Stratosphere-troposphere Exchange Research Articles

Stratosphere-Troposphere Exchange and Its Impact on the Structure of the Lower Stratosphere.

Cross tropopause air mass flux plays an important role for the exchange of minor constituents between the troposphere and stratosphere. In general, one can assume net upward air mass fluxes in the tropics and net downward flow in the extratropics. Yet the detailed picture exhibits coexisting up- and downgoing meso-scale flows in the tropopause region probably occurring at all latitudes. This feature of stratosphere-troposphere exchange is particularly addressed in the paper with emphasis on the extratropics. Tropopause depressions like folds and cut-off lows are meso-scale features heavily contributing to downward transport at middle and high latitudes. Statistics derived from global analyses for the period 12/1983 to 11/1991 reveal that folding and cut-off low activity is about twice as strong in the northern hemisphere than in the southern one. A case study of a folding event overthe North Atlantic using a mesoscale meteorological model shows that downward cross-tropopause fluxes are partly compensated by upward fluxes. The ratio of up- and downgoing air mass is about 2:3 during the simulated episode. Note that the upward flux is not a mere return flow. It is expected that this air mass exchange leads to structural and chemical modifications of the lowest stratosphere.

Estimates of stratosphere‐troposphere exchange: Sensitivity to model formulation and horizontal resolution

The vertical transport between the stratosphere and the troposphere is studied by evaluating a matrix that gives the amount of air transferred between the different model layers of a three‐dimensional off‐line transport model coupled to European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses. The latitudinal structure of vertical exchange reflects the Hadley circulation with upward transport in the tropics and with downward transport at mid‐latitudes. Maxima in downward transport occur near the subtropical and polar jet streams. The seasonal cycle of downward transport and the asymmetry between the two hemispheres are at least qualitatively represented by the model. However, it is found that the evaluated total amount of air transferred from the stratosphere to the troposphere per month is sensitive to the chosen horizontal model resolution. Refinement of this resolution from 8° × 10° to 4° × 5° leads to almost a doubling of the computed downward transport. Also, the vertical transport is found to depend strongly on changes in the formulation of the parent model (ECMWF). It is therefore important to use internally consistent meteorological input data when performing multiyear simulations with off‐line chemistry‐transport models.

A reinterpretation of the data from the NASA Stratosphere‐Troposphere Exchange Project

Data obtained during the NASA Stratosphere Troposphere Exchange Project (STEP) Mid‐Latitude Field Experiment displayed laminae of ozone, water, and condensation nuclei in the stratosphere in association with a mid‐latitude stratosphere‐troposphere folding event. Danielsen et al. (1991) constructed cross sections of these observations, and interpreted these quasi‐horizontal laminae as evidence of ultra‐low frequency gravity waves. We use a new technique to show that these laminae could have resulted from differential advection, rather than transport by ultra‐low frequency gravity waves. This new technique uses reverse domain filling back trajectories on multiple isentropic surfaces in conjunction with modified potential vorticity to reveal the qualitative details of the constituent laminae.

Evidence of meridional motion in the summer lower stratosphere adjacent to monsoon regions

Twenty‐one years of rawinsonde data were used together with 8 years of uninitialized European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses to describe the climatological structure of large‐scale circulations adjacent to monsoon regions in northern and southern hemisphere summers. In the upper troposphere and lower stratosphere, equatorward and poleward motions occur on the eastern and western sides of monsoon regions, respectively. It is shown that significant meridional velocities (>1 ms−1) penetrate the lower stratosphere up to a maximum height of 50–30 mbar. Largest meridional velocities are observed in connection with the Asian monsoon in northern summer. Although evanescent in height, these motions are relatively important for horizontal transport of constituents in the summer lower stratosphere, when planetary waves are otherwise small. Asian and Mexican monsoons in this season are displaced sufficiently far from the equator, in close proximity to the tropopause break, to have a significant role in stratosphere‐troposphere (S/T) exchange. The companion paper by Chen (1995) provides evidence of irreversible S/T exchange in the “upper middle world” during northern summer.

