TIMED satellite and MSIS-00 model data.The results showed that the annual mean density measured by lidar agreed well with SABER data,but both were lower than that of MSIS-00.All datasets exhibited dominant annual oscillation in the mesosphere.From 63 to 85 km,the annual amplitude of lidar density is larger than those of SABER and MSIS-00.PDD (Percentage of Density Difference)was calculated to investigate the mesospheric density climatology.The largest density variations of lidar,MSIS-00,and SABER occurred at around 72 km.Both lidar and SABER PDD reached their maximum in May,about one month earlier than the MSIS-00;while the minimum PDD appeared in late December for all datasets.

3 Climate and Modeling

Jiaoet al.[46]gave the full seasonal cycles of the Ni layer and Na layer,based upon the Yanqing lidar observations.The Ni and Na layers exhibit a similar annual cycle,increasing by a factor of about 3 from a mid-summer minimum to a midwinter maximum.The mean Na:Ni ratio is 8.1,which is significantly larger than their CI chondritic ratio of 1.2.This is explained by the more efficient ablation of Na from cosmic dust particles by a factor of 3,and the more rapid neutralization of Na+between 90 and 100 km,where the measured Na+:Ni+ratio is only 2.2.The Ni layer peak occurs around 84 km,8 km below that of Na.These features are simulated satisfactorily by the Whole Atmosphere Community Climate Model (WACCM) and are explained by significant differences in the neutral chemistry of the two metals below 90 km and their ion-molecule chemistry between 90 and 100 km.

Zhou and Chen[47]documented the possible influence of the 11-year solar cycle on the onset of the South China Sea Summer Monsoon (SCSSM) based on the reanalysis dataset and observational Sunspot Number(SSN) from 1948–2017.They found that the SSN is significantly correlated to the SCSSM with stronger (weaker) solar activity corresponding to later (earlier) outbreak of the SCSSM.Further composite analysis revealed that during the peak (valley) years of SSN,an anomalous anticyclone (cyclone) appears around the Philippines in May,concurrent with the westward-shifting (eastward-shifting) and intensified (declined) western Pacific Subtropical High.This can be attributed to the change of the local meridional circulation related to the convective activity over the maritime continent south of equator and the variations of the Walker circulation over the tropical Indo-Pacific region.And the solar signals on the SCSSM are suggested to originate from the temperature responses in the stratosphere.With the enhancement of solar irradiance,the upper-troposphere to the lower-stratosphere over the entire southern hemisphere is warmer in March and April.Through modulating the mean meridional circulation,there is a negative Antarctic Oscillation pattern in the lower troposphere induced by the redistribution of the atmosphere mass.The subsequent cyclonic circulation anomalies in the mid-latitudes delay the establishment of the Somali cross equatorial flows when the solar activity is stronger and the SASSM onset tends to occur later accordingly.Zhou and Chen[48]also investigated possible linkages between Antarctic sea ice and the 11-year solar cycle as well as related physical processes by using NCEP-DOE reanalysis datasets,sea ice concentration data from the Hadley Center and sunspot number data.In years with high solar activity,sea ice concentration is low in the vicinity of the Ross Sea and high in the Weddell Sea near the Antarctic Peninsula.And the Antarctic Sea Ice Dipole(SID) index is significantly negatively correlated with the Antarctic Oscillation (AAO),possibly through the mediation of the westerly jet.When the AAO is strong,the westerly stream turns southward around the Antarctic Peninsula-Weddell Sea,and northward near the Ross Sea,resulting in negative SID anomalies as warm air enters the Antarctic Peninsula-Weddell Sea region and cold air intrudes over the Ross Sea.Sea ice and circulation anomalies are reversed when the AAO is weak.These findings extend earlier ones by emphasizing the possible impact from solar cycle,which has practical use for climate prediction.

Zouet al.[49]report a statistical study on the effects of strong geomagnetic activity on the mesopause temperature over the auroral region from 2002 to 2018.They found the energetic electron precipitation was significantly enhanced in the 55°–70° geomagnetic latitude band.At the same time,the mesopause temperature increased about 4 K at 95 km immediately in the mesopause region,together with a descent of the mesopause of about 0.5–2 km.They suggested that mesopause is mainly influenced by electrons in the energy range of 30–100 keV.

Wuet al.[50]analyzed a long-term simulation of the Whole Atmosphere Community Climate Model with the chemistry of three metals (Na,K,and Fe),and presented the response of the meteoric metal layers in the mesosphere and lower thermosphere regions to the 27-day solar rotational cycle.The altitude-dependent correlation and sensitivity of the metal layers to the solar spectral irradiance demonstrate that there is a significant increase in sensitivity to solar rotational cycle with increasing altitude.

Xiaet al.[51]found that the poleward expansion of the Hadley circulation in autumn is closely related to the increase of Stratospheric Water Vapor (SWV) under greenhouse warming.The SWV increase radiatively cools the stratosphere,especially in the polar lower stratosphere,which consequently leads to widening of the Hadley cell in autumn.The SWV effect is affirmed in a set of “SWV-locking” experiments.It is found that the SWV increase leads to a poleward expansion of the Hadley circulation in autumn in both Hemispheres,which contributes about 30% of the total expansion due to quadrupling CO2in autumn.

Record ozone loss was observed in the Arctic stratosphere in spring 2020.Xiaet al.[52]found that the extreme Arctic ozone loss was likely caused by recordhigh Sea Surface Temperatures (SSTs) in the North Pacific.It is found that the record Arctic ozone loss was associated with the extremely cold and persistent stratospheric polar vortex from February to April,and the extremely cold vortex was a result of anomalously weak planetary wave activity.Further analysis reveals that the weak wave activity can be traced to anomalously warm SSTs in the North Pacific.Both observations and simulations show that warm SST anomalies in the North Pacific could have caused the weakening of wavenumber-1 wave activity,colder Arctic vortex,and lower Arctic ozone.These results suggest that for the present-day level of ozone-depleting substances,severe Arctic ozone loss could form again,as long as certain dynamic conditions are satisfied.

Raoet al.[53]assessed four Chinese models and found that the SSW frequency in most CMIP6 models is underestimated.SSWs mainly appear in midwinter in observations,but one-month climate drift is simulated in the models.The contrasting difference in the intensity for displacement and split events are well simulated by Chinese models.Rao and Garfinkel[54]included more CMIP5/6 models in their study to assess the possible future change in the SSW under a moderate emission scenario (RCP45/SSP245) and a strong emissions scenario(RCP85/SSP585).An insignificant (though positive)change in the SSW frequency from historical simulations to RCP45/SSP245 and then to RCP85/SSP585 is consistently projected by CMIP5 and CMIP6 multimodel ensembles.Further,they emphasized that the troposphere-stratosphere coupling strength during SSWs is nearly unchanged in the future scenario simulations.With the same models,Rao and Garfinkel[55]examined the possible future changes of Stratospheric Final Warming (SFW) events.Most CMIP5/6 models project a delay of SFWs in the two future scenarios in the Northern Hemisphere.In the Southern Hemisphere,the SFW date is largely unchanged as ozone recovers through the end of the century.

