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In the Earth's general circulation, polar jets act as baroclinic pumps of angular momentum and heat. Reanalysis datasets indicate that shear changes near jets induce upward displacements of the jet cores, suggesting a weakening thermodynamic pumping over the last 50 years. From secondary flow theory, a well‐established principle in fluid dynamics, an increased frequency of heatwaves and persistent winter storms is expected. The ageostrophic wind shear between 700 and 50 hPa indicates the strength of this secondary circulation. The weakening tendency during the reanalysis period also exists in multimodel simulations under the RCP8.5 emissions scenario for the 21st century. The reduction between the periods 2005–2025 and 2081–2100 reaches 18%, 5.3% and 19%, respectively, for the North America, Mid‐Europe and East Asia sectors of the Northern Hemisphere polar jet. Within this background, cold‐surge events are the result of synergic co‐working of several factors. The occurrence trend for transitional season winter extreme events also is examined. The winter extremes seemingly have larger temperature drops. However, in a warming climate, they emerge more rapidly from extreme cold states. The storm tracks, especially over North America, have equatorward extensions, indicating that winter storms can reach lower latitudes. Due to the temperature‐dependence of air viscosity, secondary flows decrease more slowly than the main zonal flow. This imposes an important adjustment to the traditional polar amplification effects on midlatitude winter extremes. During a warmer winter (the primary manifestation of a warmer climate), spatially uniform positive trends in cold extreme events are not expected. There are, however, regions experiencing more winter extremes. These regions show consistent patterns in both the reanalysis period and the remainder of the 21st century. [ABSTRACT FROM AUTHOR]

ABSTRACT The anomalous variability of extreme cases of the tropical tropopause provides insight into the stratosphere‐troposphere exchange process crucial for understanding climate change. The present study analyses the extreme variability of the tropopause and its thermal structure over the tropics using GPS radio occultation data over the period 2006–2019. The extremely cold and warm tropopauses and extremely high and low tropopauses are identified based on the cold point tropopause temperature and height, respectively, when their values exceed two standard deviations with respect to their climatological means. The analyses revealed frequent occurrence of extreme cases of tropopause over the Atlantic Ocean compared to the Western Pacific Ocean. Individually, extremely warm and low cases occur more frequently over the subtropics, while extremely cold and high cases occur frequently over the deep tropics. These extreme cases pose different thermal structures bounded within the extremely low and high tropopauses throughout the tropics. The height difference between the extremely high and low tropopause cases is wider over the Atlantic Ocean and adjoining areas compared to the western Pacific Ocean. The temperature difference between the extremely warm and cold tropopause cases is higher in the Atlantic, Central Pacific, and Indonesian regions compared to the American, Indian Ocean, and western Pacific Ocean regions. The relationship between the El Niño Southern Oscillation (ENSO) phases and extreme tropopause cases is also investigated which reveals a higher occurrence of extremely high tropopause cases during the El Niño phase while low tropopause cases during the La Niña phase. Our analysis also revealed the thermal patterns of the extreme cases characterising colder and sharper tropopause over the convective regions compared to subsidence regions. [ABSTRACT FROM AUTHOR]

The prediction skill for individual Madden–Julian oscillation (MJO) events is highly variable, but the key factors behind this remain unclear. Using the latest hindcast results from the subseasonal-to-seasonal (S2S) phase II models, this study attempts to understand the diverse prediction skill for the MJO events with an enhanced convective anomaly over the eastern Indian Ocean (IO) at the forecast start date, by investigating the preference of the prediction skill to the MJO-associated convective anomalies and low-frequency background states (LFBS). Compared to the low-skill MJO events, the high-skill events are characterized by a stronger intraseasonal convection–circulation couplet over the IO before the forecast start date, which could result in a longer zonal propagation range during the forecast period, thereby leading to a higher score for assessing the prediction skill. The difference in intraseasonal fields can further be attributed to the LFBS of IO sea surface temperature (SST) and quasi-biannual oscillation (QBO), with the high-skill (low-skill) events corresponding to a warmer (colder) IO and easterly (westerly) QBO phase. The physical link is that a warm IO could increase the low-level convective instability and thus amplify MJO convection over the IO, whereas an easterly QBO phase could weaken the Maritime Continent barrier effect by weakening the static stability near the tropopause, thus favoring eastward propagation of the MJO. It is also found that the combined effects of IO SST and QBO phases are more effective in influencing MJO prediction skill than individual LFBS. Significance Statement: Given the importance of the Madden–Julian oscillation (MJO) in the subseasonal predictability of global weather and climate, how well the MJO itself can be predicted is a matter of concern. This study reveals the critical observational factors that determine the variation in MJO prediction skill across events. Without making any presumptions about what the factors would be, the observed MJO events are separated according to their individual prediction skill, and the difference between the MJO events with higher and lower skill is then investigated. The results show that the low-frequency background states of the Indian Ocean sea surface temperature and the quasi-biannual oscillation are good indicators for MJO prediction skill, for their modulatory role in the MJO propagation range. [ABSTRACT FROM AUTHOR]

Pyrocumulonimbus clouds (pyroCbs) generated by intense wildfires can serve as a direct pathway for the injection of aerosols and gaseous pollutants into the lower stratosphere, resulting in significant chemical, radiative, and dynamical changes. Canada experienced an extremely severe wildfire season in 2023, with a total area burned that substantially exceeded those of previous events known to have impacted the stratosphere (such as the 2020 Australian fires). This season also had record-high pyroCb activity, which raises the question of whether the 2023 Canadian event resulted in significant stratospheric perturbations. Here, we investigate this anomalous wildfire season using retrievals from multiple satellite instruments, ACE-FTS (Atmospheric Chemistry Experiment – Fourier transform spectrometer), OMPS LP (Ozone Mapping and Profiler Suite Limb Profiler), and MLS (Microwave Limb Sounder), to determine the vertical extents of the wildfire smoke along with chemical signatures of biomass burning. These data show that smoke primarily reached the upper troposphere, and only a nominal amount managed to penetrate the tropopause. Only a few ACE-FTS occultations captured elevated abundances of biomass-burning products in the lowermost stratosphere. OMPS LP aerosol measurements also indicate that any smoke that made it past the tropopause did not last long enough or reach high enough to significantly perturb stratospheric composition. While this work focuses on Canadian wildfires given the extensive burned area, pyroCbs at other longitudes (e.g., Siberia) are also captured in the compositional analysis. These results highlight that despite the formation of many pyroCbs in major wildfires, those capable of penetrating the tropopause are extremely rare; this in turn means that even a massive area burned is not necessarily an indicator of stratospheric effects. [ABSTRACT FROM AUTHOR]

Flight-level airborne observations have often detected gravity waves with horizontal wavelengths λ x ≲ 10   km near the tropopause. Here, in situ and remote sensing aircraft data of these short gravity waves trapped along tropopause inversion layer and collected during a mountain-wave event over southern Scandinavia are analyzed to quantify their spectral energy and energy fluxes and to identify nonstationary modes. A series of three-dimensional numerical simulations are performed to explain the origin of these transient wave modes and to investigate the parameters on which they depend. It turns out that mountain-wave breaking in the middle atmosphere and the subsequent modification of the stratospheric flow are the key factors for the occurrence of trapped modes with λ x ≲ 10   km. In particular, the intermittent and periodic breaking of mountain waves in the lower stratosphere forms a wave duct directly above the tropopause, in which the short gravity waves are trapped. The characteristics of the trapped, downstream-propagating waves are mainly controlled by the sharpness of the tropopause inversion layer. It could be demonstrated that different settings for optimizing the numerical solver have a significantly smaller influence on the solutions. [ABSTRACT FROM AUTHOR]

Stratospheric balloons are platforms with great relevance in space missions to reach scientific observations. The payload thermal analyses of such missions are usually focused on the float phase. However, during the ascent phase, the coolest temperatures of the entire mission may be reached, mainly due to the convective cooling in the tropopause. This can be explained by the combination of relative wind speed and the harsh thermal environment. The aim of this work is to evaluate the impact of the thermal environment on the relative wind speed to determine the worst-case thermal analysis. Therefore, in order to perform a robust payload thermal during the ascent phase, the uncertainty in the thermal environment and the relative velocity must be evaluated. The former are reduced by defining the thermal environment based on real-data. The latter are reduced by evaluating the parameters involved in the ascent rate. For this purpose, a dynamic model has been developed to characterise the ascent rate and the horizontal relative velocity of the balloon-borne system. This tool has been validated with flight data from the REXUS/BEXUS programme, with the BEXUS missions launched from Esrange, Kiruna, Sweden from 2014 to 2018. The thermal analysis performed shows a temperature difference greater than 10 °C depending on the thermal worst-case selection. The work here presented reduces the uncertainties of the stratospheric payloads ascent phase thermal analysis. [ABSTRACT FROM AUTHOR]

The tropopause is an important transition layer and can be a diagnostic of upper-tropospheric and lower-stratospheric structures, exhibiting unique atmospheric thermal and dynamic characteristics. A comprehensive understanding of the evolution of fine tropopause structures is necessary and primary for the further study of complex multi-scale atmospheric physical–chemical coupling processes in the upper troposphere and lower stratosphere. A novel method utilizing the bi-Gaussian function is capable of identifying the characteristic parameters of vertical tropopause structures and providing information on double-tropopause (DT) structures. The new method improves the definition of the cold-point tropopause and detects one (or two) of the most significant local cold points by fitting the temperature profiles to the bi-Gaussian function, which defines the point(s) as the tropopause height(s). The bi-Gaussian function exhibits excellent potential for explicating the variation trends of temperature profiles. The results of the bi-Gaussian method and lapse rate tropopause, as defined by the World Meteorological Organization, are compared in detail for different cases. Results indicate that the bi-Gaussian method is able to more stably and obviously identify the spatial and temporal distribution characteristics of the thermal tropopauses, even in the presence of multiple temperature inversion layers at higher elevations. Moreover, 5 years of historical radiosonde data from China (from 2012 to 2016) revealed that the occurrence frequency and thickness of the DT, as well as the single-tropopause height and the first and second DT heights, displayed significant meridional monotonic variations. The occurrence frequency (thickness) of the DT increased from 1.07 % (1.96 km) to 47.19 % (5.42 km) in the latitude range of 16–50° N. The meridional gradients of tropopause height were relatively large in the latitude range of 30–40° N, essentially corresponding to the climatological locations of the subtropical jet and the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

