Tropopause Transition Layer Research Articles

A Modern Approach to a Stability-Based Definition of the Tropopause

Abstract Definition of the tropopause has remained a focus of atmospheric science since its discovery near the beginning of the twentieth century. Few universal definitions (those that can be reliably applied globally and to both common observations and numerical model output) exist and many definitions with unique limitations have been developed over the years. The most commonly used universal definition of the tropopause is the temperature lapse-rate definition established by the World Meteorological Organization (WMO) in 1957 (the LRT). Despite its widespread use, there are recurrent situations where the LRT definition fails to reliably identify the tropopause. Motivated by increased availability of coincident observations of stability and composition, this study seeks to reexamine the relationship between stability and composition change in the tropopause transition layer and identify areas for improvement in a stability-based definition of the tropopause. In particular, long-term (40+ years) balloon observations of temperature, ozone, and water vapor from six locations across the globe are used to identify covariability between several metrics of atmospheric stability and composition. We found that the vertical gradient of potential temperature is a superior stability metric to identify the greatest composition change in the tropopause transition layer, which we use to propose a new universally applicable potential temperature gradient tropopause (PTGT) definition. Application of the new definition to both observations and reanalysis output reveals that the PTGT largely agrees with the LRT, but more reliably identifies tropopause-level composition change when the two definitions differ greatly. Significance Statement In this study we provide a review of existing tropopause definitions (and their limitations) and investigate potential improvement in the definition of the tropopause using balloon-based observations of stability and atmospheric composition. This work is motivated by the need for correct identification of the tropopause to accurately assess upper-troposphere–lower-stratosphere processes, which in turn has far-reaching implications for our understanding of Earth’s radiation budget and climate. The result of this research is the creation of a new, universally applicable stability-based definition of the tropopause: the potential temperature gradient tropopause (PTGT).

Mixing characteristics within the tropopause transition layer over the Asian summer monsoon region based on ozone and water vapor sounding data

The structure of the tropopause transition layer (TTL) over the Asian summer monsoon (ASM) anticyclone is analyzed on the basis of high-resolution balloon soundings of temperature, ozone, and water vapor at Kunming and Lhasa, China during 11 summer monsoon seasons between 2009 and 2019. Two definitions of the TTL, one based on thermal structure and one based on tracer correlations of O3 and H2O, are analyzed and compared. In tracer–tracer space, the air masses of mixed stratospheric and tropospheric characteristics are identified by using the O3–H2O correlations. The mixed air formed a transitional layer and shows strong spatial variation in the altitude space. Statistical result shows that the altitude range of mixed air parcels spans a layer of approximately 5.5 km depth between 12.5 km and 18 km. This vertical distribution of mixed air masses is basically consistent with thermally-based TTL, which takes the level of minimum stability (LMS) and the cold point tropopause (CPT) as its lower and upper boundaries, respectively. The thermal definition regards the TTL as a thermal transition zone, but does not consider the tracer mixing features between stratosphere and troposphere. Tracer correlations can identify the mixed air mass, which is a basic feature of the TTL, whether they are from the range of the thermally-based TTL, or below the LMS, or above the CPT. Based on the O3–H2O correlations, statistical analysis shows that about 30%–50% of air masses between the LMS and the CPT are identified as mixed. And up to 1–1.5 km above the CPT, a small fraction of air masses is also mixed, which means that air masses from the troposphere can be lifted and mixed into the free stratosphere within the ASM anticyclone region.

The Influence of Atmospheric Cloud Radiative Effects on the Large-Scale Stratospheric Circulation

AbstractPrevious studies have explored the influence of atmospheric cloud radiative effects (ACRE) on the tropospheric circulation. Here the authors explore the influence of ACRE on the stratospheric circulation. The response of the stratospheric circulation to ACRE is assessed by comparing simulations run with and without ACRE. The stratospheric circulation response to ACRE is reproducible in a range of different GCMs and can be interpreted in the context of both a dynamically driven and a radiatively driven component.The dynamic component is linked to ACRE-induced changes in the vertical and meridional fluxes of wave activity. The ACRE-induced changes in the vertical flux of wave activity into the stratosphere are consistent with the ACRE-induced changes in tropospheric baroclinicity and thus the amplitude of midlatitude baroclinic eddies. They account for a strengthening of the Brewer–Dobson circulation, a cooling of the tropical lower stratosphere, a weakening and warming of the polar vortex, a reduction of static stability near the tropical tropopause transition layer, and a shortening of the time scale of extratropical stratospheric variability. The ACRE-induced changes in the equatorward flux of wave activity in the low-latitude stratosphere account for a strengthening of the zonal wind in the subtropical lower to midstratosphere.The radiative component is linked to ACRE-induced changes in the flux of longwave radiation into the lower stratosphere. The changes in radiative fluxes lead to a cooling of the extratropical lower stratosphere, changes in the static stability and cloud fraction near the extratropical tropopause, and a shortening of the time scales of extratropical stratospheric variability.The results highlight a previously overlooked pathway through which tropospheric climate influences the stratosphere.