Aerosols used as tracers for stratosphere-troposphere exchange in the arctic

Accumulation range particles as well as ozone and meteorological parameters were measured, in situ and on-line, during the 1991/1992 EASOE (European Arctic Stratospheric Ozone Experiment) aircraft measurement campaigns. Some interesting and significant findings relate to one flight in 31 January, 1992 from Kiruna (67°48′N, 20°18′E) to Spitsbergen (79°00′N 30°13′E) and back. At 390 mb the aircraft encountered stratospheric air, characterised by strong increases in concentrations of accumulation mode particles and ozone. The concentrations of particles with diameters between 0.31 and 0.79 μm were, moreover, highly correlated with ozone mixing ratios ( R = 0.95). In the regions where stratospheric air was sampled the particle number size distribution showed a predominant mode centred at around 0.5 μm diameter. This mode was the signature of the stratospheric aerosol transported down into the upper troposphere where the measurements were conducted. There are indications that the measured stratospheric air originated from a folded tropopause. An approach to study the mixing between the intrusions of stratospheric air and the free tropospheric air has been used to quantify mixing coefficients (fractions of stratospheric air) along the flight route and absolute levels of particles and ozone in the lower stratosphere. This approach can be applied to study the dilution process during stratosphere-troposphere exchange. The requirement is that at least two conservative stratospheric tracers are measured simultaneously.

Gravity wave momentum flux in the lower stratosphere over convection

This work describes a method for estimating vertical fluxes of horizontal momentum carried by short horizontal scale gravity waves (λx = 10–100 km) using aircraft measured winds in the lower stratosphere. We utilize the in situ wind vector and pressure altitude measurements provided by the Meteorological Measurement System (MMS) on board the ER‐2 aircraft to compute the momentum flux vectors at the flight level above deep convection during the tropical experiment of the Stratosphere Troposphere Exchange Project (STEP‐Tropical). Data from Flight 9 are presented here for illustration. The vertical flux of horizontal momentum in these observations points in opposite directions on either side of the location of a strong convective updraft in the cloud shield. This property of internal gravity waves propagating from a central source compares favorably with previously described model results.

The next generation of infrared sounders for stratospheric research

Infrared remote sounding is a well-established technique for measuring the temperature structure and composition of the atmosphere from space. This article discusses some recent progress, and reviews some of the new and even more advanced sensors currently under development. Major progress is expected in the next ten years particularly in the areas of stratospheric-tropospheric exchange, near-surface tropospheric pollution monitoring, and high spatial resolution studies of the coupling between dynamics and chemistry in the stratosphere.

Diabatic heating in an axisymmetric cut-off cyclone and related stratosphere-troposphere exchange

Diabatic heating in an axisymmetric cut-off cyclone and related stratosphere-troposphere exchange

The role of specific humidity fields in the diagnosis of stratosphere troposphere exchange

METEOSAT 4 satellite observations of water vapour show evidence of a rich spectrum of fine scale structure that may be indicative of the transfer of dry, ozone‐rich air from the stratosphere deep into the troposphere [Appenzeller and Davies, 1992]. The extent to which air within stratospheric intrusions is irreversibly mixed and remains in the troposphere is of considerable current interest, particularly in respect of pollution effects of increasing numbers of subsonic and supersonic aircraft flights that traverse the lower stratosphere. Here, we diagnose potential vorticity and the corresponding distributions of specific humidity from a numerical weather forecast model and show that the latter field displays many of the characteristics of the Meteosat data. Specific humidity fields on both a pressure surface which corresponds approximately to the Meteosat data sampling and on an isentropic surface are compared. We show the presence of streamer‐like features in specific humidity that are not evident in the PV fields. We also note the prediliction for the streamers to break off, be advected by the flow and to subsequently become ‘attached’ to other features. Care is therefore required in the interpretation of the Meteosat data for stratosphere troposphere exchange studies. Nevertheless, we suggest that the importance of specific humidity as an indicator of the dynamical evolution of upper level atmospheric flow has previously been overlooked.

