Gravity Wave Activity and Stratosphere-Troposphere Exchange During Typhoon Molave(2020)

To investigate the stratosphere-troposphere exchange(STE)process induced by the gravity waves(GWs)caused by Typhoon Molave(2020)in the upper troposphere and lower stratosphere,we analyzed the ERA5 reanalysis data provided by the European Centre for Medium-Range Weather Forecasts and the CMA Tropical Cyclone Best Track Dataset.We also adopted the mesoscale forecast model Weather Research and Forecasting model V4.3 for numerical simulation.Most of the previous studies were about typhoon-induced STE and typhoon-induced GWs,while our research focused on the STE caused by typhoon-induced gravity waves.Our analysis shows that most of the time,the gravity wave signal of Typhoon Molave appeared below the tropopause.It was stronger on the east side of the typhoon center(10°-20°N,110°-120°E)than on the west side,suggesting an eastward tilted structure with height increase.When the GWs in the upper troposphere and lower stratosphere region on the west side of the typhoon center broke up,it produced strong turbulence,resulting in stratosphere-troposphere exchange.At this time,the average potential vorticity vertical flux increased with the average ozone mass mixing ratio.The gravity wave events and STE process simulated by the WRF model were basically consistent with the results of ERA5 reanalysis data,but the time of gravity wave breaking was different.This study indicates that after the breaking of the GWs induced by typhoons,turbulent mixing will also be generated,and thus the STE.

Cross-Tropopause Mass Exchange Associated with a Tropopause Fold Event over the Northeastern Tibetan Plateau

A springtime tropopause fold event, found to be related to a cold trough intrusion from the north, was detected in the northeastern Tibetan Plateau （TP） based on various observations. A nested high-resolution mesoscale model was employed to investigate the effect of orography on the stratosphere-troposphere exchange. The model was found to be able to capture plausible tropopause fold properties. The propagation of the tropopause fold changed significantly when the terrain height in the model was altered. However, decreasing the terrain height had no significant effect on the morphology of folds. When a fold passed over an elevated surface, a leeside jet stream and a layer of humid air in the middle troposphere tended to develop. This strong leeside descent of air masses and high mid-level potential instability （PI） could give rise to deep upward motions in the leeside and inject tropospheric air into the lower stratosphere. Besides, when the flow encounters an elevated surface, forced lifting together with mid-level PI can trigger deep convective motions on the windward slope. The troposphere to stratosphere transport was found to be persistent and almost stationary over the windward slope of the TP during the evolution of the fold.

青藏高原地区一次深对流活动对平流层——对流层物质交换影响的模拟研究

利用WRF-Chem模式分析了2010年8月12日发生在青藏高原东南部的一次深对流过程中的平流层—对流层物质交换(STE)过程及其影响机制。结果表明,此次深对流系统具备穿透性对流特征,强上升气流能够直接将近地面含高浓度CO和低浓度O_(3)的空气输送至低平流层,使低平流层CO浓度升高、O_(3)浓度降低。同时,深对流活动激发了较强的湍流混合过程,在深对流活动结束后的3~4小时内,湍流混合作用导致上对流层下平流层(UTLS)区域持续发生STE过程,将对流层的冰晶、CO和O_(3)输送至低平流层,但受凝结脱水作用影响,湍流过程向低平流层传输的水汽减少。

Simulation of the stratosphere-troposphere exchange process in a typical cold vortex over Northeast China

A mesoscale weather research and forecasting(WRF)model was used to simulate a cold vortex that developed over Northeast China during June 19–23,2010.The simulation used high vertical resolution to reproduce the key features of the cold vortex development.Characteristics of the associated stratosphere-troposphere exchange(STE),specifically the spatiotemporal distribution of the cross-tropopause mass flux(CTF),were investigated using the Wei formula.The simulation results showed that the net mass exchange induced by the cold vortex was controlled by stratosphere-to-troposphere transport(STT)processes.In the pre-formation stage of the cold vortex(i.e.,the development of the trough and ridge),active exchange was evident.Over the lifecycle of the cold vortex,STT processes prevailed at the rear of the trough and moving vortex,whereas troposphere-to-stratosphere transport(TST)processes prevailed at the front end.This spatial pattern was caused by temporal fluctuations of the tropopause.However,because of the cancellation of the upward flux by the downward flux,the contribution of the tropopause fluctuation term to the net mass exchange was only minor.In this case,horizontal motion dominated the net mass exchange.The time evolution of the CTF exhibited three characteristics:(1)the predominance of the STT during the pre-formation stage;(2)the formation and development of the cold vortex,in which the CTF varied in a fluctuating pattern from TST to STT to TST;and(3)the prevalence of the STT during the decay stage.

