Thomas Birner

THE EXTRATROPICAL UPPER TROPOSPHERE AND LOWER STRATOSPHERE

[1]xa0The extratropical upper troposphere and lower stratosphere (Ex-UTLS) is a transition region between the stratosphere and the troposphere. The Ex-UTLS includes the tropopause, a strong static stability gradient and dynamic barrier to transport. The barrier is reflected in tracer profiles. This region exhibits complex dynamical, radiative, and chemical characteristics that place stringent spatial and temporal requirements on observing and modeling systems. The Ex-UTLS couples the stratosphere to the troposphere through chemical constituent transport (of, e.g., ozone), by dynamically linking the stratospheric circulation with tropospheric wave patterns, and via radiative processes tied to optically thick clouds and clear-sky gradients of radiatively active gases. A comprehensive picture of the Ex-UTLS is presented that brings together different definitions of the tropopause, focusing on observed dynamical and chemical structure and their coupling. This integral view recognizes that thermal gradients and dynamic barriers are necessarily linked, that these barriers inhibit mixing and give rise to specific trace gas distributions, and that there are radiative feedbacks that help maintain this structure. The impacts of 21st century anthropogenic changes to the atmosphere due to ozone recovery and climate change will be felt in the Ex-UTLS, and recent simulations of these effects are summarized and placed in context.

Fine-scale structure of the extratropical tropopause region

[1]xa0A vertically high resolved climatology of the thermal and wind structure of the extratropical tropopause region is presented. The climatology is based on data from 80 U.S. radiosonde stations covering the period 1998–2002. Time averages for each radiosonde station are computed using the tropopause as a common reference level for all vertical profiles within the mean. A strong inversion at the tropopause in the mean vertical temperature gradient is uncovered; that is, temperature strongly increases with altitude within the lowermost stratosphere. This tropopause inversion layer exists on average throughout the investigated extratropics (about 30°N to 70°N). Accordingly, the static stability parameter shows considerably enhanced values within the lowermost extratropical stratosphere compared to typical extratropical stratospheric values further aloft. Conventional averages are not able to capture the tropopause inversion layer. Mean profiles of the horizontal wind show behavior qualitatively corresponding to thermal wind balance. Winter and summer exhibit distinctly different climate states in the extratropical tropopause region. An approximated potential vorticity is considered and found to be close to well mixed within the troposphere as well as within the tropopause inversion layer. This suggests the view of the tropopause inversion layer as representing a dynamically active atmospheric layer. Some potential implications are discussed.

Multimodel assessment of the upper troposphere and lower stratosphere: Tropics and global trends

The performance of 18 coupled Chemistry Climate Models (CCMs) in the Tropical Tropopause Layer (TTL) is evaluated using qualitative and quantitative diagnostics. Trends in tropopause quantities in the tropics and the extratropical Upper Troposphere and Lower Stratosphere (UTLS) are analyzed. A quantitative grading methodology for evaluating CCMs is extended to include variability and used to develop four different grades for tropical tropopause temperature and pressure, water vapor and ozone. Four of the 18 models and the multi‐model mean meet quantitative and qualitative standards for reproducing key processes in the TTL. Several diagnostics are performed on a subset of the models analyzing the Tropopause Inversion Layer (TIL), Lagrangian cold point and TTL transit time. Historical decreases in tropical tropopause pressure and decreases in water vapor are simulated, lending confidence to future projections. The models simulate continued decreases in tropopause pressure in the 21st century, along with ∼1K increases per century in cold point tropopause temperature and 0.5–1 ppmv per century increases in water vapor above the tropical tropopause. TTL water vapor increases below the cold point. In two models, these trends are associated with 35% increases in TTL cloud fraction. These changes indicate significant perturbations to TTL processes, specifically to deep convective heating and humidity transport. Ozone in the extratropical lowermost stratosphere has significant and hemispheric asymmetric trends. O3 is projected to increase by nearly 30% due to ozone recovery in the Southern Hemisphere (SH) and due to enhancements in the stratospheric circulation. These UTLS ozone trends may have significant effects in the TTL and the troposphere.

How sharp is the tropopause at midlatitudes

[1] Ten years of high-resolution radiosonde data are contrasted with fifteen years of ECMWF reanalysis (ERA) data to explore the tropopause region above two midlatitude stations (Munich and Stuttgart) in Southern Germany. We present time-averaged vertical profiles of several meteorological parameters relative to the tropopause. A strong mean inversion at the tropopause is evident from the radiosonde profiles with a vertical extension of about 2 km and a temperature increase of about 4 K. The impact of the tropopause definition on the strength of this inversion is discussed as well as the relevance of baroclinic eddies in forming it. The climatological profiles for Munich and Stuttgart do not differ significantly. INDEX TERMS: 3362 Meteorology and Atmospheric Dynamics: Stratosphere/troposphere interactions; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3319 Meteorology and Atmospheric Dynamics: General circulation; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3364 Meteorology and Atmospheric Dynamics: Synoptic-scale meteorology

Evidence for inertia gravity waves forming polar stratospheric clouds over Scandinavia