Variation in the tropopause transition layer over China through analyzing high vertical resolution radiosonde data

AbstractIn this work, the meridional variation of the thermal tropopause layer is investigated using high vertical resolution radiosonde measurements over China. A tropopause-based mean method is used to characterize the depth of the tropopause inversion layer (TIL), and a curve-fitting method to profiles of Brunt–Vaisala frequency is applied to identify the transition sharpness of the static stability in the tropopause transition layer (TTL). The eight radiosonde stations are grouped into four latitudinal bands at intervals of 7°. Analyses show that the tropopause at higher latitudes is much more likely to be sharp and have a strong TIL, while that at lower latitudes is dominated by a thick TTL.

Clouds and Water Vapor in the Tropical Tropopause Transition Layer over Mesoscale Convective Systems

Abstract Observations from A-Train satellites and other datasets show that mesoscale convective systems (MCSs) affect the water vapor and ice content of the tropical tropopause transition layer (TTL). The largest MCSs with radar reflectivity characteristics consistent with the presence of large stratiform and anvil regions have the greatest impact. Most MCSs are associated with clouds in the TTL. Composites in MCS-relative coordinates indicate enhanced cloudiness and ice water content (IWC) extending toward the cold-point tropopause (CPT), particularly in large and connected MCSs. Widespread anvils in the lower TTL are evident in the peak cloudiness diverging outward at those levels. Upper-tropospheric water vapor concentrations are enhanced near MCSs. Close to the centers of MCSs, water vapor is suppressed at TTL base, likely because of the combined effects of reduced moistening or dehydration at the higher TTL relative humidities and subsidence above cloud top. Weak moistening is observed near the CPT, consistent with sublimation of ice crystals at the tops of the deepest MCSs. In the outflow region, moistening is observed in the lower TTL near the largest MCSs. Enhanced water vapor in the upper troposphere and lower TTL extends beyond the area of substantially enhanced cloudiness and IWC, in agreement with the observed radial outflow, indicating that MCSs are injecting water vapor into the environment and consistent with the possibility that MCS development may be favored by a premoistened environment.

Distributions of ice supersaturation and ice crystals from airborne observations in relation to upper tropospheric dynamical boundaries

AbstractIce supersaturation (ISS) is the prerequisite condition for cirrus cloud formation. To examine multiscale dynamics' influences on ISS formation, we analyze in situ aircraft observations (~200 m scale) over North America in coordinates relative to dynamical boundaries in the upper troposphere and lower stratosphere. Two case studies demonstrate that ISS formation is likely influenced by mesoscale uplifting, small‐scale waves, and turbulence. A collective analysis of 15 flights in April–June 2008 shows that the top layers of ISS and ice crystal distributions are strongly associated with thermal tropopause height. In addition, the average occurrence frequencies of ISS and ice crystals on the anticyclonic side of the jet stream are ~1.5–2 times of those on the cyclonic side. By defining five cirrus evolution phases based on the spatial relationships between ice‐supersaturated and in‐cloud regions, we find that their peak occurrence frequencies are located at decreasing altitudes with respect to the thermal tropopause: (phase 1) clear‐sky ISS around the tropopause, (phase 2) nucleation phase around 2 km below the tropopause, (phases 3 and 4) early and later growth phases around 6 km below the tropopause, and (phase 5) sedimentation/sublimation around 2–6 km below the tropopause. Consistent with this result, chemical tracer correlation analysis shows that the majority (~80%) of the earlier cirrus phases (clear‐sky ISS and nucleation) occurs inside the chemical tropopause transition layer, while the later phases happen mostly below that layer. These results shed light on the role of dynamical environment in facilitating cirrus cloud formation and evolution.