Characteristics of stratosphere‐troposphere exchange in a general circulation model

Air and trace gases are exchanged between the stratosphere and the troposphere on a variety of scales; but general circulation models (GCMs) are unable to represent the smaller scales. It would be useful to see how a GCM represents stratosphere‐troposphere exchange (STE), both to identify possible model deficiencies which would affect other studies and to see how important the smaller‐scale physics might be in the atmosphere itself. Our understanding of observed STE depends largely on inferences from tracer distributions. In this study we examine mass exchange, water vapor exchange, and the behavior of idealized tracers and parcels to diagnose STE in the National Center for Atmospheric Research GCM, the Community Climate Model (CCM2). The CCM2 correctly represents the seasonally of mass exchange across 100 hPa, but values are uniformly too strong. Water vapor, however, indicates that tropical STE is not well represented in the CCM2; even though mean tropopause temperatures are colder than observed, the lower stratosphere is too moist. Most net mass flux occurs at water vapor mixing ratios of about 4–5 parts per million by volume (ppmv), about 1 ppmv too moist. Vertical resolution has little impact on the nature of tropical STE. In midlatitudes, CCM2 more successfully represents STE, which occurs in developing baroclinic waves and stationary anticyclones. Exchange from troposphere to stratosphere does occur but only influences the lowest few kilometers of the extra‐tropical stratosphere, even for tracers with large vertical gradients.

Cross-Tropopause Mass Exchange and Potential Vorticity Budget in a Simulated Tropopause Folding

Abstract To study stratosphere–troposphere exchange, an approach based on the nonconservation of potential vorticity (PV) is developed; this approach arises naturally if one defines the tropopause in terms of PV. The evolution of a tropopause fold simulated by a mesoscale model is studied, as well as the evolution of PV at the tropopause level. The PV framework also permits the identification of the physical processes responsible for the cross-tropopause exchange as either diffusive or diabatic. In this model simulation, the diabatic processes are found to be the most important in the exchange. In particular, it is found that the negative heating gradient in the region of the warm sector of the surface cyclone is responsible for most of the diabatic exchange across the tropopause. The mass exchange during the tropopause folding event is estimated to be around 4.9 × 1014 kg in four days over the domain considered (1600 × 2000 km). This number is shown to correspond to the net difference between exchange fr...

Recent lidar developments to monitor stratosphere—troposphere exchange

The simultaneous knowledge of temperature fluctuations and mean wind profiles allows the complete description of the gravity wave field. Such data have been obtained in the altitude region 30–80 km in the past few years by using the temperature from the Rayleigh lidar and the wind either from falling spheres or from the Doppler lidar. A newly developed rotational Raman lidar and the downward extension of the Doppler lidar method make available simultaneous temperature and wind measurements in the lower height range 0–30 km independently on the presence of aerosols or thin clouds. The continuous monitoring for periods of about 10 h duration would give access not only to the mean values but also to the temporal evolution of the temperature fluctuations characteristic of the gravity waves, and the dependence upon the mean wind through which the waves propagate. This paper presents the first results obtained by these two newly available techniques.

Effect of lightning on the concentration of odd nitrogen species in the lower stratosphere: An update

Revised model estimates of the effect of lightning on the lower stratospheric NOy are presented. Several changes in the Atmospheric and Environmental Research Incorporated model were made since the last evaluation of the impact of lightning. Improvements were made in the model circulation and location of tropopause in the tropics, which is now calculated from the National Meteorological Center temperature data. Changes in model circulation reduced the mass flux from the troposphere to the tropical stratosphere. The calculated mass fluxes are found to agree better with some recent estimates. The circulation changes also reduced the advective mass flux from the tropical lower stratosphere to the midlatitudes. The change in circulation and the change in the tropopause height lead to increases in the calculated concentration of N2O, O3 and NOy in the tropical lower stratosphere. The effect of lightning is to increase the calculated concentration of NOy around 64 mbar by a factor of 2, compared to a factor of 10 enhancement in the previous calculations. Comparison with the Stratosphere Troposphere Exchange Project 1987 data indicates that the inclusion of a ligntning source brings the model results in closer agreement with the observations.