Xiaet al.[56]calculated the Stratospheric Water Vapor (SWV) climate feedback using the 150-year CO2forcing (1 pct CO2) simulations in the CMIP6 ensemble of models.All models robustly show a moistening of the stratosphere,causing a positive radiative feedback to surface warming.It is found that the stratospheric moistening rate and the SWV feedback both increase with surface warming.The moistening occurs at a rate of 0.9±0.1 ppmv·K–1and its radiative feedback measured by the fixed-dynamical-heating method is 0.11±0.02 W·m–2·K–1in the first 50 model years;the moistening rate increases to 1.2±0.2 ppmv·K–1and the feedback increases to 0.16±0.03 W·m–2·K–1in the last 50 model years when the global mean surface temperature is 3.3 K warmer.These increases are found to be caused by an amplified rate of tropical tropopause warming with respect to surface warming,which is 0.6 K·K–1and 1.1 K·K–1for the two 50-year periods,respectively.They concluded that the SWV feedback is strengthening with surface warming,which can contribute to increasing climate sensitivity in the future under global warming.

Zhanget al.[57]reported the correlation of the Low Frequency (LF) lightning sferics of two Terrestrial Gamma ray Flashes (TGFs),including one TGF associated lightning discharge at only about 28 km range.The LF lightning sferics associated with TGFs detected by Fermi Gamma-ray Burst Monitor (GBM) over equatorial thunderstorms have been recorded at a station in Melaka,Malaysia,in 2017 and 2018.By means of the lightning detection data of the WWLLN,and Vaisala’s Global Lightning Dataset (GLD360),the paper analyzed TGFs are related to the strongest pulse during the initial stage of their parent Intracloud (IC) lightning.It shows on a statistical basis that TGF related lightning is mostly located in the strong convection of equatorial thunderstorms at the mature stage,and nearly half TGFs are not produced in the strongest convection region.

Zhanget al.[58]provided evidence from both observations and model simulations that zonally asymmetric stratospheric ozone depletion gives a significant feedback on the position of the polar vortex and further favors the stratospheric polar vortex shift toward Siberia in February for the period 1980–1999.However,the polar vortex shift is not significant in the experiment forced by zonal mean ozone fields.

Xieet al.[59]investigated the effects of global and regional SST warming from the Industrial Revolution to the present on the stratosphere using a climate model,and estimated the relative contributions of SST warming in different regions.They found that the observed global SST warming and 1-K uniform global SST warming have opposite effects on the high-latitude stratosphere in both hemispheres:1-K uniform global SST warming results in warmer and weaker stratospheric zonal circulations and a corresponding increase in ozone,and vice versa for observed global SST warming.

Chenget al.[60]described the density correction of the NRLMSISE-00 using more than 15 years (2002–2016) of TIMED/SABER satellite atmospheric density data from the middle atmosphere (20–100 km).A bias correction factor dataset is established based on the density differences between the TIMED/SABER data and NRLMSISE-00.Seven height nodes are set in the range between 20 and 100 km.The different scale oscillations of the correction factor are separated at each height node,and the spherical harmonic function is used to fit the coefficients of the different timescale oscillations to obtain a spatiotemporal function at each height node.The evaluation results show that the spatiotemporal correction function proposed in this paper achieves a good correction effect on the atmospheric density of NRLMSISE-00.The ability of the model to characterize the mid-atmosphere (20–100 km) is significantly improved compared with the pre-correction performance.

Based on the atmospheric density observed by TIMED/SABER satellite from 2002–2018,the grid data of monthly average and standard deviation were calculated statistically by Chenget al.[61].Driving by the grid data,the atmospheric density is characterized as the sum of the monthly average and the large-scale disturbances and small-scale disturbances.The large-scale perturbations and small-scale perturbations are characterized by cosine functions and first-order autoregressive models,respectively.By comparing the simulated values of the model with the observations of the lidar in Dunhuang,the results show that the model values have a good agreement with the observations,which verifies that the modeling method is feasible.In addition,Monte Carlo method can be used to reproduce all possible states of atmospheric density on a given trajectory.The model can be used as a tool to provide density data for aerospace design and reentry trajectories simulations.

4 Dynamics in the Middle Atmosphere

4.1 Meteorological Process

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) cross-track wind data (version 2.0,dusk side) have been compared with winds measured by four ground-based FPIs at low and middle latitudes (PAR,Arecibo,CAR and XL),and the time span is from 2010 to 2013[62].The results showed that during geomagnetically quiet periods,GOCE crosswinds are 1.37–1.69 times larger than the ground-based FPIs winds,and the GOCE crosswind has typical seasonal variations with the largest speed around December and lowest speed around June,which is consistent with the ground-FPI measurements.The correlation coefficients between the four stations and GOCE crosswind data all reach around 0.6.During geomagnetically active periods,the relation between the GOCE and FPI derived winds are generally poorer,with average ratios of 0.85 for the Asian station (XL) and about 2.15 for the other three American stations (PAR,Arecibo and CAR).The discrepancies of absolute wind values from the GOCE accelerometer and ground-based FPIs should be mainly due to the different measurement principles of the two techniques.The results also suggested that XL FPI can provide reliable wind observations in the same quality as from established FPIs in the American sector.

Wuet al.[63]found oppositely Medium-Scale Traveling Ionospheric Disturbances (MSTID) in low latitudes during a geomagnetically quiet night.These MSTIDs showed the wave-fronts aligned from northwest to southeast.Some MSTID structures of them propagated southwestward and others propagated northeastward.In addition,these MSTID structures encountered and interacted with each other.The interactive process of these MSTIDs should be related to their polarization electric fields.Sunet al.[64]investigated the interaction between a midlatitude MSTID and a poleward moving Weddell Sea Anomaly (WSA)-like plasma patch.They provided observational evidence that Polarization Electric Field (PEF) inside passing-by MSTID or its seeding Es could frequently drive plasma patches from the Equatorial Ionization Anomaly (EIA) poleward to cause the Midlatitude Summer Nighttime Anomaly Structures(MSNAs) over China.This kind of MSNA in turn interacted with the MSTID,causing some poleward extending C-shaped airglow depletions/enhancements of the MSTID in a transition region where the ionosphere changed from a collapse region to an uplifted one.Sunet al.[65]investigated an interaction between an EMSTID and an EPB in the EIA crest region over China.The interaction changed the phase elongations and drifting velocities of both the EMSTID and EPB when they encountered.Moreover,interaction could have polarized one depletion of the postmidnight EPB,inside which freshly-generated meter-scale irregularities caused activated radar echoes and enhanced Ranged Spread F(RSF) over Fuke station.An observational evidence was provided that how an electrical couple of EMSTID and EPB events can activate a postmidnight EPB depletion over low latitudes of China.Luoet al.[66]reported a special MSTID (medium-scale traveling ionospheric disturbance) event observed by multi-instruments over midlatitude region of China.The airglow results showed that the inclination angles of MSTID bands were decreasing,resulting in the propagation direction changed from southwestward to nearly westward.It was also found that the MSTIDs disappeared partly in the airglow observation when they propagated to lower latitudes (below 40°N) in the later times.Both the observations from the FPI (Fabry-Perot Interferometer) and the simulations from the TIEGCM (Thermosphere-Ionosphere-Electrodynamics General Circulation Model) indicated that the variations of ionospheric neutral winds might be related to the variations of propagation direction and the disappearance of MSTIDs.