Identification of the tropical tropopause transition layer using the ozone-water vapor relationship

We present a method of identifying the tropical tropopause transition layer (TTL) using chemical tracer-tracer relationships. Coincident ozone (O3) and water vapor (H2O) measurements over Alajuela, Costa Rica (~10°N), in July and August 2007 are used to demonstrate the concept. In the tracer-tracer space, the O3 and H2O relationship helps to separate the transition layer air mass from the background troposphere and stratosphere. This tracer relationship-based transition layer is found to span an approximately 40 K potential temperature range between 340 and 380 K and is largely confined between the level of minimum stability (LMS) and the cold point tropopause (CPT). This chemical composition-based transition layer is, therefore, consistent with a definition of the TTL based on the thermal structure, for which the LMS and CPT are the lower and upper boundaries of TTL, respectively. We also examine the transition layer over the region of Asian summer monsoon (ASM) anticyclone using the measurements over Kunming, China (~25°N), and compare its behavior with the TTL structure in the deep tropics. The comparison shows that the transition layer over the ASM is similar to the TTL, although the data suggest the ASM transition layer lies at higher potential temperature levels and is potentially prone to the influence of extratropical processes.

The Residual-Mean Circulation in the Tropical Tropopause Layer Driven by Tropical Waves

Abstract Latent heating estimates derived from rainfall observations are used to construct model experiments that isolate equatorial waves forced by tropical convection from midlatitude synoptic-scale waves. These experiments are used to demonstrate that quasi-stationary equatorial Rossby waves forced by latent heating drive most of the observed residual-mean upwelling across the tropopause transition layer within 15° of the equator. The seasonal variation of the equatorial waves and the mean meridional upwelling that they cause is examined for two full years from 2006 to 2007. Changes in equatorial Rossby wave propagation through seasonally varying mean winds are the primary mechanism for producing an annual variation in the residual-mean upwelling. In the tropical tropopause layer, averaged within 15° of the equator and between 90 and 190 hPa, the annual cycle varies between a maximum upwelling of 0.4 mm s−1 during boreal winter and spring and a minimum of 0.2 mm s−1 during boreal summer. This variability seems to be due to small changes in the mean wind speed in the tropics. Seasonal variations in latent heating have only a relatively minor effect on seasonal variations in tropical tropopause upwelling. In addition, Kelvin waves drive a small downward component of the total circulation over the equator that may be modulated by the quasi-biennial oscillation.

Observations of Temperature, Wind, Cirrus, and Trace Gases in the Tropical Tropopause Transition Layer during the MJO*

Abstract Satellite observations of temperature, optically thin cirrus clouds, and trace gases derived from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and the Microwave Limb Sounder (MLS) are analyzed in combination with Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) wind and humidity fields in the tropical tropopause transition layer (TTL), using the Madden–Julian oscillation (MJO) as a carrier signal. MJO-related deep convection induces planetary-scale Kelvin and Rossby waves in the stably stratified TTL. Regions of ascent in these waves are associated with anomalously low temperatures, high radiative heating rates, enhanced cirrus occurrence, and high carbon monoxide and low ozone concentrations. Low water vapor mixing ratio anomalies lag the low temperature anomalies by about 1–2 weeks. The anomalies in all fields propagate eastward, circumnavigating the tropical belt over a roughly 40-day interval. Equatorial cross sections reveal that the anomalies tilt eastward with height in the TTL and propagate downward from the lower stratosphere into the upper troposphere. As MJO-related convection moves into the western Pacific and dissipates, a fast-moving Kelvin wave flanked by Rossby waves propagates eastward across South America and Africa into the western Indian Ocean. The region of equatorial westerly wind anomalies behind the Kelvin wave front lengthens until it encompasses most of the tropics at the 150-hPa level, giving rise to equatorially symmetric, anomalously low zonal-mean temperature and water vapor mixing ratio and enhanced cirrus above about 100 hPa.