The literature presents different views on how the stratosphere influences variability in surface nitrous oxide (N 2 O) and on whether that influence is outweighed by surface emission changes driven by the El Niño–Southern Oscillation (ENSO). These questions are investigated using a chemistry–climate model with a stratospheric N 2 O tracer; surface and aircraft-based N 2 O measurements; and indices for ENSO, polar lower stratospheric temperature (PLST), and the stratospheric quasi-biennial oscillation (QBO). The model simulates well-defined seasonal cycles in tropospheric N 2 O that are caused mainly by the seasonal descent of N 2 O-poor stratospheric air in polar regions with subsequent cross-tropopause transport and mixing. Similar seasonal cycles are identified in recently available N 2 O data from aircraft. A correlation analysis between the N 2 O atmospheric growth rate (AGR) anomaly in long-term surface monitoring data and the ENSO, PLST, and QBO indices reveals hemispheric differences. In the Northern Hemisphere, the surface N 2 O AGR is negatively correlated with winter (January–March) PLST. This correlation is consistent with an influence from the Brewer–Dobson circulation, which brings N 2 O-poor air from the middle and upper stratosphere into the lower stratosphere with associated warming due to diabatic descent. In the Southern Hemisphere, the N 2 O AGR is better correlated to QBO and ENSO indices. These different hemispheric influences on the N 2 O AGR are consistent with known atmospheric dynamics and the complex interaction of the QBO with the Brewer-Dobson circulation. More airborne surveys extending to the tropopause would help elucidate the stratospheric influence on tropospheric N 2 O, allowing for better understanding of surface sources. [ABSTRACT FROM AUTHOR]

The dynamical environment of the stratosphere, during the summer over North America, provides a natural integrator of the impact of convection in the lower stratosphere, as air can be confined for periods of a few days to more than a week. In situ data obtained during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) field campaign show increasing water vapor mixing ratios in background air as a function of time the air parcel spent within the North American Monsoon Anticyclone region. We find that water vapor added to the stratosphere by convection decreases with altitude and tends to drop below detectable limits by the 415 K isentrope in 2021 and the 430 K isentrope in 2022. Integrating between potential temperatures of 380 and 460 K we find that convection added between 20 and 32 Tg per summer to the stratosphere in 2021 and 2022. While the total amount is only 1%–4% of the amount ascending in the tropics across the tropical tropopause, small changes in the annual flux of water can have a significant effect on the radiation budget of the atmosphere. Locally, over North America we find that convection increased the water vapor mixing ratio at 380 K by as much as 40%. Tropopause‐penetrating convection is part of the yearly cycle of stratospheric water vapor and we suggest that it must be included in extratropical models to accurately predict future trends in stratospheric water vapor. Plain Language Summary: Water vapor in the stratosphere is important for regulating Earth's climate and also influences the amount of ozone in the stratosphere. Most air rises into the stratosphere through the tropical tropopause where the excess water freezes out leaving very dry air. But air and water can also be injected into the stratosphere via strong convective storm systems. During the summer months, strong storms over the Sierra Madre Occidental and the central United States are known to produce many overshoots that can be seen with radar and satellite. A recent NASA airborne campaign called Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) aimed to study this region. The aircraft was equipped with 12 different sensors measuring gases, aerosols, and meteorological variables. We use these measurements to look at the cumulative effect these storms have on the background air in the stratosphere and specifically calculate the amount of water vapor added by convection as a function of altitude above the tropopause. We find that water added by storms over North America is 1%–4% of the total amount ascending in the tropics. Over North America convection may account for as much as a 40% increase in lower stratospheric water vapor. Key Points: Deep convection over North America lofts significant amounts of water to the lower stratosphereThe North American Monsoon Anticyclone (NAMA) provides a natural integrator of convective perturbations to the stratosphereConvective detrainment of water vapor and ice is quantified by looking at changes in background air as a function of time spent in the NAMA [ABSTRACT FROM AUTHOR]

During summer, ammonia emissions in Southeast Asia influence air pollution and cloud formation. Convective transport by the South Asian monsoon carries these pollutant air masses into the upper troposphere and lower stratosphere (UTLS), where they accumulate under anticyclonic flow conditions. This air mass accumulation is thought to contribute to particle formation and the development of the Asian Tropopause Aerosol Layer (ATAL). Despite the known influence of ammonia and particulate ammonium on air pollution, a comprehensive understanding of the ATAL is lacking. In this modelling study, the influence of ammonia on particle formation is assessed with emphasis on the ATAL. We use the EMAC chemistry-climate model, incorporating new particle formation parameterisations derived from experiments at the CERN CLOUD chamber. Our diurnal cycle analysis confirms that new particle formation mainly occurs during daylight, with a 10-fold enhancement in rate. This increase is prominent in the South Asian monsoon UTLS, where deep convection introduces high ammonia levels from the boundary layer, compared to a baseline scenario without ammonia. Our model simulations reveal that this ammonia-driven particle formation and growth contributes to an increase of up to 80% in cloud condensation nuclei (CCN) concentrations at cloud-forming heights in the South Asian monsoon region. We find that ammonia profoundly influences the aerosol mass and composition in the ATAL through particle growth, as indicated by an order of magnitude increase in nitrate levels linked to ammonia emissions. However, the effect of ammonia-driven new particle formation on aerosol mass in the ATAL is relatively small. Ammonia emissions enhance the regional aerosol optical depth (AOD) for shortwave solar radiation by up to 70%. We conclude that ammonia has a pronounced effect on the ATAL development, composition, the regional AOD, and CCN concentrations. [ABSTRACT FROM AUTHOR]

The Thar is the most densely populated desert in the world, which supports diverse ecosystems and human endeavours such as agriculture and socioeconomic activities. Water demand and supply in the Thar play an essential role in regulating the socioeconomic activities of the region. Inland water and precipitation aid the movement of water in the Thar Desert. Precipitation in the Thar is governed by the Indian Summer Monsoon (ISM), during which the winds distribute water vapour to regulate precipitation across the region. Therefore, we analyse the water vapour, its sources and its relation with precipitation using satellite measurements and reanalysis data in the Thar during ISM. Like other regions, a clear seasonal cycle of water vapour is observed in the Thar, with very high values (> 45 kg/m2) during ISM and low in winter (< 15 kg/m2). Evapotranspiration and moisture transport have significant effects on the amount of water vapour during ISM. There is a significant increase in water vapour in the troposphere, with high trends at the surface (0.032 g/kg/year) and small at the tropopause (0.00002 g/kg/year). A significant increase in column water vapour is also estimated in the Thar during ISM, with high trends in the eastern and southern areas, at about 0.15–0.35 kg/m2/year. The rise in water vapour in the Thar can be attributed to the increase in evapotranspiration (0.03–0.07 mm/day/year) and water vapour transport (> 0.5 kg/m/s/year) from the Arabian Sea and Indian Ocean. The rise in water vapour can lead to an increase in precipitation in the Thar, as it shows significant positive trends (0.05–0.1 mm/day/year) in the eastern areas during ISM. The increase in precipitation and water vapour in the arid Thar Desert can have significant implications for the regional environment and agriculture. [ABSTRACT FROM AUTHOR]

• Observation of Multiple layer structures (Temperature Sheets) around the tropopause region and lower stratosphere. • Strong hydrostatic stability responsible for the intensity gradient in the radio refractive index. • Temperature sheets in the troposphere have significant implications for weather and atmospheric stability. • Higher-order spectral analysis has been applied to the atmospheric signals to detect the nonlinearities associated with the temperature sheets near the tropopause region. • Large shear region shows the temperature variation, which results in temperature sheet formation near the tropopause region. Temperature sheets have been observed in the lower stratosphere up to 20 km, with strong (positive) temperature gradients within very thin layers. These were observed from vertical profiles obtained from the Stratosphere and Troposphere (ST) radar located in Kolkata. The sheets tend to occur in groups and contribute to the regions of high static stability, inhibiting vertical air motion. Several experiments were carried out to understand the backscattering mechanism of the Very High Frequency (VHF) radar. Many experiments have reported the multiple layered structures at and around the tropopause regions, especially in the lower stratosphere, it is well established that these layered structures are due to presence of strong negative and positive temperature gradients, which is called as temperature sheets. In order to understand the nonlinearities associated with temperature sheets, Higher-order spectral analysis has been carried out on backscattered signals received from the Mesosphere Stratosphere and Troposphere (MST) radar. This approach allowed for a more detailed examination of the nonlinearities associated with temperature sheets. Therefore, this study proposes a new way of distinctly identifying temperature sheets and the process associated with them using Nonlinear Index based measurements. [ABSTRACT FROM AUTHOR]

Knowledge of the tracer characteristics, such as water vapor and ozone in the tropical tropopause layer (TTL), is vital in quantifying the radiation budget and critical to understanding the exchange processes between the troposphere and stratosphere. In this study, we have characterized the tropical tropopause parameters such as the cold point tropopause (CPT) height (CPT-H) and temperature (CPT-T), convective tropopause (COT) height (COT-H) and temperature (COT-T), and the tropical tropopause layer (TTL) using radiosonde observations during 2014–2019 over Chennai (13.0° N, 80.06° E) located in the northeast (NE) monsoon region. The water vapor and ozone data from the microwave limb sounder (MLS) simultaneous to the radiosonde observations are also utilized to understand their roles on the CPT variations for different convective conditions obtained from Infrared brightness temperature (IRBT) data. CPT over Chennai becomes higher (17.6 ± 0.3 km) and colder (189.7 ± 0.9 K) during the winter season and lower (16.6 ± 0.2 km) and warmer (192.1 ± 1.0 K) during the summer monsoon season, however, not in the same month. The water vapor (CPT-W) and ozone (CPT-O) mixing ratios at CPT are found to be lower (~ 70 ± 1.4 ppmv and 3.1 ± 0.4 ppmv) during the winter season and higher (153 ± 4.2 ppbv and 4.8 ± 0.6 ppmv) during summer monsoon season. COT, however, becomes lower (12.4 ± 0.3 km) and higher (13.3 ± 0.3 km) during pre-monsoon and summer monsoon seasons, respectively. The TTL thickness is lesser (3.5 ± 0.6 km) during the winter and greater (4.8 ± 0.8 km) during the summer monsoon seasons. Over Chennai, the seasonal variation of the upper troposphere and lower stratospheric temperature, water vapor, and ozone anomalies are in phase. We have categorized tropical convections as non-penetrative and penetrative using IRBT data. It is observed that the TTL temperature warms with the increasing strength of the non-penetrative convections and cools for the penetrative convection. [ABSTRACT FROM AUTHOR]