Radiosonde Penetration of an Undilute Cumulonimbus Anvil

Abstract An example is presented of the serendipitous radiosonde penetration through the western edge of a rapidly growing undilute cumulonimbus anvil above 200 mb by an operationally released radiosonde balloon. The sounding is supportive of deep convection and contains a stable layer (13°C potential temperature increase) from 210 to 185 mb with a quasi-adiabatic mixed layer from there to 135 mb. The 185–135-mb layer has a wet-bulb potential temperature of 24°C which agrees to within 1°C of the subcloud-layer wet-bulb potential temperature. The wind perturbation of approximately 30 m s−1 within the mixed layer is larger than, but consistent with, relative outflow velocities estimated from satellite imagery and density current theory. Downstream soundings through decaying anvil debris 12 h later still show evidence of the initial convective thermal perturbation even as the tropopause attempts to reform in the vicinity of 200 mb. Implications for the tropospheric-stratospheric exchange of water vapor and t...

Gravity waves generated by a tropical cyclone during the STEP tropical field program: A case study

Overflights of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere‐Troposphere Exchange Project (STEP) show the presence of two mesoscale phenomena: a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclone's entire cloud shield (wavelength of 250 km or greater). The larger feature is fairly steady, though its physical interpretation is ambiguous. The 110‐km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is comparable to that of 100‐to 150‐km‐diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield; these cells have lifetimes of 4.5 to 6 hours. Aircraft flights through the anvil show that these cells correspond to regions of enhanced convection, higher cloud altitude, and upwardly displaced potential temperature surfaces. A three‐dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variables associated with the 110‐km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400–600 m, a lifetime of 4.5–6 hours and a size comparable to the cells of low brightness temperature. The forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity waves is definitely important to the westerly phase of the stratopause semiannual oscillation and possibly important to the easterly phase of the quasi‐biennial oscillation. There is strong evidence that some of the mesoscale gravity waves break below 20 km as well. The effect of this wave breaking on the diabatic circulation below 20 km may be comparable to that of above‐cloud diabatic cooling.

The potential for stratosphere-troposphere exchange in cut-off-low systems

The potential for stratosphere-troposphere exchange in cut-off-low systems

The potential for stratosphere‐troposphere exchange in cut‐off‐low systems

AbstractThree mechanisms are discussed associated with cut‐off‐low (COL) systems which can transfer stratospheric air into the troposphere. These mechanisms are: (i) convective erosion of the tropopause, (ii) erosion of the tropopause by turbulence associated with a jet stream, and (iii) tropopause folding around the flank of a COL. Three case studies are presented which examine the first two of these processes and provide evidence that stratosphere‐troposphere exchange (STE) had occurred. Furthermore, results suggest that periodic reintensification of some COLs cause all these processes to recur or be prolonged. Their relative importance to STE is discussed and an estimate for that occurring by mechanism (i) is given for one of the case studies. Statistical data on annual COL numbers are used to calculate and annual mean transfer of ozone by this mechanism, which is found to be no more than 4% of the annual transfer for the northern hemisphere. This result suggests that convective STE in COLs is a minor contributor to the northern hemisphere STE.

The atmospheric CH<sub>4</sub> increase since the Last Glacial Maximum: (2) Interactions with oxidants