Shanget al.[67]found ionospheric irregularities measured with different instruments have obvious morphological differences at Hainan station after midnight,where the larger the scale of the irregularities,the slower its attenuation.Compared with the magnetic equator,the ionospheric irregularities near equatorial abnormal peaks can last until near dawn.The occurrence of ionospheric irregularities in Hainan and Southeast Asia is obviously related to the quasi-periodic structure of plasma bubbles observed by the C/NOFS satellite passing through the region,indicating that the seeding of atmospheric gravity waves may play an important role in the generation of ionospheric irregularities,even in the second half of the night.

Yiet al.[68]present the climatology of mesopause temperatures using high-and middle-latitude meteor radars.The daily mesopause temperatures are estimated using ambipolar diffusion coefficient data from the meteor radars at Davis Station (68.6°S,77.9°E),in Antarctica,Svalbard (78.3°N,16°E),Troms? (69.6°N,19.2°E) in the Arctic,and Mohe (53.5°N,122.3°E) and Beijing (40.3°N,116.2°E) in the northern middle latitudes.The seasonal variations in the meteor radar-derived temperatures are in good agreement with the SABER and MLS temperatures.

Yiet al.[69]report the response of tides in neutral atmospheric mesospheric winds observed by meteor radar and medium frequency radar to recurrent geomagnetic activity over Antarctica.The zonal component of the daily prevailing winds showed a westward increase as the geomagnetic activity increased.In addition,the zonal and meridional semidiurnal tides both showed a clear upward propagating phase but responded differently to geomagnetic activity.

Yuet al.[70]reported the observations of the 27-day and its harmonic 13.5-day periodic oscillations in the Es layers associated with the 27-day solar rotation period.The spectral analyses show that the 27-day periodic oscillations of Es layers are due to the recurrent geomagnetic activities.

By utilizing reanalysis data,Huanget al.[71]studied the climatology of the eastward and westward traveling 10-day waves from the surface to the middle mesosphere.The westward propagating wave with zonal wavenumber 1 and eastward propagating waves with zonal wavenumbers 1 and 2 are identified as the dominant traveling ones.Liet al.[72]analyzed global characteristics of the westward propagating quasi 16-Day Wave (Q16 DW) with zonal wavenumber 1 in the troposphere and stratosphere from December 2012 to November 2013.During the 2012/2013 Stratospheric Sudden Warming (SSW),the strong wave likely provides a forcing on the splitting of the displaced polar vortex.Tanget al.[73]investigated the climatological features of W1 Q16 DW.The wave amplitude is stronger in the NH than in the SH.The Quasi-Biennial Oscillation (QBO)signatures of the wave are mainly located in the stratosphere at low latitudes,no significant responses to ENSO and solar activity are observed;and the linear trends are generally positive,especially in the mid-upper stratosphere.Using the dataset from an incoherent scatter radar at Arecibo,Gonget al.[74]present a long-term statistical analysis of thermospheric tides in an altitude range from 150 to 400 km,including their climatological mean and seasonal variations and their response to solar activities.

Gonget al.[75]present an analysis of a quarter diurnal tide during the 2019 Arctic SSW event based on meteor radar data at Mohe.Applying meteor radar measurement,Maet al.[76]studied the excitation of quasi-6-day Waves (Q6 DWs) in both hemispheres during the September 2019 Antarctic SSW.The equatorward propagation induced by the SSW is the main reason for the enhanced Q6 DWs in the upper MLT at mid-latitudes in the SH,and the main source of the Q6 DWs in the NH is the seasonal variability.Besides,interhemispheric propagations also contribute to the amplification of Q6 DWs during the 2019 Antarctic SSW.Maet al.[77]also investigated the variations of planetary waves in the MLT during 2019/2020 Arctic winter.Quasi-10-day waves are enhanced following three of these warmings in the zonal winds in the MLT region over Mohe,but unusually weak after the SSW in February 2020,which is due to largely inhibited by the extremely strong polar vortex and a lack of mesospheric instability.

As the number of models that can simulate the QBO increases,some new progresses for the QBO studies have been achieved in the past two years.For example,Raoet al.[78]investigates the impact of the QBO on the northern winter stratosphere with CMIP5/6 outputs.Regardless of biases in QBO periodicity (25–31 months in observationvs.20–40 months in models),the Holton-Tan relationship can be well simulated in CMIP5/6 models with more planetary wave convergence in the polar stratosphere in easterly QBO winters.An Eliassen-Palm(E-P) flux divergence dipole (with poleward E-P flux) is simulated in midlatitude upper stratosphere by most models during QBO easterlies around 30 hPa.Raoet al.[79]further evaluated three dynamical pathways for impacts of the QBO on the troposphere,including the Holtan-Tan effect and the northern annular mode,the subtropical zonal wind downward arching over the Pacific,and changes in local convection over the Maritime Continent and Indo-Pacific Ocean.More than half of the models can reproduce at least one of the three pathways,but few models can reproduce all of the three routes.Based on two emission pathways,Raoet al.[80]indicated an enhanced surface response to the QBOviaa strengthened Holton-Tan relationship in the future.Using the Model of an idealized Moist Atmosphere (Mi-MA) capable of spontaneously generating the QBO,Raoet al.[81]explored the gradual establishment of the extratropical response to the QBO.When easterly QBO winds maximized around 30 hPa are relaxed,an Eliassen-Palm(E-P) flux divergence dipole quickly forms in the extratropical middle stratosphere as a direct response to the tropical meridional circulation,in contrast to the Holton-Tan mechanism.No detectable changes in upward propagation of waves in the midlatitude lowermost stratosphere are evident for at least 20 days after branching,with the first changes only evident after 20 days in perpetual midwinter and season-varying runs,but after 40 days in perpetual November runs.

Huet al.[82]found that the Antarctic stratospheric planetary wave activity in September has weakened significantly since the year 2000.Further analysis indicates that the September Sea Surface Temperature (SST) trend over 20°N–70°S induces a reduction in the tropospheric wave source,and subsequently leads to weakening in the stratospheric wave flux and Brewer-Dobson circulation,while the ozone recovery since 2000 has a minor contribution.

Quanet al.[83]reported the variation characteristics of D region in the lower ionosphere from 62 km to 82 km based on Langfang Medium Frequency (MF)radar.They focused on multiple C-level and M-level solar flare events before and after the large-scale flare event at 11:53 UT on 6 September 2017.The results show that it is difficult to detect the electron density over 70 km in Langfang during solar flares,but the electron density value can be obtained as low as 62 km,and the stronger the flare intensity,the lower the detectable electron density height.Besides,the equal electron density height,the received power of X and O waves will also be significantly reduced during the flares,and the reduction of equal electron density height has a weak linear relationship with flare intensity.