The tropopause is part of the fundamental structure of the atmosphere, separating the dynamically and chemically distinct troposphere and stratosphere. Based on the concepts as, thermal, dynamical, and chemical or ozone, three different tropopauses are defined. The thermal tropopause is defined as the lowest level above 500hPa at which the lapse rate decreases to 2 K/km or less, provided that also the average lapse rate between this level and all higher levels within 2 km does not exceed 2 K/km. The dynamical tropopause is defined in terms of sharp changes in the potential vorticity which be a measures of the stratification and wind shears in air masses. The original concept of the dynamical tropopause is based on the isentropic gradient of potential vorticity which is typically determined in a thin layer with absolute potential vorticity values within 1 and 4 potential vorticity units. Chemical tropopause is another type of tropopause that is defined based on the vertical concentrations of the trace gases such as ozone mixing ratio and water vapor. In tropical latitudes, a useful reference for distinguishing the tropopause from the stratosphere is the cold point, where the temperature minimum occurs. The cold-point tropopause is the temperature minimum in the tropopause region. The cold point tropopause this acts as the thermal boundary between the troposphere and the stratosphere whose height varies from 18 km in the tropics to about 8 km in the poles. This boundary plays a crucial role in the troposphere-stratosphere exchange. In the present study, radiosonde data used are obtained from the Integrated Global Radiosonde Archive at the NOAA National Climatic Data Center. Daily temperature profiles in January and July 2000-2022 are taken from two radiosonde stations in the central and south of Iran including Tehran (51.31°E, 35.56°N), and Shiraz stations (52.60°E, 29.53°N), and are analyzed. Using radiosonde temperature profiles (green lines) at Tehran stations in a) January and b) July 2021, cold-point tropopause and monthly mean temperature (black lines) are here presented (see text). The blue plus signs are cold point tropopauses from individual profiles and the red plus sign is the average of all blue plus signs. The black thick line is the mean temperature profile and the purple plus sign is the coldest point of the mean temperature profile. ΔT is the difference between the red and purple plus signs. Although we only showed two months of data in January and July here to visualize the difference between the mean of cold point tropopause and the coldest point of the mean temperature profile, our calculation for ΔT is for the 2000-2022 period. The results showed that Shiraz station's lowest temperature of the tropical cold point tropopause is colder than the Mehrabad station of Tehran, and the lowest temperature of the tropical cold point tropopause is estimated in July. This difference is related to combining geographical-latitudinal characteristics of the two stations and the development of thermal low pressure in July over Iran, which has caused an increase in height and a decrease in the temperature of the tropical tropopause. Also, the results showed the average tropical cold point tropopause temperature in the stations is 2.5 degrees kelvin lower than the average cold point of the vertical temperature profile, and the thermal low pressure and cold high pressure are the cause of this difference. [ABSTRACT FROM AUTHOR]

Climate models robustly project acceleration of the Brewer‐Dobson circulation (BDC) in response to climate change. However, the BDC trends simulated by comprehensive models are poorly constrained by observations, which cannot even determine the sign of potential trends. Additionally, the changing structure of the troposphere and stratosphere has received increasing attention in recent years. The extent to which vertical shifts of the circulation are driving the acceleration is under debate. In this study, we present a novel method that enables the attribution of advective BDC changes to structural changes of the circulation and of the stratosphere itself. Using this method allows studying the advective BDC trends in unprecedented detail and sheds new light into discrepancies between different data sets (reanalyses and models) at the tropopause and in the lower stratosphere. Our findings provide insights into the reliability of model projections of BDC changes and offer new possibilities for observational constraints. Plain Language Summary: The large‐scale interhemispheric meridional overturning circulation in the middle atmosphere determines the composition of this region, including the distribution of radiatively important trace gases. The long‐term change of this circulation is a subject of ongoing debate, and an area of disagreement between models and observations. In our study we present a method that provides an unprecedented insight into the change and disentangles the individual factors behind it. Hence, the method introduces new constraints on the circulation change and can aid the reconciliation between models and observations. Key Points: A method for attributing net upwelling trends to changes in circulation, air density and structure of the upwelling region is establishedModels agree largely on contributions from vertical shifts in pressure surfaces/tropopause and changes in vertical advectionFor reanalyses, on the other hand, the uncertainty is larger, not allowing direct constraints on model trends [ABSTRACT FROM AUTHOR]

In the present study, the tropical tropopause inversion layer (TIL) Kelvin waves are extracted from the Global Navigation Satellite System (GNSS) radio occultation (RO) temperature data of multiple missions from January 2007 to December 2020. We focus on the variations of TIL Kelvin waves in two longitude regions, the Maritime Continent (MC; 90°–150°E) and the Pacific Ocean (PO; 170°–230°E). The results show that over both regions, ENSO leads to the opposite variations of TIL Kelvin wave temperature amplitude during different ENSO phases. Specifically, during La Niña, the strong (weak) deep convection over MC (PO) leads to strengthened (weakened) static stability. With enhanced easterly (westerly) winds and strengthened (weakened) static stability, the TIL Kelvin wave temperature amplitudes are stronger (weaker) over MC (PO). The opposite phenomenon occurs during El Niño. The zonal-mean zonal winds affect TIL Kelvin wave temperature amplitudes by two mechanisms. First, the prevalence of easterlies (westerlies) in the upper troposphere affects the upward propagation of Kelvin waves, resulting in stronger (weaker) TIL Kelvin wave temperature amplitudes over MC (PO). Second, the TIL Kelvin wave temperature amplitude peaks about 2 months before the zero-wind line of the descending westerly QBO phase occurs, due to dissipation on the critical line. Additionally, the rapid increase of zonal-mean static stability significantly affects the annual variation of TIL Kelvin wave temperature amplitudes. They both reach maxima during DJF and minima during JJA, which should be related to the annual cycles of temperature and ozone mixing ratio in the TIL. Significance Statement: Recent studies indicate that the Kelvin wave temperature amplitudes in the tropical tropopause inversion layer (TIL) exhibit distinct characteristics compared with those in other height levels, while the modulation mechanisms of the TIL Kelvin waves need further investigation. The present study aims to study the differences in the variabilities and the modulation factors of TIL Kelvin waves over two longitude regions. Our findings suggest that the different responses of background conditions during ENSO phases influence the spatiotemporal distribution of the TIL Kelvin waves. Besides, the zonal winds and the static stability significantly affect the temporal variations of TIL Kelvin waves. Our work fills the research gap of TIL Kelvin waves and contributes to understanding the dynamics of tropical tropopause variations. [ABSTRACT FROM AUTHOR]

We present data and analysis of a set of balloon‐borne sounding profiles, which includes co‐located O3, CO, CH4, and particles, over the northern Tibetan Plateau during an Asian summer monsoon (ASM) season. These novel measurements shed light on the ASM transport behavior near the northern edge of the anticyclone. Joint analyses of these species with the temperature and wind profiles and supported by back trajectory modeling identify three distinct transport processes that dominate the vertical chemical structure in the middle troposphere, upper troposphere (UT), and the tropopause region. The correlated changes in profile structures in the middle troposphere highlight the influence of the strong westerly jet. Elevated constituent concentrations in the UT identify the main level of convective transport at the upstream source regions. Observed higher altitude maxima for CH4 characterize the airmasses' continued ascent following convection. These data complement constituent observations from other parts of the ASM anticyclone. Plain Language Summary: Asian summer monsoon deep convection transports surface pollutants to the stratosphere. Although satellite data have provided clear evidence of this transport, in situ measurements are critical for characterizing how monsoon is vertically re‐distributing the regional emissions. We report new balloon‐borne measurements over the Tibetan Plateau that provide a unique data set on the northern edge of the anticyclone, complementing other observations. Key Points: A novel set of in‐situ profile measurements of O3, CO, CH4 and particles from Tibetan Plateau during Asian summer monsoon are presentedJoint analyses of the profiles provide insights into transport processes controlling the northern edge of the Asian monsoon anticycloneObserved CO profile maxima at 13–14 km (∼360–370 K) identify the level of convective transport at the upstream source regions [ABSTRACT FROM AUTHOR]

Tropical tropopause layer (TTL) clouds have a significant impact on the Earth's radiative budget and regulate the amount of water vapor entering the stratosphere. Estimating the total coverage of tropical cirrus clouds is challenging, since the range of their optical depth spans several orders of magnitude, from thick opaque cirrus detrained from convection to sub-visible clouds just below the stratosphere. During the Strateole-2 observation campaign, three microlidars were flown on board stratospheric superpressure balloons from October 2021 to late January 2022, slowly drifting only a few kilometers above the TTL. These measurements have unprecedented sensitivity to thin cirrus and provide a fine-scale description of cloudy structures both in time and in space. Case studies of collocated observations with the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) show very good agreement between the instruments and highlight the Balloon-borne Cirrus and convective overshOOt Lidar's (BeCOOL) higher detection sensitivity. Indeed, the microlidar is able to detect optically very thin clouds (optical depth τ<2×10-3) that are undetected by CALIOP. Statistics on cloud occurrence show that TTL cirrus appear in about 50 % of the microlidar profiles and have a mean geometrical depth of 1 km. Ultrathin TTL cirrus (τ<2×10-3) have a significant coverage (23 % of the profiles), and their mean geometrical depth is 0.5 km. [ABSTRACT FROM AUTHOR]