Two studies of the effect of changing CH4 fluxes on global tropospheric oxidant levels, O3, OH, H2O2, have been performed with a multi-box photochemical model. (1) A sensitivity study is made by scaling back CH4, CO and NO emissions relative to present-day budgets. When the CH4 ice core record is compared to calculated CH4 abundances, corresponding CH4 fluxes for the pre-industrial Holocene (PIH) and Last Glacial Maximum (LGM) are fairly well-constrained: 175–225 Tg CH4/yr for PIH and 100–130 Tg CH4/yr for LGM. Except for OH at fluxes of 100–200 Tg CH4/yr, preindustrial oxidant concentrations levels are not narrowly defined by the CH4 record. The small range of CH4 flux and OH abundance at the LGM is due to strong CH4–OH feedbacks. (2) Specific scenarios for CH4/CO/NO are selected to represent sources for the PIH and LGM. The CH4 budget is taken from an evaluation of wetlands and other natural sources. For CO and NO, apparent O3 levels and ice-core-derived H2O2 for the PIH are used to constrain PIH CO and NO fluxes. These fluxes are further scaled back to simulate the LGM, and perturbed temperature, precipitation and stratosphere-troposphere exchange are prescribed according to the GISS GCM. For the PIH changes for global abundance relative to present day are: 44% less O3; 20% more OH and 56% less H2O2. For the LGM, with 120 Tg CH4/yr and surface temperature 4–5 K lower than today, global changes are 56% less O3, 32% more OH and 59% less H2O2. Calculated preindustrial oxidant changes are in reasonable agreement with other studies based on one-, two- and three-dimensional models, although differences among model physics preclude a definitive comparison. There is consensus that OH has decreased since the Last Glacial Maximum, in contrast to projections for future OH, on which models are in disagreement. Model validation of oxidant concentrations requires more ice-core data. Preindustrial OH would be inferred best by using a model with ice-core measurements of species that are uncoupled from the CH4–CO—OH—O3 cycle. Candidates might be marine gases of moderately long lifetime, e.g., CH3Cl or OCS.

A Global Analysis of Stratospheric–Tropospheric Exchange during Northern Winter

The global and the regional distributions of the time-averaged stratospheric-tropospheric (ST) mass exchange were analyzed. Using a mathematical formulation of ST exchange, the cross-tropopause mass flux was diagnosed globally for January 1979, and contributions by physical mechanisms including the diabatic transport and the quasi-horizontal adiabatic transport along isentropes that intersect the tropopause surface were evaluated. Two regions of mass flux zonally integrated into the stratosphere were found: one over tropical latitudes associated with diabatic transports, and a second one over subpolar latitudes associated with adiabatic transport. The results are consistent with independent analyses of trace atmospheric constituents.

Tropospheric ozone annual variation and possible troposphere-stratosphere coupling in the Arctic and Antarctic as derived from ozone soundings at Resolute and Amundsen-Scott stations

The tropospheric ozone annual variation in the northern and southern polar regions is analyzed from ozone sounding data obtained at Resolute (74.41°N, 94.59°W) during a 15-year period (1974–1988) and Amundsen-Scott (South Pole) during a 7-year period (1967–1971, 1986–1987). Tropospheric ozone is always less abundant in the southern than in the northern polar region. The difference is greatest in spring in the tropopause layer where the Arctic ozone mixing ratio can be 5 × as large as the mixing ratio in Antarctica. The phase of ozone annual variation above Resolute changes (increases) gradually from the stratosphere across the tropopause to the middle troposphere. Unlike this, the phase of the Antarctic ozone annual harmonic has a discontinuity in the layer of the changing tropopause level, so that the annual harmonic in the upper troposphere, lower stratosphere is 4-to-5 months out of phase (earlier) to that above and beneath. Above both the Arctic and Antarctic stations, the ozone mixing ratio and its vertical gradient evolve in a similar manner in the wide layer from the lower stratosphere to the middle troposphere. This likely points out that ozone in this layer is controlled from above. An indication of the stratospheric-tropospheric ozone exchange above Resolute is noted from mid-winter to spring. The analysis of columnar tropospheric ozone changes gives an lower estimate of the cross-tropopause ozone flux up to 5. 10 10 mol cm -2 s -1 . Above the South Pole, the cross-tropopause ozone flux is not usually large. However, in winter, when the tropopause is not strongly pronounced, it can rise to 4.5. 10 10 mol cm -2 s -1 , but does not penetrate into the middle troposphere. There is also some evidence that early in the spring, when the stratospheric ozone “hole” is developed, the stratospheric-tropospheric exchange conducts the influence of the “hole” into the upper troposphere, where the integrated ozone destruction is estimated to be 8. 10 10 mol cm -2 s -1 . Correlation analysis gives no ozone-tropopause correlation in the Antarctic in winter, while in other seasons as well as during all seasons in the Arctic, there are negative correlation peaks just above the tropopause. DOI: 10.1034/j.1600-0889.1993.t01-1-00001.x