An estimation of ozone flux in a stratosphere-troposphere exchange event

A new method based on mass fluxes and observed ozone profiles was developed to estimate crosstropopause ozone flux. Using this method, we estimated the cross-tropopause ozone flux in a stratospheric-tropospheric exchange event that occurred over East Asia in March 2001.The result revealed that the ozone flux across the tropopause in this event was an order of magnitude higher than the global and hemispheric average. Compared to the traditional method using a linear relationship between ozone mixing ratio and potential vorticity near the tropopause, the cross-tropopause ozone flux evaluated with ozonesonde data was somewhat higher, although the orders of the two values were the same.

青藏高原地区上对流层—下平流层区域水汽分布和变化特征

利用2005-2008年青藏高原（下称高原）地区微波临边探测器MLS（MicrowaveLimbSounder）、高光谱分辨率大气红外探测仪AIRS（AtmosphereInfraredSounder）、ECMWF的ERA-Interim资料，以及NCEP／NCAR再分析数据和NOAAHYSPLIT（Hybrid；Single-ParticleLagrangianIntegratedTrajec-toryModel）轨迹模式资料，讨论了高原上空对流层顶附近的水汽分布和变化特征及高原上空平流层与对流层之间的物质交换。结果表明，3-4月高原南侧对流层顶附近100hPa存在一个水汽低值带，而7-8月和9-10月此处存在一个明显的水汽高值区。3-4月夏季风未发展之前，受高原大地形抬升和西风气流的影响，高原以南地区存在对流层与平流层的物质交换，而215hPa的高原中部地区（80°E-90°E）则由于空气的下沉运动将上层的干空气向下输送而出现一个水汽低值中心。7-8月，受印度夏季风和高原上空反气旋式环流的影响，高原上空有明显的水汽穿过对流层顶向平流层输送，反气旋环流中心的水汽经过2～4天的上升过程可以从对流层进入平流层。高原及其以东、以西地区的水汽在对流层顶附近的季节变化基本一致，100hPa三个不同区域的水汽在3月达到最低。

亚洲夏季平流层-对流层水汽交换年际变化与亚洲夏季风的联系

亚洲地区是物质由对流层向平流层输送的主要通道,在平流层-对流层交换中扮演着积极的角色.本文主要利用卫星资料和欧洲中心ERA40再分析资料,借助Wei诊断模式研究亚洲地区夏季上对流层-下平流层(UTLS)水汽分布和平流层-对流层水汽交换特征,重点着眼于水汽交换的年际变化,并探讨其与亚洲夏季风的联系.结果表明,季风区UTLS水汽较赤道地区偏多,且通过磁带记录信号的传播,可穿越对流层顶影响下平流层水汽的多寡.夏季平流层-对流层水汽交换表现出明显的年际特征,其年际变化与亚洲季风强弱变化有密切联系,尤其与南亚夏季风的关系更为显著.在亚洲夏季风影响下,亚洲地区出现异常的大气环流和垂直运动,从而影响平流层-对流层之间水汽的交换.这些结果对认识其它大气成分的输送过程也具有重要的指示意义.

Advances in Studies of the Middle and Upper Atmosphere and Their Coupling with the Lower Atmosphere

Recent advances in studies of the middle and upper atmosphere and their coupling with the lower atmosphere in China are briefly reviewed. This review emphasizes four aspects: (1) Development of instrumentation for middle and upper atmosphere observation; (2) Analyses and observation of middle and upper atmosphere; (3) Theoretical and modeling studies of planetary wave and gravity wave activities in the middle atmosphere and their relation to lower atmospheric processes; (4) Study on the coupling between the stratosphere and the troposphere.

