Tropopause Transition Layer Research Articles

A global view of the extratropical tropopause transition layer from Atmospheric Chemistry Experiment Fourier Transform Spectrometer O3, H2O, and CO

The global behavior of the extratropical tropopause transition layer (ExTL) is investigated using O3, H2O, and CO measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) on Canada's SCISAT‐1 satellite obtained between February 2004 and May 2007. The ExTL depth is derived using H2O‐O3 and CO‐O3 correlations. The ExTL top derived from H2O‐O3 shows an increase from roughly 1–1.5 km above the thermal tropopause in the subtropics to 3–4 km (2.5–3.5 km) in the north (south) polar region, implying somewhat weaker troposphere‐stratosphere‐transport in the Southern Hemisphere. The ExTL bottom extends ∼1 km below the thermal tropopause, indicating a persistent stratospheric influence on the troposphere at all latitudes. The ExTL top derived from the CO‐O3 correlation is lower, at 2 km or ∼345 K (1.5 km or ∼335 K) in the Northern (Southern) Hemisphere. Its annual mean coincides with the relative temperature maximum just above the thermal tropopause. The vertical CO gradient maximizes at the thermal tropopause, indicating a local minimum in mixing within the tropopause region. The seasonal changes in and the scales of the vertical H2O gradients show a similar pattern as the static stability structure of the tropopause inversion layer (TIL), which provides observational support for the hypothesis that H2O plays a radiative role in forcing and maintaining the structure of the TIL.

Tropospheric O3 from photolysis of O2

Photolytic dissociation of molecular oxygen (O2) at wavelengths about 205 nm produces ozone (O3) in the upper tropical troposphere. In tropospheric chemistry models that ignore this process, the O3 abundance above 14 km in the tropics (a.k.a. Tropopause Transition Layer) is underestimated by 5 to 20 ppb. Even for models including O2 photolysis, uncertainty in the O2 cross sections yields similar uncertainty in TTL O3. The related impact on global atmospheric chemistry is small, i.e., ±0.2% in CO and CH4 budgets, but the change in the O3 column, ±1.6 DU in the tropics, may be important in calculating heating rates and climate forcing.

Cirrus Microphysical Properties and Air Motion Statistics Using Cloud Radar Doppler Moments. Part II: Climatology

AbstractThe algorithm described in Part I has been applied to the millimeter cloud radar observations from January 1999 to December 2005 at the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) and Tropical Western Pacific (including Manus and Nauru) sites. Approximately 10 000 cirrus hours from each of these sites were analyzed. Retrieved cloud properties including condensed mass, particle size, optical depth, and in-cloud vertical air motions were analyzed in terms of their geographical, seasonal, and diurnal variations. The analysis shows that tropical ice clouds observed by millimeter radar are very different from ice clouds at SGP, with the tropical clouds having slightly larger particle sizes and greater ice masses and being more likely to be associated with ascending air motions, in addition to being colder and higher in altitude. A positive residual of derived in-cloud air motion found in the tropical data likely provides evidence for lofting of air into the tropopause transition layer as a result of radiative heating. The midlatitude cirrus demonstrate strong seasonal variations with more frequent, thicker clouds occurring during the summer than during the winter. Very subtle seasonal variations are found for tropical ice clouds, and evidence is presented that cirrus properties vary interannually and are correlated with El Niño oscillations. In addition, it is found that tropical cirrus demonstrate a stronger diurnal cycle than cirrus of the midlatitudes, with the in-cloud updrafts peaking in the early afternoon.

A set of diagnostics for evaluating chemistry‐climate models in the extratropical tropopause region

Three related diagnostics are used to evaluate the representation of chemical transport processes in the extratropical upper troposphere and lower stratosphere (UTLS) by chemistry‐transport and chemistry‐climate models. The diagnostics are based on in situ observations of ozone, carbon monoxide, water vapor profiles (obtained on board the NASA ER‐2 research aircraft, near 65°N and during 1997), and their interrelationships in the UTLS. The first diagnostic compares the observed and modeled UTLS trace gas profiles in a relative altitude coordinate. The second one compares the observed and modeled UTLS tracer relationships. The third one compares the observed and modeled thickness of the tropopause transition layer. Together, they characterize the model's ability to reproduce the observed chemical distribution in the UTLS region and chemical transition across the extratropical tropopause. These are key indicators of whether the contributions of dynamics and chemistry to this region are correctly represented in the models. These diagnostics are used to evaluate the performance of an NCAR chemistry‐transport model (CTM), MOZART‐3, and a chemistry‐climate model (CCM), WACCM3. Results from four model runs with different meteorological fields and grid resolution are examined. Overall, the NCAR models show qualitative agreement with the observations in the location of the chemical transition across the extratropical tropopause. Quantitatively, there are significant differences between the modeled and the observed chemical distributions. Both the meteorological field and grid resolutions are contributing factors to the differences.