Caiet al.[84]analyzed the dependence of the 11-year solar cycle on horizontal winds in the local mesosphere and lower thermosphere.They found that the zonal wind is positively correlated with solar activity during spring at 80–84 km,and during summer at 80–82 km;the meridional wind is positively correlated with solar activity during spring at 84–88 km and during summer at 84–90 km.They explained the correlations in terms of the changes in stratospheric temperature and the net flux of gravity waves during solar activities.In addition,annual and semiannual oscillations of the zonal/meridional wind were found by using the least squares fitting method on daily horizontal winds,which show negative correlations with solar activity at heights of 80–90 km.

Tianet al.[85]reported the momentum flux of shortperiod (less than 2 h) Gravity Waves (GWs) in the Mesosphere and Lower Thermosphere (MLT),using meteor radar data collected over Langfang,China.They found that seasonal variations in GW momentum flux exhibited Annual Oscillation (AO),Semiannual Oscillation (SAO),and quasi-4-month oscillation.The mean flow acceleration of zonal winds,estimated from the divergence of this flux,was in the same direction as the observed acceleration of zonal winds for quasi-4-month oscillation winds,with GWs contributing more than 69%.In addition,the estimated acceleration due to Coriolis forces to the zonal wind was opposite to the estimated acceleration of high-frequency GWs for quasi-4-month oscillation winds.

Tianet al.[86]showed the diurnal and seasonal variations in atmospheric short-period (less than 2 h) Gravity Waves (GWs) in the Mesosphere and Lower Thermosphere (MLT).They found that GW activity was strong over a 24-hour period,above the 95% confidence level,during almost every month of the year.A 12-hour period of particularly high activity was also evident in April and October for zonal wind variance,as well as in January,April,May,and December for meridional wind variance.Additional periods were observed for the first time,including 4-,6-,and 8-hour cycles with confidence intervals greater than 95%.These results suggest the possibility that GW activity could be modulated by solar-heating tidal wave harmonics.

Shiet al.[87]analyzed a case of CGWs detected simultaneously by the AIRS (Atmospheric Infrared Sounder) and the VIIRS/DNB (Day/Night Band of the Visible Infrared Imager Radiometer Suite) in the stratosphere and mesosphere.Results showed that Gravity Waves (GWs) were generated by the collocated Hurricane Bejisa on the island of Mauritius.The AIRS data showed arc-like phase fronts of GWs with horizontal wavelengths of 190 and 150 km at 21:08 Coordinated Universal Time (UTC) on 1 January 2014 and at 10:00 UTC on 2 January 2014,whereas the DNB observed arced GWs with horizontal wavelengths of 60 and 150 km in the same geographic regions at 22:24 UTC.The characteristics of CGW parameters in the stratosphere(about 40 km) and the mesosphere (about 87 km),such as the vertical wavelength,intrinsic frequency,and intrinsic horizontal phase speed,were first derived together with the background winds from ERA5 reanalysis data and Horizontal Wind Model data through the dispersion relationship of GWs and the wind-filtering theory.

Guoet al.[88]analyzed the global distribution of stratospheric gravity wave activity intensity and occurrence frequency using the 79th channel’s observation data of the AIRS in January and July between 2012 and 2014.The study shows that the gravity wave intensity varies significantly with latitude.In the low latitude area(0–30°),the value of the gravity wave intensity in winter hemisphere is low,but is high in summer hemisphere.In the middle and high latitudes,winter hemisphere has strong gravity waves activity but the summer hemisphere gravity wave is weak.In January,there are four prominent hot spots in the global range,located at 50° north latitude,the continental and Atlantic intersections,and the North American and Atlantic intersections,and at 20° south latitude,the South American and Atlantic intersections,and Africa.Intersection with the Indian Ocean.In July,the gravity wave activity was prominent in Patagonia to Antarctic Peninsula region and the Indian Ocean region near 50° south latitude and 75° east longitude.

4.2 Influence of Lower Atmospheric Perturbation on the Thermosphere/Ionosphere

Sunet al.[89]further analyzed the effect of the MJO on the Northern Hemisphere (NH) mesosphere.Both observations and simulations suggest the anomalous PWs lagging MJO P4 by 25 days lead to the weaker eastward zonal wind in the upper stratosphere and lower mesosphere approximately 5 days later.Due to critical-level filtering,the mesosphere meridional circulation is suppressed due to both anomalous PWs and GWs,and this suppression causes polar mesospheric cooling in 10 days.The interaction between PWs,zonal wind and GWs results in the 15-day lag between the stratospheric and mesospheric response.Yanget al.[90]revealed the effect of the MJO on springtime Antarctic ozone variations for the first time from multi-satellite reanalysis and model simulations.Twenty to 30 days after MJO Phase 8 (P8),Antarctic Total Column Ozone (TCO) anomalies significantly decrease by up to–15 DU,associated with a wave-1 response at around 60°S.The MJO suppressed the upward and poleward propagation of Planetary Waves (PWs) and lead to weakened Brewer-Dobson circulation in the SH stratosphere.This in turn results in less ozone transport from midlatitudes into the polar region and thus a negative polar TCO response.Dynamical transport plays a dominant role rather than chemical processes in modulating the Antarctic TCO after MJO P8.

By using meteor radar,radiosonde observations and reanalysis data,Chenget al.[91]reported a dynamical coupling from the tropical lower atmosphere to the Mesosphere and Lower Thermosphere (MLT) through a quasi-27-day intraseasonal oscillation.Maet al.[92]studied an enhancement of a quasi-27-day wave during recurrent geomagnetic storms in the autumn of 2018 based on the zonal wind observations in the Mesosphere and Lower Thermosphere (MLT) region over Beijing.Combining radiosonde and satellite observations and reanalysis data,Baiet al.[93]investigated anomalous changes in temperature and ozone QBOs from the lower to middle stratosphere.Anomalous changes of temperature and ozone QBOs due to unexcepted QBO zonal wind variation are well explained according to thermal wind balance and thermodynamic balance.Duet al.[94]reported an extremely negative anomaly of atmospheric water vapor in the tropical western Pacific during the super El Ni?o winter of 2015/16,and revealed the contributions of the anomalies in the Hadley,Walker and monsoon circulations to the observed water vapor anomalies in the eastern-Pacific and central-Pacific El Ni?o events.

Using the high-precision wind and temperature observations from lidar,Huanget al.[95]and Liet al.[96]investigated the fundamental features of three-dimensional wind and temperature spectra.The wavenumber spectral slopes of the horizontal winds and temperature are systematically less negative than–3,and their frequency spectrum slopes have more or less deviated from the universal spectral index of–5/3.In the saturated spectrum region,the frequency and vertical wavenumber spectra of the vertical wind have the shallower slopes than those of the horizontal winds.In the spectral tail region,the frequency and wavenumber spectrum slopes are far steeper relative to their saturated spectrum slopes.Although the vertical wind spectrum is almost always separable,the horizontal wind spectra are separable only at high frequencies.Based on the wind measurements from MST radar at Xianghe,Ninget al.[97]statistically analyzed the Inertia-Gravity Wave (IGW) activity in the troposphere and lower stratosphere,and estimate directly the intensity of wave momentum flux.The annual average of momentum flux indicates that the IGWs in the lower stratosphere can apply a persistent eastward drag on the background flow at higher altitudes through momentum transported by these waves.