[1]xa0At three successive days at the end of January 2000 the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) airborne lidar Ozone Lidar Experiment explored mountain-wave-induced polar stratospheric clouds above the Scandinavian mountain ridge. Global analyses and mesoscale modeling are applied to explain their complex internal structure and their day-to-day variability. Depending on the synoptical-scale meteorological conditions, stratospheric temperature anomalies of different amplitude and horizontal extent are generated by the upward propagating mountain waves. Short-term excitation of about 6 hours resulted in localized stratospheric temperature anomalies directly above the mountain ridge as for 25 January 2000. In this case, the elevation of the observed clouds differed not much from the synoptic-scale clouds upstream above the Norwegian Sea. On the other hand, long-lasting flow past the Scandinavian mountain ridge formed huge 400-km horizontally extending stratospheric ice clouds in altitudes as much as 5 km above the elevation of the upstream clouds just 1 day later. Inertia gravity waves with horizontal wavelengths of about 350 km are responsible for their formation. For the first time a predicted temperature minimum far downstream of the mountains could be proofed by the observation of an isolated stratospheric ice cloud above Finland. The observed particles are classified in terms of their measured optical properties such as backscatter ratio and depolarization. In all cases, mountain waves generated ice clouds. In contrast to the nitric acid trihydrate tail of the ice cloud on 25 January the same classification results in a tail of liquid supercooled ternary solutions droplets 1 day later. The particle structure downstream of the mountains is very complex and needs detailed microphyical modeling and interpretation.

Defining Sudden Stratospheric Warmings

AbstractSudden stratospheric warmings (SSWs) are large, rapid temperature rises in the winter polar stratosphere, occurring predominantly in the Northern Hemisphere. Major SSWs are also associated with a reversal of the climatological westerly zonal-mean zonal winds. Circulation anomalies associated with SSWs can descend into the troposphere with substantial surface weather impacts, such as wintertime extreme cold air outbreaks. After their discovery in 1952, SSWs were classified by the World Meteorological Organization. An examination of literature suggests that a single, original reference for an exact definition of SSWs is elusive, but in many references a definition involves the reversal of the meridional temperature gradient and, for major warmings, the reversal of the zonal circulation poleward of 60° latitude at 10 hPa.Though versions of this definition are still commonly used to detect SSWs, the details of the definition and its implementation remain ambiguous. In addition, other SSW definitions h...

Residual Circulation and Tropopause Structure

Abstract The effect of large-scale dynamics as represented by the residual mean meridional circulation in the transformed Eulerian sense, in particular its stratospheric part, on lower stratospheric static stability and tropopause structure is studied using a comprehensive chemistry–climate model (CCM), reanalysis data, and simple idealized modeling. Dynamical forcing of static stability as associated with the vertical structure of the residual circulation results in a dominant dipole forcing structure with negative static stability forcing just below the tropopause and positive static stability forcing just above the tropopause. This dipole forcing structure effectively sharpens the tropopause, especially during winter. Furthermore, the strong positive lowermost stratospheric static stability forcing causes a layer of strongly enhanced static stability just above the extratropical tropopause—a tropopause inversion layer (TIL)—especially in the winter midlatitudes. The strong positive static stability for...

A Global Survey of Static Stability in the Stratosphere and Upper Troposphere

Abstract Static stability is a fundamental dynamical quantity that measures the vertical temperature stratification of the atmosphere. However, the magnitude and structure of finescale features in this field are difficult to discern in temperature data with low vertical resolution. In this study, the authors apply more than six years of high vertical resolution global positioning system radio occultation temperature profiles to document the long-term mean structure and variability of the global static stability field in the stratosphere and upper troposphere. The most pronounced feature in the long-term mean static stability field is the well-known transition from low values in the troposphere to high values in the stratosphere. Superposed on this general structure are a series of finer-scale features: a minimum in static stability in the tropical upper troposphere, a broad band of high static stability in the tropical stratosphere, increases in static stability within the core of the stratospheric polar ...

Recent widening of the tropical belt from global tropopause statistics: Sensitivities

[1]xa0Several recent studies have shown evidence for a widening of the tropical belt over the past few decades. One line of evidence uses statistics of the tropopause height to distinguish between tropics and extratropics and defines tropical edge latitudes as those latitudes at which the number of days per year with tropopause heights greater than 15 km exceeds a certain threshold (typically 200 days/yr). This definition involves two somewhat arbitrary thresholds. Here the sensitivity of the resulting widening trend of the tropical belt to these thresholds is investigated using four different reanalysis data sets. Widening trends are found to be particularly sensitive to changes in the tropopause height threshold. Ways to objectively determine appropriate thresholds to define tropical edge latitudes based on tropopause statistics are presented. Trend estimates for the width of the tropical belt from different reanalysis data sets are found to be mostly inconsistent with each other despite consistent seasonal and interannual variations.

The tropopause inversion layer in models and analyses

Recent high-resolution radiosonde climatologies have revealed a tropopause inversion layer (TIL) in the extratropics: temperature strongly increases just above a sharp local cold point tropopause. Here, it is asked to what extent a TIL exists in current general circulation models (GCMs) and meteorological analyses. Only a weak hint of a TIL exists in NCEP/NCAR reanalysis data. In contrast, the Canadian Middle Atmosphere Model (CMAM), a comprehensive GCM, exhibits a TIL of realistic strength. However, in data assimilation mode CMAM exhibits a much weaker TIL, especially in the Southern Hemisphere where only coarse satellite data are available. The discrepancy between the analyses and the GCM is thus hypothesized to be mainly due to data assimilation acting to smooth the observed strong curvature in temperature around the tropopause. This is confirmed in the reanalysis where the stratification around the tropopause exhibits a strong discontinuity at the start of the satellite era.