The purpose of this first article is to provide a physical basis for atmospheric Rossby waves at the tropopause to clarify their properties and improve our knowledge of their role in the genesis of extreme precipitation and heat waves. By analogy with the oceanic Rossby waves, the role played by the pycnocline in ocean Rossby waves is replaced here by the interface between the polar jet and the ascending air column at the meeting of the polar and Ferrel cell circulation or between the subtropical jet and the descending air column at the meeting of the Ferrel and Hadley cell circulation. In both cases, the Rossby waves are suitable for being resonantly forced in harmonic modes by tuning their natural period to the forcing period. Here, the forcing period is one year as a result of the variation in insolation due to solar declination. A search for cause-and-effect relationships is performed from the joint representation of the amplitude and phase of (1) the velocity of the cold or warm modulated airflows at 250 mb resulting from Rossby waves, (2) the geopotential height at 500 mb, and (3) the precipitation rate or ground air temperature. This is for the dominant harmonic mode whose period can be 1/16, 1/32, or 1/64 year, which reflects the intra-seasonal variations in the rising and falling air columns at the meeting of the polar, Ferrel, and Hadley cell circulation. Harmonics determine the duration of blocking. Two case studies referring to extreme cold and heat waves are presented. Dual cyclone–anticyclone systems seem to favor extreme events. They are formed by two joint vortices of opposite signs reversing over a period, concomitantly with the involved modulated airflows at the tropopause. A second article will be oriented toward (1) the examination of different case studies in order to ascertain the common characteristics of Rossby wave patterns leading to extreme events and (2) a map of the globe revealing future trends in the occurrence of extreme events. [ABSTRACT FROM AUTHOR]

Since the tropopause was first identified, the quality and resolution of weather balloons has dramatically improved. NOAA Earth System Research Laboratories (ESRL) have provided high resolution and very high quality ozonesondes from eight locations: Fiji; American Samoa; Greenland; Antarctica; and several locations in USA (Hawai'i, Colorado, California and Alabama). These locations collectively cover polar regions, mid‐latitudes and tropics. Using this publicly archived data set, we studied the variability of the tropopause for all eight locations for one complete year (2016). Along with the standard estimates of the tropopause provided by NOAA ESRL, we developed four alternative tropopause definitions each based on changes in one of the following: (a) molar density; (b) temperature lapse rates; (c) water vapor content; (d) ozone content. These old and new tropopause definitions appear to hold over all eight locations—for all seasons and from the tropics to the poles. The cohesiveness between all five of these independent tropopause definitions is remarkable, although the NOAA ESRL estimates sometimes identify higher tropopause onsets than the other estimates. Therefore, each tropopause definition could potentially be used as proxies for other tropopause definitions. However, it also confirms that the troposphere/tropopause transition is a multi‐faceted physical and chemical phenomenon associated with more than just temperature changes. Finally, these high‐resolution results suggest that the original term "tropopause" might be a misnomer since they suggest increases in temperature lapse rate variability, rather than the "pausing" implied by lower resolution data or by lapse rates that are averaged over large distances. Plain Language Summary: In the lower atmosphere ("the troposphere"), the temperature decreases with height. This is why it is colder on mountaintops. Since 1902, it has been known that higher up in the atmosphere, in the region near where commercial airplanes fly, temperatures remain relatively constant with height. This region is called "the tropopause." In 1957, the World Meteorological Organization (WMO) defined the start of the tropopause in terms of a set of rules, that were complex, but designed to be applicable for all latitudes and seasons. However, these rules are often hard to implement and can give ambiguous results. So, other definitions have been developed for specific regions, such as the tropics, that are more straightforward and easier to interpret. In this paper, we have developed four new tropopause definitions that can be used with modern high resolution "ozonesondes" (weather balloons that also measure ozone). These four independent definitions describe different aspects of the atmosphere but each give very similar estimates of the tropopause. This reveals that the tropopause involves many different but synchronous changes in the atmosphere. It also shows that the name "tropopause" is confusing because it is a part of the atmosphere involving lots of changes instead of "pausing." Key Points: Four new tropopause definitions have been developed for use with high‐resolution ozonesondes that work for all latitudes and seasonsAll these definitions yield almost identical tropopause estimates yet are derived from independent physical or chemical metricsSurprisingly, the "tropopause" marks a region with increased lapse rate variability over short altitudes as opposed to long‐term averages [ABSTRACT FROM AUTHOR]

This paper presents the first quantitative relationship between the cold point tropopause (CPT) and tropical easterly jet (TEJ) using radiosonde observations over Gadanki (13.45° N, 79.2° E) during the Indian summer monsoon season 2006–2014. CPT and TEJ peak altitudes (HCPTandHTEJ) show amalgams of two categories of variability on the day‐to‐day scale. In category1 HTEJ occurs close to HCPT and they show in‐phase variation. While in Category2 HTEJ occurs far apart from HCPT and they do not show any relationship. For Category1 HCPT and HTEJ are strongly correlated (0.70), as well as HCPT and TCPT (CPT temperature) are moderately anticorrelated (−0.55) significant at a 95% confidence level, indicating the dominance of adiabatic processes. Whereas in Category2 HCPT and TCPT are not significantly anti‐correlated. Thus, when TEJ and CPT are close to each other, it may serve as an indicator for the prevalence of the synoptic‐scale effect. [ABSTRACT FROM AUTHOR]

The tropopause serves a critical role in shaping global and regional weather and climate dynamics. Changes in tropopause characteristics can significantly impact other atmospheric components, thereby influencing Earth's climate systems. In the long run, variations in tropopause features can lead to shifts in the thermal, dynamic, and chemical properties of the tropospheric layer. This study aims to investigate the descriptive attributes of tropopause pressure levels (TPLs) during different months, as well as the temporal and spatial trends in TPL across the Northern Hemisphere spanning from 1979 to 2022. Utilizing ERA5 temperature data for the 700 to 50 hPa range, the tropopause was identified using the lapse rate of tropopause (LRT), and its changes were analyzed employing the linear regression model with the least squares error approach. The results indicated that the spatial pattern of TPLs changed across various latitudes varies seasonally. Generally, the changes in TPLs did not exhibit a linear relationship with latitude, and in most observed months, the highest and lowest TPLs did not correspond to the lowest and highest latitudes, respectively. Examination of the trend in TPLs revealed that in numerous significant areas across different seasons, the trends were statistically insignificant. Where significant, the trends predominantly indicated negative changes (decreases), suggesting a reduction in pressure and potentially an increase in tropopause altitude in these regions, possibly reflecting the influence of global warming. [ABSTRACT FROM AUTHOR]

Aerosol particles play a critical role in the tropical tropopause layer (TTL) through cloud formation and heterogeneous chemistry, influencing the radiative and chemical balance of the stratosphere. However, aerosol measurements in the TTL are sparse, resulting in poor knowledge of aerosol abundance and distribution in this important region. Here, we present in situ aircraft measurements over the western tropical Pacific, revealing a persistent and altitude‐dependent enhancement of aerosol mass in the TTL compared to the convectively influenced troposphere below. Notably, our data demonstrate a striking positive correlation between aerosol mass and ozone. Model simulations suggest that organic materials constitute a substantial fraction of the total aerosol mass within the TTL. We further derived an empirical parameterization of TTL aerosol mass as a function of ozone based on their linear relationship. This framework holds potential for estimating the TTL aerosol abundance but requires further validation and refinement through future measurements. Plain Language Summary: We investigated tiny particles called aerosols in a specific atmospheric layer called the tropical tropopause layer (TTL). These particles are crucial because they affect cloud formation and chemical processes in the atmosphere, influencing how energy is distributed. Unfortunately, there hasn't been much research on aerosols in the TTL, leading to gaps in our understanding of their abundance and distribution in this important region. To fill this knowledge gap, we conducted measurements using aircraft over the western tropical Pacific. Our findings revealed that aerosol mass in the TTL is consistently higher compared to the lower troposphere, which is influenced by upward air movement. What's interesting is that we observed a clear connection between the amount of aerosol and ozone. Our model simulations indicated that a significant portion of the aerosol mass in the TTL is made up of organic materials. To make it easier to estimate aerosol levels and their impact on climate, we developed a way to predict TTL aerosol mass based on ozone measurements. Since ozone is relatively straightforward to measure and model, our method could provide a useful framework for understanding aerosol abundance in the TTL and its effects on the climate. Key Points: Aircraft measurements reveal persistent enhancement of aerosol mass in the TTLThe TTL aerosol enhancement tightly correlates with ozone. An empirical parameterization of TTL aerosol as a function of ozone is derivedModeling suggests that TTL aerosol particles are mainly composed of organics and sulfate [ABSTRACT FROM AUTHOR]

An enhanced aerosol layer, known as the Asian Tropopause Aerosol Layer (ATAL), has been observed within the seasonal Asian monsoon anticyclone (AMA) since the late 1990s. Given the apparently abrupt appearance of this layer based on observations, it has been speculated that it originates from increasing human made emissions in Asia. However, the ATAL confinement is a result of a dynamical feature and does not purely consist of human made components. We herein investigate the possible existence of an ATAL earlier than the late 1990s. We exploit earliest possible, high quality space‐based aerosol observations from Stratospheric Aerosol and Gas Experiment, or SAGE (1979–1981), SAGE III/ISS (2017, ongoing) and revisit SAGE II (1984–2005) data analysis. We find that seasonal averaged solar occultation aerosol measurements (past and present) can neither be used to exclude the existence of the ATAL, nor to infer a significant trend. However, first CAM5‐MAM7 simulations indicate the presence of an ATAL signal for the tested years 1979 and 1980, with a human made component. We hypothesize that the human made component of the ATAL likely occurred since at least the 1970s, while the natural ATAL component (e.g., from dust) has always existed. Extended simulation based ATAL evolution studies are therefore the most reliable source for early ATAL investigations. Plain Language Summary: An enhanced aerosol layer in the Upper Troposphere and Lower Stratosphere (UTLS), known as ATAL (Asian Tropopause Aerosol Layer), was discovered during the seasonal Asian monsoon. Initial observations from space‐borne aerosol data suggested its first appearance in the late 1990s. However, our study reveals that the data set used (from SAGE instruments) is inadequate for ATAL evolution studies and to conclusively determine its first appearance. The limited sampling within the Asian monsoon region and cloud filtering procedure do not provide sufficient data points for definitive conclusions. Nevertheless, the dynamical structure leading to an ATAL has always been present, leading us to hypothesize that the non‐human‐made ATAL component has existed previously. Additionally, our first model simulations for 1979 and 1980 indicate the presence of an ATAL. Key Points: Early space‐borne observations (SAGE instruments) cannot be used for ATAL evolution studies, contrary to previous claimsSAGE lacks sufficient data in the tropics and an effective cloud capture method to quantitatively detect or exclude the presence of ATALFirst model simulations indicate an ATAL signal, without volcanic influence, in 1979 and 1980 [ABSTRACT FROM AUTHOR]