Using the temperature profiles measured by the SABER instrument (2002–2021),Liuet al.[98]derived the global GW action and its scale height and identified a persistent layer of enhanced GW dissipation centered at 80–85 km with a vertical extent of 8–20 km.The possible mechanisms of enhanced dissipation include wave refraction,wind filtering,and reduced static stability.According to the dispersion and polarization relations of linear GWs and the SABER temperature data,Liuet al.[99]proposed a method of deriving GW-perturbed wind shears and showed that the magnitudes of the GWperturbed shears agreed with the lidar and sounding rocket observations in mesosphere and lower thermosphere.Liuet al.[100]studied the global atmospheric static stability in the middle atmosphere and its relation to GWs.They found that the correlation coefficients between static stability and GW amplitudes was about 0.8 and indicated that large static stability supported large-amplitude GWs.The background wind is difficult to be measured in the stratosphere and mesosphere.To fill this gap,Liuet al.[101]developed a dataset of the monthly mean zonal wind in the height range of 18–100 km and at latitudes of 50°S–50°N from 2002 to 2019,derived from the gradient balance wind theory and the temperature and pressure observed by the SABER instrument.

Zouet al.[102]gave the spectrum of a gravity wave in the turbulence region,by using the high-resolution data of Yanqing Lidar.This is the first report on the properties of the turbulence spectrum and their seasonal variations in the northern hemisphere over China.The observed gravity wave turbulence spectra well fitted with the theoretical prediction.These results achieved new field detections,which will lead us to further discover the nature and essence of a gravity wave and deeply study the behavior of upper atmosphere circulation on a large scale.

5 Coupling between Stratosphere and Troposphere

Heet al.[103]showed that the mixing ratio of carbon monoxide (CO) within the Asian Summer Monsoon Anticyclone (ASMA) is significantly higher at both 100 hPa and 147 hPa in the easterly phase of QBO than the westerly phase of QBO,which is related to the variations of location and strength of the ASMA.Additionally,the enhanced ascending motions over the Tibetan Plateau favor the high CO at 100 hPa during easterly QBO.

Wanget al.[104]found that a decrease of Arctic stratospheric ozone in March favors anomalous tropospheric cyclones and negative sea surface temperature anomalies over the western North Pacific in early summer via influencing the tropospheric circulation and airsea interaction processes.These results suggested that the Arctic stratospheric ozone signal in March has implications for the predictions of the circulation and SST variations over the western North Pacific in early summer.

Zhanget al.[105]found that the stratospheric Quasi-Biennial Oscillation (QBO) plays an important role in the mid-latitude surface temperature variations through modifying the stratospheric polar vortex.The downward extension of stratospheric negative northern annular mode signals is more than twice as large during the easterly phase of the QBO as those during the westerly phase of the QBO.The effects of QBO on stratospheric polar vortex are carried out through both the tropospheric and stratospheric paths.

Xuet al.[106]found that the tropospheric and stratospheric pathways contribute roughly equally to the latewinter cooling over Eurasia induced by the Barents-Kara Seas (BKS) sea-ice loss.Their study further identified the underlying mechanisms responsible for the two pathways:for tropospheric pathway,the reduced sea-ice could strengthen the Siberian High and induce two cold air-mass streams over Eurasia;for a stratospheric pathway,a sea-ice-induced extension of stratospheric polar vortex toward Eurasia could intensify the Eurasian cooling.Xuet al.[107]found that the sea ice loss over Barents-Kara Seas (BKS) could lead to a deepened East Asian trough (EAT) in late winter,while the EAT axis tilt is not sensitive to the BKS sea ice reduction.Their analysis indicates that stratosphere-troposphere coupling contributes to around 70% of the sea-ice-induced deepening of EAT,while the remaining 30% is from tropospheric processes.

Maet al.[108]found that the East Asian Winter Monsoon (EAWM) in the early winter months (November to December) is weaker in the Easterly phase of the QBO(EQBO) than in the westerly phase of the QBO(WQBO).During EQBO,the northerly monsoon flow is weakened,and anomalously warm temperatures occur over East Asia.Moreover,components of the EAWM circulation system,including the Siberian High,Aleutian Low,East Asian trough,and East Asian jet stream,are all weakened.The QBO is suggested to influence the EAWM directly via a subtropical pathway.The EQBO tends to weaken the East Asian jet stream and shift it poleward,and the changes in the East Asian jet stream lead to a weakened EAWM.The activities of planetary waves are also changed in association with the QBO.Examinations of individual zonal Wavenumbers (WNs)of planetary waves in the SLP field reveal that WN 1 is strengthened while WNs 2 and 3 are weakened during EQBO.Specifically,the anomalous WN 2 leads to weakening of the Siberian High,Aleutian Low,and East Asian trough,and the anomalous WN 3 reduces the pressure gradient between the East Asia landmass and the western Pacific.The changes in WN 2 and WN 3 associated with the QBO play a dominant role in anomalous EAWM.

Chenet al.[109]further found that the relationship between the spring NAO and the following EASM varied with the 11-year solar cycle.The analysis of the time-lag relationship between the NAO in May and the following summer rainfall indicates that during the Low Solar activity (LS) summers,a more robust relationship is established,with below-normal rainfall anomalies in South China and the Sunda Islands in relation to a positive NAO.And the May NAO-related atmospheric circulation anomalies during boreal summer over East and Southeast Asia may well explain the distinct rainfall difference.The possible mechanism for this solar modulation is attributed to the changes in structure of the spring NAO and associated tripole SST anomalies in the North Atlantic.On the other hand,Xueet al.[110]revealed the solar cycle modulation of the connection between boreal winter ENSO and following summer South Asia High(SAH).For El Ni?o accompanied by a low solar cycle(EL&LS) or La Ni?a accompanied by a low solar cycle(LA&LS),the boreal winter ENSO prominently affects the following summer’s SAH variation.The SAH is enhanced (weakened) and expanded (narrowed) in the meridional and zonal directions in the EL&LS (LA&LS)phase.The composite difference in the SAH between El Ni?o and La Ni?a events is obviously stronger in the low solar cycle phase.Further investigation based on longer historic data also ensures that the 11-year solar cycle modulates the ENSO-SAH connection,with a more robust connection in the low solar cycle phase.