Dynamical and Chemical Features of a Cutoff Low over Northeast China in July 2007:Results from Satellite Measurements and Reanalysis

The European Centre for Medium-Range Weather orology and measurements from the Microwave Limb Forecasts Re-Analysis Interim （ERA-Interim） meteSounder, High Resolution Dynamics Limb Sounder, and Ozone Monitoring Instrument onboard the Earth Observing System Aura satellite were applied to analyze the dynamical and chemical features of a cutoff low （COL） event over northeast China in early July 2007. The results showed the polar stratospheric origin of an upper-level warm-core cyclone at 100 300 hPa, associated with a funnel-shaped tropopause intruding into the mid-troposphere just above the COL center. The impacts of the stratospheric intrusion on both column ozone and ozone profiles were investigated using satellite measurements. When the intensity of the COL peaked on 10 July 2007, the total column ozone （TCO） increase reached a maximum （40-70 DU）. This could be dynamically attributed to both the descent of the tropopause （~75%） and the downward transport of stratospheric ozone across the tropopause （~25%）. Analysis of the tropospheric ozone profiles provided evidence for irreversible transport/mixing of ozone-rich stratospheric air across the tropopause near the upper-level front region ahead of the COL center. This ozone intrusion underwent downstream transport by the upper tropospheric winds, leading to further increase in TCO by 12 16 DU over broad regions extending from east China toward the northern Japan Sea via South Korea. Meteorological analysis also showed the precedence of the stratospheric intrusion ahead of the development of cyclones in the middle and lower troposphere.

ADVANCES ON STUDY OF MIDDLE AND UPPER ATMOSPHERE AND THEIR COUPLING WITH LOWER ATMOSPHERE

In this paper advances on study of middle and upper atmosphere and their cou pling with lower atmosphere in China in recent two years are briefly reviewed.This review emphasized three aspects, ie. (1) analysis and observation of mid and upper atmosphere over China; (2) theoretical and modelling study of grav ity wave activities in middle atmosphere and their relation to lower atmospheric processes; (3) coupling between the stratosphere and troposphere.

Ten-Year Climatological Features and Air Origin of Midlatitude Double Tropopauses

The 10-year climatological features related to midlatitude double tropopause events （DTs） are examined using ERA- Interim data from 2003 to 2012. The analysis is based on tropopauses defined by lapse rate. Results show that DTs are permanent or semi-permanent in the midlatitudes, and high DT frequency bands move poleward in winter and equatorward in summer, which is consistent with the seasonal movement of the subtropical jet. Based on our statistics, the second tropopause is found at about 100 hPa in the subtropics and at slightly lower altitudes in sub-polar regions. The thickness between the first and second tropopause is smaller in the subtropics and increases with latitude. Next, the origin of air sandwiched between the first and second tropopause of DTs is studied with a revised version of the UK Universities Global Atmospheric Modelling Programme Offline Trajectory Code （Version 3） diabatic trajectory model. The results show that, in the lower or middle troposphere, air is transported into the DTs from lower latitudes, mainly in the tropics. The dominant source regions are mainly areas of deep convection and steep orography, e.g., the western Pacific and Himalayan Mountains, and they show strong seasonality following the seasonal shift of these strong upwelling regions.

Recent Enhanced Deep Troposphere-to-Stratosphere Air Mass Transport Accompanying the Weakening Asian Monsoon

The Asian monsoon(AM) region is a well-known region with prevailing stratosphere–troposphere exchange(STE).However,how the STE across this region changes with the weakening AM remains unclear.Here,we particularly diagnose the air mass transport between the planetary boundary layer(PBL) and the stratosphere over the AM region during 1992–2017 using the Lagrangian particle dispersion model FLEXPART based on the ERA-Interim reanalysis data.The results show that both the downward and upward deep STEs exhibit a detectable increasing trend,while the latter,namely,the deep troposphere-to-stratosphere transport(DTST),is relatively more significant.Further analysis reveals that the long-term trend of DTST over the AM region could be partly attributed to changes in the Pacific Walker circulation and the air temperature(especially at upper levels).Additionally,it is found that DTST increases markedly over the tropical oceanic regions,while the increasing DTST into the stratosphere can be attributed to the enhanced air masses originated from the PBL over the terrestrial regions,where large amounts of pollutant emissions occur.The results imply that the influence of the DTST on the chemical composition and the climate of the stratosphere over the AM region is expected to become increasingly important,and is thereby of relevance to climate projection in an evolving climate.