An entropy-based measure of mixing at the tropopause

Qualitative analysis of tracer correlations is now a well established technique for investigating mixing in the tropopause transition layer. Generally, these studies rely on in situ data retrieved over short distances and time-scales. Using satellite data from HALOE, we introduce a simple quantitative analysis technique, based on entropy, to provide a climatology of mixing, from H2O:O3 tracer correlations. Results compare well with previous studies using Lagrangian-based estimates of effective diffusivity and equivalent lengths and also climatologies of tropopause folding. Further analysis techniques reveal characteristics of the transition layer, including its depth and position, leading to the distinction of three primary forms of mixing. Within our analysis domain, we estimate that half of total entropy can be attributed to subtropical ‘Rossby-driven’ tropopause folding events. The majority of the remaining entropy is associated with tropical convective mixing and a third form, possibly linked to shear-induced mixing at subtropical jet streaks. Copyright © 2006 Royal Meteorological Society

Convective formation of pileus cloud near the tropopause

Abstract. Pileus clouds form where humid, vertically stratified air is mechanically displaced ahead of rising convection. This paper describes convective formation of pileus cloud in the tropopause transition layer (TTL), and explores a possible link to the formation of long-lasting cirrus at cold temperatures. The study examines in detail in-situ measurements from off the coast of Honduras during the July 2002 CRYSTAL-FACE experiment that showed an example of TTL cirrus associated with, and penetrated by, deep convection. The TTL cirrus was enriched with total water compared to its surroundings, but was composed of extremely small ice crystals with effective radii between 2 and 4 μm. Through gravity wave analysis, and intercomparison of measured and simulated cloud microphysics, it is argued that the TTL cirrus originated neither from convectively-forced gravity wave motions nor environmental mixing alone. Rather, it is hypothesized that a combination of these two processes was involved in which, first, a pulse of convection forced pileus cloud to form from TTL air; second, the pileus layer was punctured by the convective pulse and received larger ice crystals through interfacial mixing; third, the addition of this condensate inhibited evaporation of the original pileus ice crystals where a convectively forced gravity wave entered its warm phase; fourth, through successive pulses of convection, a sheet of TTL cirrus formed. While the general incidence and longevity of pileus cloud remains unknown, in-situ measurements, and satellite-based Microwave Limb Sounder retrievals, suggest that much of the tropical TTL is sufficiently humid to be susceptible to its formation. Where these clouds form and persist, there is potential for an irreversible repartition from water vapor to ice at cold temperatures.

Seasonal cycles and variability of O<sub>3</sub> and H<sub>2</sub>O in the UT/LMS during SPURT

Abstract. Airborne high resolution in situ measurements of a large set of trace gases including ozone (O3) and total water (H2O) in the upper troposphere and the lowermost stratosphere (UT/LMS) have been performed above Europe within the SPURT project. SPURT provides an extensive data coverage of the UT/LMS in each season within the time period between November 2001 and July 2003. In the LMS a distinct spring maximum and autumn minimum is observed in O3, whereas its annual cycle in the UT is shifted by 2–3 months later towards the end of the year. The more variable H2O measurements reveal a maximum during summer and a minimum during autumn/winter with no phase shift between the two atmospheric compartments. For a comprehensive insight into trace gas composition and variability in the UT/LMS several statistical methods are applied using chemical, thermal and dynamical vertical coordinates. In particular, 2-dimensional probability distribution functions serve as a tool to transform localised aircraft data to a more comprehensive view of the probed atmospheric region. It appears that both trace gases, O3 and H2O, reveal the most compact arrangement and are best correlated in the view of potential vorticity (PV) and distance to the local tropopause, indicating an advanced mixing state on these surfaces. Thus, strong gradients of PV seem to act as a transport barrier both in the vertical and the horizontal direction. The alignment of trace gas isopleths reflects the existence of a year-round extra-tropical tropopause transition layer. The SPURT measurements reveal that this layer is mainly affected by stratospheric air during winter/spring and by tropospheric air during autumn/summer. Normalised mixing entropy values for O3 and H2O in the LMS appear to be maximal during spring and summer, respectively, indicating highest variability of these trace gases during the respective seasons.

Temperature Fields in the Tropical Tropopause Transition Layer

Abstract There has been increasing recognition of the role of the tropical tropopause layer (TTL) in determining stratospheric water vapor concentrations—the TTL being a layer of transition between air showing tropospheric properties below and stratospheric properties above. This study investigates the spatial structure of temperatures in the TTL. A dehydration index based on the atmospheric region with temperatures colder than a specific reference temperature was defined to examine the TTL temperature structure and possible influences on stratospheric water vapor. The results indicate that dehydration regions with cold temperatures (e.g., <190 K) occur mainly over the western Pacific and are about 1.5–2.0 km in depth during Northern Hemisphere winter. The dehydration index is mainly dependent on the annual cycle of the TTL temperatures, but is strongly affected by interannual variations associated with the quasi-biennial oscillation (QBO) and El Nino–Southern Oscillation (ENSO). Dehydration regions with ...

The Implications of Tropical Rossby Waves for Tropical Tropopause Cirrus Formation and for the Equatorial Upwelling of the Brewer–Dobson Circulation

This study puts forward a mechanism for the observed upwelling in the tropical upper troposphere and lower stratosphere. In this hypothesis, the tropical upwelling is driven by momentum transport by Rossby waves that are generated by tropical convection. To test this hypothesis, model runs are conducted using an axisymmetric, global, primitive equation model. In these runs, the effect of Rossby waves is included by driving the model with observed fields of large-scale eddy momentum flux convergence. The resulting overturning circulation includes both meridional flow from the intertropical convergence zone (ITCZ) to the equator and rising motion in the tropical tropopause transition layer (TTL). This circulation therefore helps to explain the transport of moisture from the lower portion of the TTL in the ITCZ to the equatorial cold-point tropopause, where tropopause cirrus layers frequently occur.