Huangfuet al.[111]investigated the process by which the QBO modulates tropospheric circulation and convection during summer (between July and October),when Tropical Cyclone (TC) activities enter their peak period.Concurrent with the western phase of the QBO(QBOW),significant tripole pattern modulations over the tropical Indo-Pacific Ocean are regressed onto the residual part of U70 after removing the ENSO signal,with enhanced convection observed over its central branch (0°–10°N,120°E–180°) and the inactive convection branches located to both sides.A ventilation opening-like effect is exerted on the monsoon trough,which is shifted southward under the QBOW phase.According to the QBO-associated changes in the circulations over the tropical Western North Pacific (WNP),equatorial environments tend to be favorable for TC genesis.Consequently,the off-equatorial TC tracks show a significantly decreased occurrence frequency in the northern monsoon trough region.In addition,Wanget al.[112]identified the significant modulation of the QBO on the winter Tropical Cyclone Precipitation (TCP) in the coastal regions of the Western North Pacific (WNP).In the westerly QBO winter,the zonal wind vertical shear anomalies in the stratosphere strengthen (weaken) convective activities around the East China Sea (the Philippines) and cause middle-level easterly (westerly) anomalies of the middle (low) latitudes in the troposphere,leading to more (less) TC activities around the East China Sea (the Philippines).Consequently,a TCP dipole pattern can be observed.The TCP increases in East China,Korean peninsula,Japan and Russian Far East,but decreases in Indo-China Peninsula,South China and the Philippines.These results not only improve the knowledge of QBO-TC activity relationship but also provide a potential indicator for the seasonal prediction of the TC variation around the WNP due to the high predictability of the QBO.

Luet al.[113]investigated the impact of the stratospheric polar vortex shift on the tropospheric Arctic Oscillation (AO) in winter.Their results showed that a shift in the stratospheric polar vortex toward the Eurasian continent is favorable for the occurrence of the negative phase of the AO,the extension of duration of AO events and enhancement in the AO intensity.The polar vortex shift leads to changes in the intensity and position of the three action centers in the AO spatial pattern.

Zhanget al.[114]analyzed the impacts of Arctic Stratospheric Polar Vortex (SPV) on wintertime precipitation over the Northern Hemisphere.They found that the SPV-induced changes in precipitation over the North Atlantic are stronger than those over the North Pacific.The convective (large-scale) precipitation changes play a major role in the total precipitation changes over the southern (northern) parts of middle latitudes associated with SPV changes.

Huet al.[115]showed that the interannual relationship between the SAV (Stratospheric Arctic Vortex) in early winter and the Arctic Oscillation (AO) in late winter during 1958–2018 is unsteady and has experienced a remarkable interdecadal change around the 2000 s,with large and statistically significant positive correlations before the 2000 s but small and insignificant correlations after the 2000 s.The weakened linkage between the SAV and AO after the 2000 s is possibly caused by (i)the weakened downward effect of SAV on the North Atlantic and polar sectors of AO,and (ii) the opposite signs of Sea Level Pressure (SLP) in the North Pacific and North Atlantic.The links of SAV-AO are enforced(weakened) when AO-related SLP anomalies over the North Pacific are the same (opposite) signs of those over the North Atlantic before (after) the 2000 s.Differences in the tropospheric state over the North Pacific during two periods might be jointly contributed by the SAV and the SAV-related significant (insignificant) sea surface temperature anomalies over central North Pacific in the earlier (latter) period.

Huanget al.[116]investigated the impact of the first two tropical Indian Ocean SST modes on the stratospheric water vapor and stratospheric polar vortex.It is found that the first (second) Indian Ocean SST mode servers to dry (moisten) the tropical stratosphere on the annual average and the responses are seasonally dependent.Both modes enhance (weaken) northern (southern)stratospheric polar vortex in boreal winter (austral spring).

Huanget al.[117]provided corroborative evidence that weak polar vortex conditions in the lower stratosphere substantially increase the risk of severe Cold Air Outbreaks (CAOs) over most continental regions of the Northern Hemisphere,as compared to moderate CAOs.By analyzing the stream of polar cold air mass,this study showed that the polar vortex affects severe cold air outbreaks by modifying the inter-hemispheric transport of cold air mass,which sheds light on the spatial and temporal response of CAOs following anomalous stratospheric conditions.Huanget al.[118]explored the stratospheric influence on the synoptic development of the 2018 later winter European cold air outbreak by running a regional Weather Research and Forecasting (WRF)model.Based on a set of nudged WRF simulations,they showed that capturing the evolution of the stratospheric child vortex over northeastern Canada,particularly in the lowermost stratosphere,is of great importance for predicting this CAO,since it substantially affects the strength and location of the Atlantic ridge that in turn determines the evolution and severity of this CAO.

Lianget al.[119]separated the dry and moist components of the Moist Isentropic Mass Circulation (MIMC)on a daily timescale and investigates their relationships with extratropical surface temperature changes in winter(November to February).Results from ERA5 reanalysis data set (1979–2018) show that the MIMC is composed of a poleward warm branch in the upper layers and an equatorward cold branch below.The Warm Branch(WB) is dominated by the moist component in the midlatitude troposphere,but by the dry component in other regions.The stronger moist component of WB at 50°N–70°N (WB_M) is a better precursory indicator than the dry component (WB_D) for the Arctic surface warming as a result of its dominant role in modifying the downward longwave radiation via water-vapor-related processes.The stronger WB_D is coupled with a negative Arctic Oscillation and a stronger,longer-lasting equatorward transport of colder air,therefore a better precursory indicator of mid-latitude cold events.

Liuet al.[120]examined the relationship between Sea Surface Temperatures (SSTs) in the North Pacific and the concentration of stratospheric ozone in the northern hemisphere in February-March from 1980 to 2017 using reanalysis and satellite datasets.Our results show that the concentration of stratospheric ozone can be modulated by the principal mode of the North Pacific SSTs—that is,there are negative and zonally asymmetrical ozone anomalies in the Arctic stratosphere,but positive ozone anomalies in the lower stratosphere at midlatitudes during the positive phases of the North Pacific SSTs.The North Pacific SST anomalies account for about 20% of the linear variance of ozone concentrations in the lower stratosphere over the Arctic.Negative SST anomalies in the central North Pacific tend to result in a strengthened Western-Pacific-like teleconnection,which favors the propagation of more planetary wavenumber-1 and -2 waves into the stratosphere.The North Pacific SSTs-related upwelling branch of the Brewer-Dobson circulation in the midlatitude stratosphere strengthens,but its downwelling branch in the Arctic stratosphere weakens.This results in decreased ozone anomalies in the lower stratosphere over the Arctic and increased ozone anomalies in the lower stratosphere at mid-latitudes.The zonally asymmetrical distribution of ozone related to the positive phase of SSTs over the North Pacific may be related to the shifting and strengthening of the stratospheric Arctic vortex.

Xiaet al.[121]found that Arctic ozone loss may lead to a decrease in surface Ultraviolet (UV) radiation over the Siberian Arctic in spring using ERA5 reanalysis.It is found that Arctic ozone loss is associated with an increase in high clouds by modifying static stability in the upper troposphere.Stratospheric ozone loss allows more UV radiation to reach the surface.On the contrary,the increase in high clouds results in a reduction of surface UV radiation.Interestingly,a composite analysis suggests that this cloud masking effect is found to be stronger than that from stratospheric ozone loss over the Siberian Arctic in spring.These results suggest that more attention should be paid to the high-ozone events which would lead to more surface UV radiation by the cloud effects.