This study presented an analysis of the geometric and optical properties of cirrus clouds with data produced by Compact Cloud-Aerosol Lidar (ComCAL) over Koror, Palau (7.3°N, 134.5°E), in the Tropical Western Pacific region. The lidar measurement dataset covers April 2018 to May 2019 and includes data collected during March, July and August 2022. The results show that cirrus clouds occur approximately 47.9% of the lidar sampling time, predominantly between altitudes of 15 and 18 km. Seasonal variations in cirrus top height closely align with those of the cold point tropopause. Most cirrus clouds exhibit low cloud optical depth (COD < 0.1), with an annual mean depolarization ratio of 31 ± 19%. Convective-forming cirrus clouds during the summer monsoon season exhibit a larger size by notably lower values in terms of color ratio. Extremely thin cirrus clouds (COD < 0.005) constituting 1.6% of total cirrus occurrences are frequently observed at 1–2 km above the cold point, particularly during winter and summer, suggesting significant stratosphere–troposphere exchange. The coldest and highest tropopause over Palau is persistent during winter, and related to the pathway of tropospheric air entering the stratosphere through the cold trap. In summer, the extremely thin cirrus above the cold point is likely correlated with equatorial Kelvin waves induced by western Pacific monsoon convection. [ABSTRACT FROM AUTHOR]

Tropical-tropopause-layer cirrus clouds and their radiative effects represent a major uncertainty in the evaluation of Earth's energy budget. High-altitude aircraft offer an opportunity to provide observations at cirrus cloud altitudes, most commonly using in situ measurements of ice particle optical properties and composition. In particular, remote sensing of scattering properties and near-IR ice water absorption in the limb can provide unique insights into thin and sub-visible cirrus clouds. Here we present novel spectroscopic observations of path-averaged ice water absorptions on board NASA's Global Hawk aircraft during the Airborne Tropical TRopopause Experiment (ATTREX), which took place in 2011, 2013, and 2014. The University of California Los Angeles and University of Heidelberg mini-differential optical absorption spectroscopy (mini-DOAS) instrument provided multi-angle limb-scanning observations of scattered solar radiation in the near-IR (900–1726 nm), allowing the identification of ice and liquid water, O 2 , CO 2 , and H 2 O. The VLIDORT-QS radiative transfer (RT) code was specifically developed for this study and used to simulate high-altitude limb observations for varied cloud scenarios. We performed a comprehensive sensitivity study, developing a fundamental understanding of airborne near-IR limb observations of cirrus clouds. We identified two general distinct cases: a linear regime for optically thin clouds, where the ice absorption is proportional to ice water content (IWC), and a regime for optically thick cirrus clouds, where ice absorption is in saturation and independent of IWC. Results also demonstrate how molecular oxygen absorption can be used to infer information on optical properties of ice particles in the second regime only, with minimal information for thin cirrus clouds. We also explored the feasibility of retrieving IWC from mini-DOAS path-averaged ice water absorption (SIWP) measurements. This innovative interpolation-based approach requires a small number of RT calculations per observation to determine the sensitivity of SIWP to IWC. Spectral retrievals were applied for a particularly interesting case during Science Flight 2 over Guam in February 2014, during which the aircraft flew in circles in the same general area for an extended period of time. Retrieved IWC results are consistent with independent in situ measurements from other instruments on board. The measurements of ice particle scattering and absorption at different azimuths relative to the sun and at different altitudes represent a unique opportunity to test our approach and to infer properties of the ice particles, together with information on cirrus cloud radiative transfer. [ABSTRACT FROM AUTHOR]

Aerosol particles with diameters larger than 40 nm were collected during the flight campaign StratoClim 2017 within the Asian tropopause aerosol layer (ATAL) of the 2017 monsoon anticyclone above the Indian subcontinent. A multi-impactor system was installed on board the aircraft M-55 Geophysica, which was operated from Kathmandu, Nepal. The size and chemical composition of more than 5000 refractory particles/inclusions of 17 selected particle samples from seven different flights were analyzed by use of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy dispersive X-ray (EDX) microanalysis. Based on chemical composition and morphology, the refractory particles were assigned to the following particle groups: extraterrestrial, silicates, Fe-rich, Al-rich, Hg-rich, other metals, C-rich, soot, Cl-rich, and Ca-rich. Most abundant particle groups within the refractory particles are silicates and C-rich (non-volatile organics). In samples taken above the tropopause, extraterrestrial particles are becoming increasingly important with rising altitude. The most frequent particle sources for the small (maximum in size distribution DP-max=120 nm) refractory particles carried into the ATAL are combustion processes at the ground (burning of fossil fuels/biomass burning) and the agitation of soil material. The refractory particles in the ATAL represent only a very small fraction (< 2 % by number for particles > 40 nm) of the total aerosol particles, which are dominated by species like ammonium, sulfate, nitrate, and volatile organics. During one flight, a large number of very small (DP-max=25 nm) cinnabar particles (HgS) were detected, which are supposed to originate from a ground source such as coal combustion or underground coal fires. [ABSTRACT FROM AUTHOR]

The tropopause variation is very complex and affects climate change in the upper-troposphere lower-stratosphere (UTLS). In this study, the relationship and interaction between global gravity wave (GW) activities in terms of potential energy (Ep) over an altitude range from 5 km below the lapse-rate tropopause (LRT) to 10 km above the LRT and LRT characteristics are investigated using data from 12 global navigation satellite system radio occultation (GNSS-RO) missions from June 2001 to June 2022. The LRT height (LRT-H) and LRT temperature (LRT-T) display an increasing trend, while the LRT pressure (LRT-P) depicts a decreasing trend. In height Band 1, which corresponds to a height range from 5 km below LRT to LRT level, the GW Ep is maximum, with an upward trend of about 0.62 J/kg per decade. In addition, in height Band 2, which corresponds to a height range from LRT level to 5 km above LRT level, the GW Ep shows an increasing trend of about 0.18 J/kg per decade. In the case of height Band 3, which corresponds to a height range from 5 km above LRT level to 10 km above LRT level, the GW Ep is minimum with a downward trend of about − 0.17 J/kg per decade. The highest correlation coefficients among GW Ep and LRT parameters (H, T, and P) are at the height Band 2, at about − 0.52, 0.76, and 0.6 for LRT-H, LRT-T, and LRT-P, respectively. The results of the empirical orthogonal function (EOF) analysis of LRT parameters and GW activity at all height bands indicate that all the studied parameters have an increasing variability pattern. In addition, the leading mode of spatial variability of GW Ep at height Band 2 agrees with that of LRT-T and LRT-P and has an opposite sign in the case of LRT-H. The singular value decomposition (SVD) analysis results reveal that the leading temporal variability paired mode of Ep and all LRT parameters has a significant positive correlation, while the spatial one shows a strong coupling between the patterns of the combined Ep in height Band 2 and LRT parameters. Both global GW Ep and tropopause parameters have a clear seasonal variation. The results illustrate that the variations of LRT parameters are most closely related to the variations of GW Ep at height Band 2. Furthermore, the close effect of GW's activity on tropopause variation characteristics is clearly evident. [ABSTRACT FROM AUTHOR]

• Middle atmosphere dynamics and coupling using three decades of Indian MST radar and complimentary observations. • Highlighted recent results on waves, tides, planetary waves, tropopause, STE processes and low-latitude mesospheric dynamics. • Gap areas and scope for future study using other complementary techniques are mentioned. The genesis for Indian Mesosphere-Stratosphere-Troposphere (MST) radar came from the Indian Middle Atmospheric Programme (IMAP) to study the middle atmospheric dynamics and its underlying processes. During the early 1990s VHF radar operating at 53 MHz has been set up at Gadanki (13.5°N, 79.2°E), India, which started operating initially in the stratospheric and tropospheric (ST) mode. It is upgraded to MST mode in 1993. Since then, it has served various scientific experiments not only from India but under international collaboration. Numerous focused campaigns that have national and international importance were conducted. Using this unique data in the last 30 years along with complementary measurements, several stimulating results has been reported covering a wide range of topics that includes gravity wave characteristics, tides, planetary waves, oscillations, tropopause dynamics and stratosphere-troposphere exchange processes, monsoon dynamics and circulations, mesospheric structure and dynamics, vertical and lateral coupling and long-term trends. The present communication will highlight the unique results obtained using long-term observations of Indian MST radar and complementary techniques covering the complete middle atmosphere. Unique results include quantification of the momentum carried by the high-frequency waves generated in the troposphere up to the mesosphere, demarcating the complete tropical tropopause layer using MST radar observations alone, indication for the strengthening of the tropical easterly jet in the recent decade, unusual behavior of the Hadley circulation during drought and El Nino years, the revelation of the mechanisms for the occurrence of mesospheric echoes at the tropical latitudes, quantification of the turbulence, evidence for the role of anthropogenic changes on the dynamics of the mesosphere and generation of the empirical model by combing multi-instrument data sets. These data set also served in validating numerous ground-based and space-borne measurements including several re-analysis products over a tropical latitude. Issues remained unresolved and ways to tackle in future experiments are highlighted. [ABSTRACT FROM AUTHOR]