The signature of the stratospheric Brewer‒Dobson circulation in tropospheric clouds

The signature of the stratospheric Brewer‒Dobson circulation (BDC) in tropospheric cloudiness is investigated in CloudSat and CALIPSO data from June 2006 through April 2011. During the Northern Hemisphere winter, periods of enhanced stratospheric wave driving are associated with increased cloud incidence in the tropical tropopause transition layer (TTL) juxtaposed against decreased cloud incidence in the Arctic troposphere. The results are consistent with the physical linkages between (1) the BDC and near‒tropopause temperature and (2) near‒tropopause temperature and upper tropospheric cloud incidence. The key finding of the work is that changes in the stratospheric circulation not only influence cloud amounts in the troposphere but also that they do so in a coupled manner that links climate variability in the Arctic and upper tropical troposphere. The results provide a pathway through which stratospheric processes influence tropospheric climate that is in addition to stratosphere/troposphere dynamical coupling.

The Structure of O3/H2O Mixing Relationships in the Tropopause Transition Layer at Middle and High Latitudes of the Northern Hemisphere

AbstractUsing HALOE O3 and H2O vertical profiles in 40°N~50°N of the Northern Hemisphere, considering chemical lifetime and transport temporal constant of O3 and H2O, the structure and seasonal characteristic of O3/H2O mixing relationships between 300 K to 390 K near the tropopause and the lowest stratosphere have been constructed in the isentropic coordinates. The results are as follows: (1) The slope of O3 mixing ratios profile has a spatial sudden change between 320~380 K isentropic surfaces of the tropopause translation layer, while the slope of H2O mixing ratios profile exhibits a spatial gradual change. The source region of O3 is stratosphere but H2O is troposphere near tropopause. So on the two sides of 320~380 K isentropic surfaces, mixing ratios of O3 and H2O show distinctly different vertical gradient. (2) In the vicinity of the tropopause where O3/H2O relationships reach best least square fitting, the troposphere and stratospheric branch of the non‐regular “L” relationship show seasonal and intra‐seasonal changes with a negative slope in tropospheric branch, but positive and negative slopes alter with seasons in stratospheric branch. At the same time, based on the “L” corner, the characteristic of chemopause with seasons can be confirmed. (3) The O3/H2O mixing relationship indicates that the transition layer is formed by stratospheric mixing with tropospheric air masses from different regions, and the mixing line of the transition layer is not constant. The transition layer mixing is equivalent in January, April 2003 and in January 2005, about 30 K isentropic thickness, that is between 320~350 K isentropic surfaces. Mixing height increases in November 2005, and the H2O coming into the stratosphere is less than in January 2003 and 2005, with about 30 K isentropic thickness, between 330~360 K isentropic surfaces. The change of mixing isentropic thickness is small in different seasons, but mixing height changes with seasons. The stratospheric branch of O3/H2O mixing relationship obviously changes with seasons, for which the significant reason is the lowest stratospheric air dehydration in January.

Effect of Tropical Waves on the Tropical Tropopause Transition Layer Upwelling

Abstract An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause transition layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field, and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, suggesting that this may also contribute to the TTL upwelling in that region. The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL (resembling the observed “cold trap”) occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the meridional momentum flux of the Rossby wave, passes through the cold trap generated by the Kelvin wave. Since in reality the background flow is shaped by the convective heating, the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the cold trap; however, both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves.

Annual, Interannual, and Intraseasonal Variability of Tropical Tropopause Transition Layer Cirrus

Abstract Cloud fields based on the first three years of data from the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission are used to investigate the relationship between cirrus within the tropical tropopause transition layer (TTL) and the Madden–Julian oscillation (MJO), the annual cycle, and El Niño–Southern Oscillation (ENSO). The TTL cirrus signature observed in association with the MJO resembles convectively induced, mixed Kelvin–Rossby wave solutions above the Pacific warm pool region. This signature is centered to the east of the peak convection and propagates eastward more rapidly than the convection; it exhibits a pronounced eastward tilt with height, suggestive of downward phase propagation and upward energy dispersion. A cirrus maximum is observed over equatorial Africa and South America when the enhanced MJO-related convection enters the western Pacific. Tropical-mean TTL cirrus is modulated by the MJO, with more than twice as much TTL cirrus fractional coverage equatorward of 10° latitude when the enhanced convection enters the Pacific than a few weeks earlier, when the convection is over the Indian Ocean. The annual cycle in cirrus clouds around the base of the TTL is equatorially asymmetric, with more cirrus observed in the summer hemisphere. Higher in the TTL, the annual cycle in cirrus clouds is more equatorially symmetric, with a maximum in the boreal winter throughout most of the tropics. The ENSO signature in TTL cirrus is marked by a zonal shift of the peak cloudiness toward the central Pacific during El Niño and toward the Maritime Continent during La Niña.