Xiaet al.[122]showed that the anomalous surface warming in the Siberian Arctic in spring 2020 was likely related to the severe stratospheric ozone loss.The dramatic Arctic ozone loss in March was shifted to Siberia in April and May,which largely cools the lower stratosphere and leads to an increase of high clouds by modifying the static stability in the upper troposphere.This further results in an increase of longwave radiation at the surface which likely contributes to surface warming.Multiple linear regression demonstrates that ozone loss contributes most of the surface warming in April,while the Arctic Oscillation and ice-albedo feedback play a minor role.In May,both ozone loss and ice-albedo feedback contribute to the surface warming.These results support that surface warming in the Siberian Arctic could occur in April and May when severe ozone loss occurs in March.

That SSW tends to follow certain phases of the Madden-Julian Oscillation (MJO) was documented.Maet al.[123]investigated the modulation of El Ni?o-Southern Oscillation (ENSO) on the relationship between the MJO and major SSW in the Northern Hemisphere winter.They found that there is a much stronger MJO-SSW relationship during La Ni?a winters than El Ni?o winters.Further analysis indicates that ENSO influences the MJO-related geopotential height anomalies in middle to high latitudes of the upper troposphere,which leads to an increase in wavenumber 2 in La Ni?a winters.And wavenumber 2 plays a key role in generating the strong heat flux anomalies in the stratosphere.The results may help explain that,although the stratospheric polar vortex is stronger on a seasonal mean timescale during La Ni?a winters,some extreme events,such as SSWs,may not decrease as suggested in observational studies.Xuet al.[124]further identified two leading modes of SSWs by using height-time domain Empirical Orthogonal Function (EOF) analysis.They found that the EOF1 is associated with polar vortex state before the occurrence of SSW,and the EOF2 indicates the intensity of the SSW.And those SSWs with weak initial polar vortex or stronger warming intensity may lead to evident surface pressure anomalies of negative North Atlantic Oscillation pattern,whereas the SSWs with strong initial polar vortex or weaker warming intensity tend not to exert influence on the surface.Different activity of planetary waves leads to discrepancy among SSWs.These results could help to better predict the tropospheric response following SSWs.Kovalet al.[125]analyzed the changes in the vertical and meridional Residual Mean Meridional Circulation (RMC) velocity components at different stages of SSW events with ensemble simulation of the atmospheric general circulation at altitudes up to the lower thermosphere.Their obtained statistically significant results on the evolution of RMC and eddy circulation at different SSW stages at altitudes up to the lower thermosphere may provide a better understanding of the mechanisms of planetary wave impacts on the mean flow and for the diagnostics of the transport of conservative tracers in the atmosphere.

Maet al.[126]systematically studied the stratospheric and tropospheric evolutions during the lifecycle of both strong and weak Stratosphere Polar Vortex events(SPV and WPV events,respectively) based on the reanalysis data for the period of 1958–2017.Moreover,the atmospheric circulation and dynamical characteristics of two types of WPV events,namely events with and without SSW,have also been analyzed.The results show that the formation of SPV event follows a slow development and then a rapid intensification stage,while a WPV event is established dramatically.Compared with the SPV events,the WPV events are stronger and have a higher anomaly center when they reach a peak.And the occurrence of SPV and WPV events is closely related to the positive feedback of wave-mean flow interaction.In addition,for the WPV events with SSW,enhanced upward wave-1 Eliassen-Palm (EP) flux in the stratosphere occurs in the growth stage.Through the positive feedback of wave-mean flow interaction,both the upward propagating wave-1 and wave-2 EP fluxes are increased,which leads to the breakdown of the polar vortex.For the WPV events without SSW,the upward propagating wave-1 EP flux is weak in the growth stage,while the wave-2 flux plays an important role.Hence,the total upward propagating planetary waves are much smaller than the WPV events with SSW.And the tropospheric influence of the WPV events with SSW is not robust,and the magnitude of induced AO index in the troposphere is much smaller than that for the WPV events without SSW.

Weiet al.[127]further investigated the longitudinal asymmetry of planetary wave propagation and its role in stratosphere-troposphere dynamical coupling,since three-dimensional (3D) planetary wave analysis can provide more regionalized information on stratospheric-tropospheric dynamic interactions.They found that the upward wave flux from the troposphere to the stratosphere is maximized above northeastern Eurasia,and the downward flux occurs mainly over the North America and North Atlantic (NANA) region,which is much stronger during mid-to-late winter.The upward wave flux anomalies in early winter are negatively correlated with the strength of the Stratospheric Polar Vortex (SPV).During the mid-to-late winter months,the strength of the SPV is positively correlated with the first mode of the 3D wave flux and has a leading relationship of approximately one month.The stronger SPV corresponds to a stronger upward wave flux above northern Eurasia and a stronger downward flux over the NANA region.The interannual variations in wave flux during early winter are closely associated with the Scandinavian wave train pattern.In contrast,the wave flux variations are related to the circulation anomaly corresponding to the Arctic Oscillation during mid-to-late winter,which causes climate anomalies across the Northern Hemisphere,especially coherent temperature changes in northern Europe,eastern United States and northeastern China.Weiet al.[128]also suggested that stratosphere may amplify the global climate effect of wildfires and volcano eruptions.In the Earth’s long historical evolution,large-scale wildfires,active volcanic activities,meteorites that hitting the Earth,are likely to inject aerosol particles such as black carbon and sulfate into the stratosphere.The dynamical stability and horizontal circulation in the stratosphere can spread aerosols globally,and extend the lifetime of aerosols in the stratosphere,affecting the radiation balance of the Earth system on a global scale.Therefore,the stratosphere can be an “ amplifier” of climate change events.They called on that the scientific community needs to pay more attention to the stratospheric process,since there are still many problems related to aerosol radiation,photochemistry and transport dynamics in the stratosphere that need further study.

Luet al.[129]explored a Sudden Stratospheric Warming (SSW) in January 2021,its favorable conditions,and the near surface impact.Wavenumbers 1 and 2 alternately contributed to the total eddy heat flux from mid-December 2020 to late January 2021,and the wavenumber 2 during the onset period nearly split the stratospheric polar vortex.In mid-December 2020 and during the 2021 New Year period (1–5 January 2021),a blocking developed over the Urals,which enhanced the local ridge and the climatological wavenumber 2.Composite results confirm that the Arctic sea ice loss in autumn and La Ni?a favor the deepening of the high latitude North Pacific low and the increase of the Urals height ridge,which together enhance the planetary waves and hence disturb the stratospheric polar vortex.However,the Madden-Julian Oscillation (MJO) in the tropics was dormant in mid-to-late December 2020 and early January 2021,and the well-established statistical relationship between the MJO convection over the western Pacific and the SSW is not applicable to this special case.The cold air outbreak in China during the 2021 New Year period before the January 2021 SSW onset is not explained by the SSW signal which developed in the stratosphere.In contrast,the downward-propagating signal reached the near surface in mid-February 2021,which may contribute to the cold air outbreak in US and may help to explain the extreme coldness of Texas in middle February.