Initial conditions of current numerical weather prediction systems insufficiently represent the sharp vertical gradients across the midlatitude tropopause. Data assimilation may provide a means to improve tropopause structure by correcting the erroneous background forecast towards the observations. In this paper, the influence of assimilating radiosonde observations on tropopause structure, i.e., the sharpness and altitude, is investigated in the ECMWF's Integrated Forecasting System. We evaluate 9729 midlatitude radiosondes launched during 1 month in autumn 2016. About 500 of these radiosondes, launched on request during the North Atlantic Waveguide Downstream Impact Experiment (NAWDEX) field campaign, are used to set up an observing system experiment (OSE) that comprises two assimilation forecast experiments, one run with and one without the non-operational soundings. The influence on the tropopause is assessed in a statistical, tropopause-relative evaluation of observation departures of temperature, static stability (N2), wind speed, and wind shear from the background forecast and the analysis. Temperature is overestimated by the background at the tropopause (warm bias, ∼ 1 K) and underestimated in the lower stratosphere (cold bias, - 0.3 K) leading to an underestimation of the abrupt increase in N2 at the tropopause. The increments (differences in analysis and background) reduce these background biases and improve tropopause sharpness. Profiles with sharper tropopause exhibit stronger background biases but also an increased positive influence of the observations on temperature and N2 in the analysis. Wind speed is underestimated in the background, especially in the upper troposphere (∼ 1 m s -1), but the assimilation improves the wind profile. For the strongest winds the background bias is roughly halved. The positive influence on the analysis wind profile is associated with an improved vertical distribution of wind shear, particularly in the lower stratosphere. We furthermore detect a shift in the analysis tropopause altitude towards the observations. The evaluation of the OSE highlights that the diagnosed tropopause sharpening can be primarily attributed to the radiosondes. This study shows that data assimilation improves wind and temperature gradients across the tropopause, but the sharpening is small compared with the model biases. Hence, the analysis still systematically underestimates tropopause sharpness which may negatively impact weather and climate forecasts. [ABSTRACT FROM AUTHOR]

The tropopause is important as a diagnostic of the upper troposphere and lower stratosphere structures, with unique atmospheric thermal, dynamic structures. A comprehensive understanding of the evolution of the fine tropopause structures is necessary and primary to further study the complex multi-scale atmospheric physicochemical coupling processes in the upper troposphere and lower stratosphere. Utilizing the bi-Gaussian function, a novel method is capable of identifying the characteristic parameters of tropopause vertical structures, as well as providing the information of double tropopauses (DT) structures. The new method improves the definition of cold point tropopause, and detects one (or two) most significant local coldest point(s) in mathematical statistics by fitting the temperature profiles to the bi-Gaussian function, which is (are) defined as the tropopause height(s). The bi-Gaussian function exhibits remarkable potential for explicating the variation trend of temperature profiles. The recognition results of the bi-Gaussian method and lapse rate tropopause, as defined by World Meteorological Organization, are compared in detail for different cases. Results indicate that the bi-Gaussian method possesses a lower missed detection rate and false detection rate than lapse rate tropopause, because it is not restricted by thresholds, even in the presence of multiple temperature inversion layers at higher elevations. Five-year (from 2012 to 2016) historical radiosondes in China revealed that the occurrence frequency and thickness of DT, as well as the single tropopause height, and the first and second DT height displayed significant meridional monotonic variations. The occurrence frequency (thickness) of DT increased from 2.93 % (2.61 km) to 72.45 % (6.84 km) in the latitude range [16° N, 50° N]. At mid-latitudes [30° N, 40° N], the meridional gradients of tropopause height were relatively large, essentially corresponding to the climatological location of the subtropical jet and Tibetan Plateau. The average DT thickness reported in this study is approximately 1–2 km thicker than that in previous studies, particularly in the mid-high latitudes [45° N, 50° N], which may be related to the different vertical resolution of temperature profiles provided by various data sources. DT structure occurs most frequently and has the largest meridional gradient in the mid-latitudes, formatted by a combination of poleward advection in the low-latitude upper troposphere and equatorward advection in the high-latitude lower stratosphere. In addition, although DT is thick in winter, the DT temperature difference is small, even the case of the first tropopause temperature is lower than the second tropopause temperature happens occasionally. [ABSTRACT FROM AUTHOR]

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat provided infrared limb emission spectra, which were used to infer global distributions of CFC-11, CFC-12, and HCFC-22. Spectra were analysed using constrained non-linear least-squares fitting. Changes with respect to earlier data versions refer to the use of version 8 spectra, the altitude range where the background continuum is considered, details of the regularization and microwindow selection, and the occasional joint fitting of interfering species, the use of new spectroscopic data, the joint fit of a tangent-height-dependent spectral offset, and the use of 2D temperature fields. In the lower stratosphere the error budget is dominated by uncertainties in spectroscopic data, while above this measurement noise is the leading error source. The vertical resolution of CFC-11 and CFC-12 is 2–3 km near the tropopause, about 4 km at 30 km altitude, and 6–10 km at 50 km. The vertical resolution of HCFC-22 is somewhat coarser, 3–4 km at the tropopause and 10–12 km at 35 km altitude. In the altitude range of interest, the horizontal resolution is typically limited by the horizontal sampling of the measurements, not by the smearing of the retrievals. Horizontal information displacement does not exceed 150 km , which can become an issue only for comparisons with model simulations with high horizontal resolution or localized in situ observations. Along with the regular data product, an alternative representation of the data on a coarser vertical grid is offered. These data can be used without consideration of the averaging kernels. The new data version provides improvement with respect to reduction of biases and improved consistency between the full- and reduced-resolution mission period of MIPAS. [ABSTRACT FROM AUTHOR]

The steady response of the stratosphere to tropospheric thermal forcing via an SST perturbation is considered in two separate theoretical models. It is first shown that an SST anomaly imposes a geopotential anomaly at the tropopause. Solutions to the linearized quasigeostrophic potential vorticity equations are then used to show that the vertical length scale of a tropopause geopotential anomaly is initially shallow, but significantly increased by diabatic heating from radiative relaxation. This process is a quasi-balanced response of the stratosphere to tropospheric forcing. A previously developed, coupled troposphere–stratosphere model is then introduced and modified. Solutions under steady, zonally symmetric SST forcing in the linear β-plane model show that the upward stratospheric penetration of the corresponding tropopause geopotential anomaly is controlled by two nondimensional parameters: 1) a dynamical aspect ratio and 2) a ratio between tropospheric and stratospheric drag. The meridional scale of the SST anomaly, radiative relaxation rate, and wave drag all significantly modulate these nondimensional parameters. Under Earthlike estimates of the nondimensional parameters, the theoretical model predicts stratospheric temperature anomalies 2–3 larger in magnitude than that in the boundary layer, approximately in line with observational data. Using reanalysis data, the spatial variability of temperature anomalies in the troposphere is shown to have remarkable coherence with that of the lower stratosphere, which further supports the existence of a quasi-balanced response of the stratosphere to SST forcing. These findings suggest that besides mechanical and radiative forcing, there is a third way the stratosphere can be forced—through the tropopause via tropospheric thermal forcing. Significance Statement: Upward motion in the tropical stratosphere, the layer of atmosphere above where most weather occurs, is thought to be controlled by weather disturbances that propagate upward and dissipate in the stratosphere. The strength of this upward motion is important since it sets the global distribution of ozone. We formulate and use simple mathematical models to show the vertical motion in the stratosphere can also depend on the warming in the troposphere, the layer of atmosphere where humans live. We use the theory as an explanation for our observations of inverse correlations between the ocean temperature and the stratosphere temperature. These findings suggest that local stratospheric cooling may be coupled to local tropospheric warming. [ABSTRACT FROM AUTHOR]

We introduce a climate intervention strategy focused on decreasing water vapor (WV) concentrations near the tropopause and in the stratosphere to increase outbound longwave radiation. The mechanism is the targeted injection of ice-nucleating particles (INP) in air supersaturated with respect to ice at high altitudes in the tropical entryway to the stratosphere. Ice formation in this region is a critical control of stratospheric WV. Recent airborne in situ data indicate that targeting only a small fraction of air parcels in the region would be sufficient to achieve substantial removal of water. This "intentional stratospheric dehydration" (ISD) strategy would not counteract a large fraction of the forcing from carbon dioxide but may contribute to a portfolio of climate interventions by acting with different time and length scales of impact and risk than other interventions that are already under consideration. We outline the idea, its plausibility, technical hurdles, and side effects to be considered. [ABSTRACT FROM AUTHOR]

Abstract Climate models robustly project acceleration of the Brewer‐Dobson circulation (BDC) in response to climate change. However, the BDC trends simulated by comprehensive models are poorly constrained by observations, which cannot even determine the sign of potential trends. Additionally, the changing structure of the troposphere and stratosphere has received increasing attention in recent years. The extent to which vertical shifts of the circulation are driving the acceleration is under debate. In this study, we present a novel method that enables the attribution of advective BDC changes to structural changes of the circulation and of the stratosphere itself. Using this method allows studying the advective BDC trends in unprecedented detail and sheds new light into discrepancies between different data sets (reanalyses and models) at the tropopause and in the lower stratosphere. Our findings provide insights into the reliability of model projections of BDC changes and offer new possibilities for observational constraints.