A deep stratospheric intrusion associated with an intense cut-off low event over East Asia

European Centre for Medium-Range Weather Forecasts Re-Analysis Interim(ERA-Interim)reanalysis data and satellite data,and trajectory model were applied to analyze the dynamical,thermo-dynamical,and chemical structure in the upper troposphere and lower stratosphere(UTLS)of an intense cut-off low(COL)event occurring over East Asia during June 19-23,2010,and to characterize the process and transport pathway of deep stratospheric intrusion.The Atmospheric Infrared Sounder(AIRS)ozone data and the Global Positioning System Ozone(GPSO3)sonde data showed that the air mass originating from the polar formed a region with relatively high values of potential vorticity(PV)and ozone in the center of COL,and a secondary ozone peak appeared in the upper troposphere during mature stage of the COL.Forward trajectory simulation suggested that during the first stage of COL,deep stratospheric intrusion associated with strong northerly wind jet on the west side of the upper-level trough transported ozone-rich air from the polar lower stratosphere into the middle and lower troposphere in the mid-latitude,and increased the ozone concentration there.During the mature stage of the COL,stratospheric air was transported counterclockwise into the troposphere.Backward trajectory model was used to find the source regions of air mass within the COL during its mature stage.Model results show that air masses with high ozone concentration in the center of the COL have two source regions:one is the subpolar vortex which lies in northern part of Center Siberia,where ozone-rich air plays a major role in increasing the ozone concentrations,and the other is the strong shear region which is near by the cyclonic side of the extratropical jet axis(west of 90°E,near 50°N).The air masses with low ozone concentration around the COL also have two source regions:one is the anticyclonic side of the extratropical jet axis,where the air mass with the relatively low ozone concentration at the bottom of the COL is mainly controlled by horizontal movement,and the other is the warm area of the south side of COL,where the air mass on the east and west side of the COL is mainly dominated by upward motion.

Stratospheric entry point for upper-tropospheric air within the Asian summer monsoon anticyclone

The Asian summer monsoon(ASM) anticyclone isolates upper-tropospheric air within the interior of the anticyclone from the outside. Forward trajectory simulations in previous studies have shown that much of the air within the ASM anticyclone can be trapped for up to two or three weeks, not only laterally but also vertically. Here, we investigate the locations of exit points for upper-tropospheric air trapped within the ASM anticyclone, especially the preferred tropopause-crossing locations, using a 3-dimensional trajectory model. Forward trajectory calculations show two-thirds of the air crosses the tropopause at the southern part of the anticyclone via upward diabatic transport. Furthermore, some air crosses at northern and eastern parts via isentropic shedding, but air crosses rarely through the center of the anticyclone. However, calculations also show that many stratospheric parcels within the anticyclone are traceable from the upper-tropospheric anticyclone. This implies they cannot break through the tropopause directly overhead but instead enter the stratosphere via other entry points.

Characteristics of the vertical structure of the atmospheric turbulence in the Tibetan Plateau

The Tibetan Plateau(TP)has unique atmospheric dynamics and thermal structures that originate from its giant terrain and complex climate.High vertical-resolution thermal radiosondes were launched near the central(Lhasa,91°06′E,29°36′N,3670 m above sea level(ASL))and marginal(Da Qaidam,95°21′E,37°51′N,3180 m ASL)areas of the TP during the summers of 2018 and 2020,respectively.Atmospheric turbulence parameters were calculated,and the characteristics of the atmospheric turbulent vertical structure at sunset in these two areas were analyzed and compared.Affected by TP thermal forcing and stably controlled by the summer monsoon anticyclone,the atmospheric refractive index structure constant(C_(n)^(2))tended to increase and then decrease with increasing height,reaching a maximum at the tropopause(~18 km ASL)at the Lhasa site.Although C_(n)^(2) at the Da Qaidam site also tended to increase at the tropopause,the position of the strong turbulent band(STB)(5–7 km ASL)was below the tropopause height corresponding to the potential temperature lapse rate minimum.The vertical distribution of C_(n)^(2) at the two sites,particularly regarding the position of the STB,was highly correlated with the atmospheric stability(Ri)and the thermal mixing scale(L_(T)).The significant correlations among the three parameters(STB,Ri,and L_(T))indicated that the strong fluctuations in temperature caused by thermal mixing were the dominant factor causing the Ri to be less than its critical value of 0.25.Moreover,the suppression strength involving the upward transport of the heat sources was the main reason for the different turbulent vertical structures and STB positions at the two sites.The zonal mean thermodynamic and dynamical fields derived from the reanalysis data also showed a height difference in the heat sources transported to the troposphere at the two sites.In the marginal TP,the material and energy in the lower troposphere were transported by the turbulent atmosphere upward along the slope of the mountain and converged at the central TP(28°N–35°N)with strong thermal forcing up to the tropopause.In the STB of the Lhasa site,the turbulent dissipation rate and eddy diffusion coefficient increased sharply,indicating that the turbulent atmosphere in this central site was highly diffused,and the small-scale turbulence transported the material and energy upward.