Raoet al.[130]analyzes the prediction of the downward propagation and surface impact of the 2018 and 2019 SSWs by using the real-time predictions from 11 subseasonsal models.These two SSWs differed both in their morphology types and magnitudes,with the former more splitting in the vortex shape and stronger in the westerly wind deceleration.With abundant samples,they also showed that the strength of the SSW is more important than the vortex morphology and wavenumbers to determine the magnitude of its downward impact.Strong SSWs are more likely and easily to propagate downward than weak SSWs.The multimodel ensemble analysis supports that the observed strong SSW in February 2018 had a stronger and more continuous downward impact than the January 2019 SSW.For another SSW event during January 2021,predictability[131]of the event was investigated by adopting reanalysis,observations;and subseasonal to seasonal forecasts of the event was conducted concurrently.They found that this SSW occurred under the tropical westerly Quasi-Biennial Oscillation (QBO) and weak convection over tropical Pacific.Alternate wave pulses by the wavenumber 1 and 2 finally led to the January 2021 SSW onset.Due to an unfavorable condition in the tropics with QBO westerlies,the predictability for the occurrence of the January 2021 SSW is not beyond two weeks with the required hit ratio ＞50%.The observed Arctic sea ice loss since the autumn in 2020 is unlikely to extend the predictability of this event in subseasonal models.For the Southern Hemisphere (SH) SSW event in September 2021,Raoet al.[132]found that the predictive limit to this SSW is around 18 days in some S2 S models with favorable tropical forcing conditions.

In addition to weak polar vortex events such as SSWs,Rao and Garfinkel[133]also compared the possible maximum predictability of strong polar vortex events in early spring coupled to evident ozone loss in the Arctic.Three ozone loss events in springs of 1997,2011,and 2020 were accompanied by an extremely strong and cold polar vortex.Rao and Garfinkel[134]further adopted realtime forecasts and found that the strong stratospheric polar vortex in March 2020 coupled with Arctic ozone loss is related to the anomalously weak wave activities.Weakened Brewer-Dobson circulation contributed around 40% of the total Arctic ozone loss in March.The empirical model using the S2 S outputs tends to underestimate the Arctic total ozone.Yuet al.[135]further explored the possible impact of Arctic ozone loss coupled with a strong polar vortex on the Eurasia.

The cancelling effect of the tropical Indian Ocean forcing on the stratospheric ENSO signal traditionally attributed to the Pacific force has been well understood in recent years.Rao and Ren[136]further revealed the destructive interference of the sea surface temperature forcing in the southern winter stratosphere between the tropical Indian and Pacific Oceans by designing sensitivity experiments with WACCM4.The Southern Hemisphere zonal wind responses to the tropical SST forcings from the Indian and Pacific Oceans exhibit a dipole pattern.Specifically,a warm Pacific SST forcing favors an equatorward shift of the polar jet,while a warm Indian Ocean SST forcing induces a poleward shift.Therefore,no significant observed ENSO signal in the southern winter stratosphere is mainly caused by the destructive interference from the tropical Indian Ocean.In the Southern Hemisphere,warm Pacific forcing excites a positive Pacific-South America (PSA)-like pattern,while warm Indian Ocean forcing induces a negative PSA circulation pattern.

Renet al.[137]and Xiaet al.[138]found that the Eastern Asian (EA) topographic forcing has a dominant weakening effect on the stratospheric polar vortex,while the North American (NA) topography has a trivial or an even opposite effect.By designing sensitivity experiments with WACCM4,they proved the dominant role of the EA topography in its interference with the NA.The joint effects of EA and NA topography,rather than being a linear superimposition of their independent effects,are largely dominated by the effects of EA.Yuet al.[139]continued to investigate the topographic dynamical effect of EA and NA on the winter isentropic meridional mass circulation and further confirmed the dominant role of the EA as compared with NA topography.

Xieet al.[140]diagnosed the dynamics for the February 2018 SSW event and analyzed its possible impact on the weather over North America.They found that the ridge over Alaska and the trough over the northeastern North America are the prominent tropospheric precursors before the SSW.A cold wave occurs in the northwestern North America within 10 days after the 2018 SSW.

Huanget al.[141]showed that a PM2.5 diffusion event in December 2015–January 2016 includes two stages:rapid diffusion stage (December 22–28,2015)and persistent diffusion stage (1–23 January 2016),controlled by different tropospheric and stratospheric meteorological conditions.The stratosphere-troposphere coupling effectively favors and extends the duration of PM2.5 diffusion,and the weakening of SPV provides a 1–2 weeks lead information to the meteorological diffusion condition.

Luet al.[142]compared the PM2.5 concentration in Beijing-Tianjin-Hebei during two SSWs (11 February 2018 and 2 January 2019).They found that the PM2.5 concentration in the pre-SSW period,SSW-occurrence period,and post-SSW period is different.The February 2018 SSW shows stronger downward propagating signals in the post-SSW period,which favors weakening of the pressure contrast between Arctic and midlatitudes and strengthening of the East Asian winter monsoon systems in the middle troposphere,resulting in diffusion and dilution of the pollutants.In contrast,the PM2.5 concentration is still high in the post-SSW period during the 2019 non-downward propagating event.

Chenet al.[143]used the WRF model to simulate the typical case of a cut-off low over northeast Asia,and then applied the output of the WRF model to drive a FLEXPART-WRF model to carry out both forward and backward simulations.They revealed the detailed trajectories and sources of air masses within the cut-off lows.Their trajectories illustrate the multi-time scale deep intrusion processes in the upper troposphere and lower stratosphere caused by the cut-off low.The processes of air intrusion from the lower stratosphere to the middle troposphere can be divided into three stages:a slow descent stage,a rapid intrusion stage and a relatively slow intrusion stage.The ozone-rich air in the COL primarily originated from an extratropical cyclone over central Siberia and from the extratropical jet stream.

Xiaoet al.[144]found that surface nitrogen oxides(NOx) emissions in East Asia can be transported to the tropics and East Asian Upper Troposphere and Lower Stratosphere (UTLS) region during summer and autumn.In summer,the south Asia anticyclone can transport the NOxin the East Asian UTLS region to the lower latitudes.In the UTLS regions,with the increase of surface NOxemissions in East Asia,ozone concentration increases in the lower latitudes while ozone concentration decreases in the middle latitudes.

Tianet al.[145]investigated the anomalous signals near the tropopause before the overshooting convective system onset over the Tibetan Plateau.They found that the tropopause height is stable at the maximum height seven and five days before the onset of overshooting convection.It then decreases significantly one day before and on the day of the onset.The upward motion in the troposphere is the strongest five days before the onset.From one day before and after the onset,there are strong ascending motions at 500–300 hPa but weak descending motions near the tropopause.