The dynamical tropopause as a transport barrier between the tropical upper troposphere and extratropical lowermost stratosphere is characterized by steep gradients in potential vorticity (PV) along an isentropic surface. Hence, the latitudinal separation between the dynamical tropopause in the northern and southern hemispheres can be used as a metric of upper tropospheric width for assessing climate change impacts. Here, we obtain the PV gradient-based dynamical tropopause (PVG tropopause) from four meteorological satellite-era reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2) and investigate its climatology, variability and long-term trends ranging from 1980 to 2017. Our results show a distinct seasonal cycle with larger PV values and a poleward movement of the PVG tropopause in summer. The climatological tropopause PV values are substantially different between different reanalyses, but the tropopause latitude is similar. Significant inter-annual variability in the PVG tropopause latitude is related to El Niño Southern Oscillation (ENSO) and weaker variability also to the Quasi-Biennial Oscillation (QBO), and is consistently represented in reanalyses. In particular, El Niño causes equatorward shifts of the PVG tropopause, hence a decrease of upper tropospheric width. Long-term trends in the PVG tropopause over the period 1980–2017 exhibit a distinct vertical structure with poleward shifts below 340 K potential temperature, equatorward shifts between 340–370 K and poleward shifts between 370–380 K, implying an expansion of the troposphere at lower levels, a narrowing at upper levels and an expansion near the tropical tropopause. [ABSTRACT FROM AUTHOR]

Water vapor and cirrus clouds in the tropical tropopause layer (TTL) are important for the climate and are largely controlled by temperature in the TTL. On interannual timescales, both stratospheric and tropospheric modes of the large‐scale variability could affect temperatures in the TTL. Here multiple linear regression (MLR) is used to investigate explained variance in the cold point tropopause temperature (CPT), cold point tropopause height (CPZ), 83 hPa water vapor (WV83), 83 hPa ozone (O383), and total cirrus cloud fraction with cloud base (TTLCCF) and top (ALLCF) above 14.5 km, all averaged over 15°S‐15°N. Predictors of the MLR are a set of stratospheric and tropospheric large‐scale modes of variability. The MLR explains significant variance in CPT (76%), CPZ (78%), WV83 (65%), O383 (62%), TTLCCF (52%), and ALLCF (36%). The interannual variability of CPT and WV83 is dominated by stratospheric processes associated with the Quasi‐Biennial Oscillation (QBO) and Brewer‐Dobson Circulation (BDC), whereas the variability of CPZ, O383, TTLCCF and ALLCF is also controlled by 500 hPa temperature (T500). Residual variability in CPT and CPZ not captured by the MLR are further significantly correlated to stratospheric temperature. It is shown that the portion of the BDC's shallow branch missed by the eddy heat flux based BDC index contributes significant amounts of the explained variances. Plain Language Summary: Between the tropical upper troposphere and lower stratosphere, water can exist as either vapor or ice. The amount of water that enters the stratosphere depends on the portion of vapor that is frozen out by the coldest temperature that air experiences in this region, which on interannual timescales could be modulated by both large‐scale stratospheric and tropospheric modes of variability. Here we show that 76%, 65%, and 52% of the interannual variance in cold point temperature, water vapor at 83 hPa, and ice cloud fraction in this region can be explained using a multiple linear regression (MLR), where the predictors are the modes of the large‐scale variability. Stratospheric processes are much more important in controlling the interannual variance of cold point temperature and water vapor at 83 hPa, but notably, the height of the cold point is controlled by both stratospheric and tropospheric processes. Residual variability of the cold point temperature not captured by the MLR is still connected to temperature variability in the stratosphere. Key Points: The interannual variability in the cold point tropopause temperature averaged over 15°S‐15°N is driven by stratospheric processesThis cold point tropopause temperature residual after regressing out large‐scale modes is still correlated with stratospheric temperatureThe portion of the BDC's shallow branch, which is independent of our eddy heat flux BDC index, is an important source of TTL variability [ABSTRACT FROM AUTHOR]

As one of the most important greenhouse gases, water vapor in the upper troposphere and lower stratosphere (UTLS) has a significant impact on the global earth-atmosphere system. The Tibetan Plateau (TP) is an important high terrain which exerts a profound impact on the change of weather and climate, and mass exchange. Tropopause folds occur frequently over the TP due to the impact of the subtropical westerly jet, which affects water vapor transport between the stratosphere and the troposphere. In this paper, the spatial and temporal distribution characteristics of tropopause folds over the TP are examined by applying an improved three-dimensional (3D) labeling algorithm to the ERA5 reanalysis data (1979 to 2019). The effects of different fold depths in various regions over the TP on the variations of UTLS water vapor are further studied. The results of a case study (25 February 2008) suggest that there is a good continuity in identification of the fold depth for the same fold event using the improved 3D labeling algorithm. The fold depth and height are consistent with the results of radiosonde data and ERA5 reanalysis data. The fold frequency over the TP shows an increasing trend in the last 41 years, with slightly lower frequency of medium folds than that of shallow folds, and lowest frequency of deep folds. There is increasing water vapor in the UTLS over the TP due to tropopause folds. The results indicate that tropopause folds enhance the horizontal divergence of water vapor in the UTLS and increase the vertical water vapor flux in the UTLS region. The folding over the plateau leads to increased moisture in the UTLS. It is argued that vertical velocity anomalies in the vicinity of the fold and subgrid perturbations have a significant impact on the increase of UTLS water vapor over the TP. The results of this work provide a scientific basis for a better understanding of the stratosphere-troposphere exchanges due to tropopause folds over the TP. [ABSTRACT FROM AUTHOR]

Cirrus clouds play an important role in the radiation budget of the Earth; nonetheless, the radiative effect of ultra-thin cirrus clouds in the tropopause region and in the lowermost stratosphere remains poorly constrained. These clouds have a small vertical extent and optical depth and are frequently neither observed even by sensitive sensors nor considered in climate model simulations. In addition, their short-wave (cooling) and long-wave (warming) radiative effects are often in approximate balance, and their net effect strongly depends on the shape and size of the cirrus particles. However, the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere instrument (CRISTA-2) allows ultra-thin cirrus clouds to be detected. Here we use CRISTA-2 observations in summer 1997 in the Northern Hemisphere midlatitudes together with the Suite Of Community RAdiative Transfer codes based on Edwards and Slingo (SOCRATES) radiative transfer model to calculate the radiative effect of observed ultra-thin cirrus. Using sensitivity simulations with different ice effective particle size and shape, we provide an estimate of the uncertainty in the radiative effect of ultra-thin cirrus in the extratropical lowermost stratosphere and tropopause region during summer and – by extrapolation of the summer results – for winter. Cloud top height and ice water content are based on CRISTA-2 measurements, while the cloud vertical thickness was predefined to be 0.5 or 2 km. Our results indicate that if the ice crystals of these thin cirrus clouds are assumed to be spherical, their net cloud radiative effect is generally positive (warming). In contrast, assuming aggregates or a hexagonal shape, their net radiative effect is generally negative (cooling) during summer months and very likely positive (warming) during winter. The radiative effect is in the order of ± (0.1–0.01) Wm-2 for a realistic global cloud coverage of 10 %, similar to the magnitude of the contrail cirrus radiative forcing (of ∼ 0.1 Wm-2). The radiative effect is also dependent on the cloud vertical extent and consequently the optically thickness and effective radius of the particle size distribution (e.g. effective radius increase from 5 to 30 µm results in a factor ∼ 6 smaller long- and short-wave effects, respectively). The properties of ultra-thin cirrus clouds in the lowermost stratosphere and tropopause region need to be better observed, and ultra-thin cirrus clouds need to be evaluated in climate model simulations. [ABSTRACT FROM AUTHOR]

The presence of an aerosol layer in the upper troposphere/lower stratosphere (UT/LS) in South America was identified with the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2). This layer, which we shall refer to as the South American tropopause aerosol layer (SATAL), was identified over the Amazon basin at altitudes between 11 and 14 km. It exhibits a seasonal behavior similar to the Asian tropopause aerosol layer (ATAL) and the North American tropopause aerosol layer (NATAL). The SATAL is observed from October to March, coinciding with the presence of the South American monsoon. It forms first in the eastern Amazon basin in October, then moves to the southern Amazon, where it weakens in December-January and finally dissipates in February-March. We hypothesize that two main factors influence the SATAL formation in the UT/LS: 1) the source of aerosols from Africa and 2) the updraft mass flux from deep convective systems during the active phase of the South American monsoon system that transports aerosols to the UT/LS. Further satellite observations of aerosols and field campaigns are needed to provide useful information to find the origin and composition of the aerosols in the UT/LS during the South American monsoon. [ABSTRACT FROM AUTHOR]

Satellite-based observations require independent sources of data to monitor and evaluate their precision and accuracy. For the temperature and water vapor profiles produced by satellite-based sounders, this typically results in comparisons to operational radiosonde observations. However, polar-orbiting satellite overpasses are frequently misaligned with the global synoptic launch times. The routine airborne in situ observations of temperature and water vapor from the Airborne Meteorological Data Relay (AMDAR) program and the Water Vapor Sensing System-II (WVSS-II) instrument greatly enhance opportunities to make precise matchups due to the far greater temporal frequency and spatial density of aircraft flights. The potential for the use of aircraft-based observations as a source of evaluation of tropospheric satellite sounder profiles is explored through a year-long intercomparison with the Infrared Atmospheric Sounding Interferometer (IASI) level-2 profiles produced from both the Metop-A and Metop-B satellites. Results using 1 h and 50 km match criteria indicating good agreement between the satellites and the aircraft-based observations with temperature, specific humidity, and relative humidity biases generally less than 0.5 K, 0.8 gkg-1 , and 5 %, respectively; both IASI instruments perform nearly identically. While the intercomparisons are generally limited to the troposphere as aircraft typically reach their maximum height at the tropopause, the substantially larger number of intercomparison points enable characterization as a function of season, scan angle, and other characteristics heretofore unexplored due to a lack of sufficient validation data. [ABSTRACT FROM AUTHOR]

In this study we analyse the physical processes causing the weakening of the tropical circulation in the Coupled Model Intercomparison Project 6 (CMIP6). We apply a diagnostic model for the large-scale tropical circulation (vertical motion) based on the moist static energy for the first baroclinic mode (MSEB) and evaluate the sensitivity of the tropical circulation changes to the changes in advection of moisture and heat, the net radiation, the moist static stability, the baroclinic mode, and to the height of the tropopause. Based on the CMIP6 model simulations we find that the tropical circulation weakens by about 10-15% over the twenty-first century. The analysis of the MSEB model suggests that the primary cause for this weakening of the tropical circulation is the lifting of the tropopause height. This effect is fairly uniform throughout the tropics and present in all model simulations. The tropopause height increase shifts the first baroclinic mode away from lower levels with unstable air masses into high levels of the troposphere where stable air masses lead to a stabilisation of the large-scale circulation. Other factors such as changes in the advection of moisture and heat, increased net heating or increased instability of the lower tropospheric gross moist stability do have strong regional differences, and mostly increase tropical circulations, counteracting the weakening caused by the lifting of the tropopause. [ABSTRACT FROM AUTHOR]