Tropical Tropopause Transition Layer Cirrus as Represented by CALIPSO Lidar Observations

Abstract The spatial and temporal variability of cirrus cloud fraction within the tropical tropopause transition layer (TTL) is investigated based on three years of data from the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, analyzed in conjunction with fields from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA)-Interim and temperature profiles from radiosondes launched at Manus Island, Papua New Guinea (2°S, 147°E). TTL cirrus is found to be mainly confined to the rising branch of the Hadley cell within ∼15° of the equator, with maximum cloud fraction between 14 and 15 km. The time-varying spatial pattern of cloud fraction within this belt does not resemble the pattern of cloud fraction in the layer below, as would be expected if the TTL cirrus were formed by the spreading of the anvils of convective clouds. On the contrary, within the stably stratified layer above ∼13 km, cirrus cloud fraction and temperature both appear to be modulated by the planetary-scale vertical velocity field. The time-varying spatial patterns are reminiscent of the vertical-propagating Kelvin wave response to an equatorial heat source, with the coldest, cloudiest air in the TTL centered approximately 30° of longitude to the east of the strongest heating.

Transport timescales and tracer properties in the extratropical UTLS

Abstract. A comprehensive evaluation of seasonal backward trajectories initialized in the northern hemisphere lowermost stratosphere (LMS) has been performed to investigate the factors that determine the temporal and spatial structure of troposphere-to-stratosphere-transport (TST) and it's impact on the LMS. In particular we explain the fundamental role of the transit time since last TST (tTST) for the chemical composition of the LMS. According to our results the structure of the LMS can be characterized by a layer with tTST<40 days forming a narrow band around the local tropopause. This layer extends about 30 K above the local dynamical tropopause, corresponding to the extratropical tropopause transition layer (ExTL) as identified by CO. The LMS beyond this layer shows a relatively well defined separation as marked by an aprupt transition to longer tTST indicating less frequent mixing and a smaller fraction of tropospheric air. Thus the LMS constitutes a region of two well defined regimes of tropospheric influence. These can be characterized mainly by different transport times from the troposphere and different fractions of tropospheric air. Carbon monoxide (CO) mirrors this structure of tTST due to it's finite lifetime on the order of three months. Water vapour isopleths, on the other hand, do not uniquely indicate TST and are independent of tTST, but are determined by the Lagrangian Cold Point (LCP) of air parcels. Most of the backward trajectories from the LMS experienced their LCP in the tropics and sub-tropics, and TST often occurs 20 days after trajectories have encountered their LCP. Therefore, ExTL properties deduced from CO and H2O provide totally different informations on transport and particular TST for the LMS.

Research Spotlight: Large atmospheric study investigates tropical tropopause transition layer

The tropical tropopause transition layer (TTL), starting at about 12 kilometers in altitude, is a transition layer between the tropical troposphere (the layer nearest Earth) and the stratosphere above. The TTL can influence the chemistry of the stratosphere, including the global ozone budget. In addition, understanding how water and other greenhouse gases behave in the TTL is important to understanding the greenhouse effect and global climate change.

Extratropical tropopause transition layer characteristics from high‐resolution sounding data

Accurate determination of the tropopause is important for applications such as dynamical analysis and forecasting, radiative transfer calculations, and the diagnosis of chemical transport in the atmosphere. In this paper, we examine how well the extratropical tropopause is determined in the National Centers for Environmental Prediction high‐resolution Global Forecast System (GFS) model analysis over the continental United States using high‐resolution aircraft and radiosonde data. The GFS analyses and sounding data compare well, with RMS differences of approximately 600 m, which is comparable to the vertical resolution of the model. The GFS tropopause is a good proxy in areas without in situ observations, but near the subtropical jet the GFS analysis often mistakenly identifies the secondary rather than the primary tropopause. We also explore an alternative method to identify the tropopause by fitting a smoothed step function to the static stability profile. This new approach provides a measure of the depth of the troposphere‐stratosphere transition and facilitates the study of the dynamical behavior of the tropopause region. In particular, using the transition depth, we are able to identify the statistical behavior of temperature in profiles with deep or shallow tropopause transition layers.