The Asian tropopause aerosol layer (ATAL) is present during the Asian summer monsoon season affecting the radiative balance of the atmosphere. However, the source regions and transport pathways of ATAL particles are still uncertain. Here, we investigate transport pathways from different regions at the model boundary layer (MBL) to the ATAL by combining two Lagrangian transport models (CLaMS, Chemical Lagrangian Model of the Stratosphere; MPTRAC, Massive-Parallel Trajectory Calculations) with balloon-borne measurements of the ATAL performed by the Compact Optical Backscatter Aerosol Detector (COBALD) above Nainital (India) in August 2016. Trajectories are initialised at the measured location of the ATAL and calculated 90 d backwards in time to investigate the relation between the measured, daily averaged, aerosol backscatter ratio and source regions at the MBL. Different simulation scenarios are performed to find differences and robust patterns when the reanalysis data (ERA5 or ERA-Interim), the trajectory model, the vertical coordinate (kinematic and diabatic approach) or the convective parameterisation are varied. The robust finding among all scenarios is that the largest continental air mass contributions originate from the Tibetan Plateau and the Indian subcontinent (mostly the Indo-Gangetic Plain), and the largest maritime air mass contributions in Asia come from the western Pacific (e.g. related to tropical cyclones). Additionally, all simulation scenarios indicate that the transport of maritime air from the tropical western Pacific to the region of the ATAL lowers the backscatter ratio (BSR) of the ATAL, while most scenarios indicate that the transport of polluted air from the Indo-Gangetic Plain increases the BSR. While the results corroborate key findings from previous ERA-Interim-based studies, they also highlight the variability in the contributions of different MBL regions to the ATAL depending on different simulation scenarios. [ABSTRACT FROM AUTHOR]

Future changes in extratropical cyclone structure and dynamics may lead to important impacts but are not yet fully understood. In the first part of this study, we have applied a composite approach together with potential vorticity (PV) inversion to study such changes in the dynamics of North Atlantic cyclones. Here, this is complemented with the help of a Lagrangian perspective, making use of air parcel trajectories to investigate the causes of altered PV anomalies as well as the role that cyclone airstreams play in shaping these changes. Intense cyclones in the extended winter seasons of two periods, 1990–2000 and 2091–2100, are studied in Community Earth System Model Large Ensemble (CESM-LE) simulations, and backward trajectories are calculated from the cyclone area as a basis to construct cyclone-centered composites of Lagrangian tendencies and their projected future changes. Our results show that diabatic processes on a timescale of 24 h shape the cyclones' low-level PV distribution and corroborate that the increasing moisture content along with enhanced ascent in warm conveyor belts leads to amplified latent heat release and larger low- and mid-level PV anomalies near the cyclone center in a warmer climate. In contrast, projected upper-level PV changes are due to a combination of several processes. These processes include cloud diabatic PV changes, anomalous PV advection, and likely also radiative PV generation in the lower stratosphere above the cyclone center. For instance, enhanced poleward advection is the primary reason for a projected decrease in upper-level PV anomalies south of the cyclone center. Warm conveyor belt outflow regions are projected to shift upward, but there is not robust change in the associated upper-level PV anomalies due to compensation between enhanced low-level PV generation and upper-level PV destruction. In summary, our two-part study points to future changes in the relative importance of different processes for the dynamics of intense North Atlantic cyclones in a warming climate, with important consequences for the near-surface wind pattern. In particular, a larger role of cloud diabatic processes is projected, affecting the cyclones through PV production in the lower troposphere. The role of other mechanisms, in particular radiative changes near the tropopause, should be investigated in more detail in future studies. [ABSTRACT FROM AUTHOR]

The tropopause region is crucial for the stratosphere‐troposphere exchange (STE) and acts as an indicator of climate change. Double tropopauses (DTs) act to increase the STE process but their driving mechanisms remain an open question. The present assessment offers for the first time the linkage between tropospheric gravity waves (GWs) and DT events by exploring a global data set of multi‐year radiosonde measurements. In the extratropics, the occurrence frequency of DT events keeps a remarkably consistent spatial‐temporal structure with GW total energy. Under the DT scenario, GW total energy has increased by 37.67% compared to single tropopause events. Based on a statistical assessment, the upward propagating GWs throughout the second tropopause region can probably raise Kelvin‐Helmholtz instability or turbulence, leading permanent irregularities in thermodynamic structure, and consequently, increasing the likelihood of DT. Plain Language Summary: Atmospheric disturbances above the troposphere are predominantly associated with waves generated in the troposphere. Among others, gravity waves (GWs) play major roles in transport of energy and momentum from the lower to the upper atmosphere. The double tropopause (DT) caused by irregular temperature changes is a common phenomenon in the lowermost stratosphere, which has an indicative significance for climate change. The upward propagating GWs sourced from the troposphere can probably alter the temperature variability in the tropopause region, and therefore, they are likely associated with the occurrence of DTs. By analyzing a large amount of global radiosonde data, simultaneous measurements of tropospheric GWs and irregularities in the tropopause enable us to argue that GWs sourced from troposphere could increase the likelihood of DTs. Key Points: A good level of agreement in spatial‐temporal variability has been identified between Double tropopause (DT) events and tropospheric gravity wave (GW) total energySubstantial tropospheric GW and Kelvin‐Helmholtz instability frequency enhancements in the tropopause region can be observed with DT phenomenaThe tropospheric GWs may increase the likelihood of DT events by introducing permanent localized irregularities or instabilities [ABSTRACT FROM AUTHOR]

We examine geographical distribution pattern of convective overshooting and its internal microphysical three-dimensional structure of precipitation over East China by matching Global Precipitation Measurement Dual-frequency Precipitation Radar instrument (GPM DPR) with European Centre for Medium-Range Weather Forecasts 5th Reanalysis (ERA5). Convective overshooting events mainly occur over NC (Northeast China) and northern MEC (Middle and East China), with a magnitude of only 10−3; Radar reflectivity of convective overshooting over NC accounts for a higher proportion below the zero level, while MEC and SC (South China) account for a higher proportion above the zero level, indicating stronger upward motion and more ice crystal particles; The microphysical processes within convective overshooting are unique, leading to various properties of the droplets in precipitation. Droplets of convective overshooting are large, but sparse. And its effective radius of droplet, below 10 km altitude, is almost exceeding 2.5 mm, which is about twice than normal precipitation. Convective overshooting humidifies air below the cloud top and obviously increases the ozone near tropopause as a result of influx of ozone from lower troposphere and sinking of air with high concentration ozone in the stratosphere. Findings of this study may have important implications for the microphysical evolution associated with convective overshooting, and provide more accurate precipitation microphysical parameters as the input of the model simulation. [ABSTRACT FROM AUTHOR]

Recurving tropical cyclones (TCs) in the western North Pacific often cause heavy rainfall events (HREs) in East Asia. However, how their interactions with midlatitude flows alter the characteristics of HREs remains unclear. The present study examines the synoptic–dynamic characteristics of HREs directly resulting from TCs in South Korea with a focus on the role of midlatitude baroclinic condition. The HREs are categorized into two clusters based on midlatitude tropopause patterns: strongly (C1) and weakly (C2) baroclinic conditions. C1, which is common in late summer, is characterized by a well-defined trough–ridge couplet and jet streak at the tropopause. As TCs approach, the trough–ridge couplet amplifies, but is anchored by divergent TC outflow. This leads to phase locking of the upstream trough with TCs and thereby prompts substantial structural changes of TCs reminiscent of extratropical transition. The synergistic TC–midlatitude flow interactions allow for widely enhanced quasigeostrophic forcing for ascent to the north of the TC center. This allows HREs to occur even before TC landfall with more inland rainfall than C2 HREs. In contrast, C2, which is mainly observed in midsummer, does not accompany the undulating tropopause. In the absence of strong interactions with midlatitude flows, TCs rapidly dissipate after HREs while maintaining their tropical features. The upward motion is confined to the inherent TC convection, and thus HREs occur only when TCs are located in the vicinity of the country. These findings suggest that midlatitude baroclinic condition determines the spatial extent of TC rainfall and the timing of TC-induced HREs in South Korea. Significance Statement: This study suggests that the midlatitude flows can substantially modulate heavy rainfall events directly caused by tropical cyclones. By analyzing the 42-yr tropical cyclone–induced heavy rainfall events in South Korea, it is found that tropical cyclones and midlatitude flows strongly interact with each other, especially when the midlatitude flows meander in conjunction with a strong jet stream. Their synergistic interactions result in a poleward expansion of the tropical cyclones' precipitation shields, leading to heavy rainfall events even before they make landfall in the country. Consequently, it is advisable to carefully monitor the midlatitude conditions as well as tropical cyclones themselves as earlier heavy rainfall warnings may be necessary depending on the former. [ABSTRACT FROM AUTHOR]

Abstract Since the tropopause was first identified, the quality and resolution of weather balloons has dramatically improved. NOAA Earth System Research Laboratories (ESRL) have provided high resolution and very high quality ozonesondes from eight locations: Fiji; American Samoa; Greenland; Antarctica; and several locations in USA (Hawai'i, Colorado, California and Alabama). These locations collectively cover polar regions, mid‐latitudes and tropics. Using this publicly archived data set, we studied the variability of the tropopause for all eight locations for one complete year (2016). Along with the standard estimates of the tropopause provided by NOAA ESRL, we developed four alternative tropopause definitions each based on changes in one of the following: (a) molar density; (b) temperature lapse rates; (c) water vapor content; (d) ozone content. These old and new tropopause definitions appear to hold over all eight locations—for all seasons and from the tropics to the poles. The cohesiveness between all five of these independent tropopause definitions is remarkable, although the NOAA ESRL estimates sometimes identify higher tropopause onsets than the other estimates. Therefore, each tropopause definition could potentially be used as proxies for other tropopause definitions. However, it also confirms that the troposphere/tropopause transition is a multi‐faceted physical and chemical phenomenon associated with more than just temperature changes. Finally, these high‐resolution results suggest that the original term “tropopause” might be a misnomer since they suggest increases in temperature lapse rate variability, rather than the “pausing” implied by lower resolution data or by lapse rates that are averaged over large distances.