Convective distribution of tropospheric ozone and tracers in the Central American ITCZ region: Evidence from observations during TC4

During the Tropical Composition, Clouds and Climate Coupling (TC4) experiment that occurred in July and August of 2007, extensive sampling of active convection in the ITCZ region near Central America was performed from multiple aircraft and satellite sensors. As part of a sampling strategy designed to study cloud processes, the NASA ER‐2, WB‐57 and DC‐8 flew in stacked “racetrack patterns” in convective cells. On July 24, 2007, the ER‐2 and DC‐8 probed an actively developing storm and the DC‐8 was hit by lightning. Case studies of this flight, and of convective outflow on August 5, 2007 reveal a significant anti‐correlation between ozone and condensed cloud water content. With little variability in the boundary layer and a vertical gradient, low ozone in the upper troposphere indicates convective transport. Because of the large spatial and temporal variability in surface CO and other pollutants in this region, low ozone is a better convective indicator. Lower tropospheric tracers methyl hydrogen peroxide, total organic bromine and calcium substantiate the ozone results. OMI measurements of mean upper tropospheric ozone near convection show lower ozone in convective outflow. A mass balance estimation of the amount of convective turnover below the tropical tropopause transition layer (TTL) is 50%, with an altitude of maximum convective outflow located between 10 and 11 km, 4 km below the cirrus anvil tops. It appears that convective lofting in this region of the ITCZ is either a two‐stage or a rapid mixing process, because undiluted boundary layer air is never sampled in the convective outflow.

Transport and Mixing in the Extratropical Tropopause Region in a High-Vertical-Resolution GCM. Part I: Potential Vorticity and Heat Budget Analysis

Abstract A high-vertical-resolution general circulation model (GCM) output has been analyzed to clarify transport and mixing processes in the extratropical tropopause region. The high-resolution GCM, with a vertical resolution of about 300 m above the extratropical upper troposphere, allows simulation of fine atmospheric structures near the tropopause, such as the extratropical tropopause transition layer (ExTL) and the tropopause inversion layer (TIL). The high-resolution GCM realistically simulates fine thermal and dynamic structures in the extratropical tropopause region. The thickness and maximum stability of the simulated TIL are consistent with observations. The high-resolution output was analyzed using a zonal mean potential vorticity (PV) equation to identify dominant transport processes in the extratropical tropopause region. In the Northern Hemisphere extratropics during winter, mean downward advection sharpens the PV gradient between the tropopause and 20 K below it, whereas latitudinal variation in isentropic mixing sharpens the vertical PV gradient between the tropopause and 10 K above it. During summer, vertical mixing substantially sharpens the vertical PV gradient at 10–25 K above the tropopause. These sharpening effects may be strongly related to the formation mechanisms of strong concentration gradients of chemical tracers around the ExTL. Mechanisms of evolution of the TIL are also discussed by analyzing a thermodynamic equation. Downward heat advection and radiation processes primarily determine the seasonality of the TIL. The analysis results suggest that the locations of the TIL and the ExTL can be similar because of common dynamic processes and interactions between constituent distributions and thermal structure.

Multimodel assessment of the upper troposphere and lower stratosphere: Extratropics

A multimodel assessment of the performance of chemistry‐climate models (CCMs) in the extratropical upper troposphere/lower stratosphere (UTLS) is conducted for the first time. Process‐oriented diagnostics are used to validate dynamical and transport characteristics of 18 CCMs using meteorological analyses and aircraft and satellite observations. The main dynamical and chemical climatological characteristics of the extratropical UTLS are generally well represented by the models, despite the limited horizontal and vertical resolution. The seasonal cycle of lowermost stratospheric mass is realistic, however with a wide spread in its mean value. A tropopause inversion layer is present in most models, although the maximum in static stability is located too high above the tropopause and is somewhat too weak, as expected from limited model resolution. Similar comments apply to the extratropical tropopause transition layer. The seasonality in lower stratospheric chemical tracers is consistent with the seasonality in the Brewer‐Dobson circulation. Both vertical and meridional tracer gradients are of similar strength to those found in observations. Models that perform less well tend to use a semi‐Lagrangian transport scheme and/or have a very low resolution. Two models, and the multimodel mean, score consistently well on all diagnostics, while seven other models score well on all diagnostics except the seasonal cycle of water vapor. Only four of the models are consistently below average. The lack of tropospheric chemistry in most models limits their evaluation in the upper troposphere. Finally, the UTLS is relatively sparsely sampled by observations, limiting our ability to quantitatively evaluate many aspects of model performance.