Observations of barriers to mixing in the stratosphere (original) (raw)
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Journal of Geophysical Research, 2002
1] A high-resolution three-dimensional off-line chemical transport simulation has been performed with the SLIMCAT model to examine transport and mixing of ozone depleted air in the lower stratosphere on breakup of the polar vortex in spring/summer 2000. The model included ozone, N 2 O, and F11 tracers and used simplified chemistry parameterizations. The model was forced by T106 European Centre for Medium-Range Weather Forecasts analyses. The model results show that, by the end of June, above 420 K, much of the ozonedepleted air is transported from polar regions to the subtropics. In contrast, below 420 K, most of the ozone-depleted air remains poleward of approximately 55°N. It is suggested that the influence of the upper extension of the tropospheric subtropical jet provides a transport barrier at lower levels, while strong stirring on breakup of the polar vortex is important at upper levels. The mean meridional circulation modifies the distribution of ozone-depleted air by moving it up the subtropics and down in the extratropics. The model simulation is validated by comparing vertical profiles of ozone loss against ozonesonde measurements. The model results are consistent with many of the features present in the ozonesonde measurements. F11-N 2 O correlation plots are examined in the model and they show distinct canonical correlation curves for the polar vortex, midlatitudes, and the tropics. Comparison against balloon and aircraft measurements show that the model reproduces the separation between the vortex and midlatitude curves; however, the ratio of N 2 O to F11 lifetimes is somewhat too small in the model. It is shown that anomalies from the midlatitude canonical correlation curve can be used to identify remnants of polar vortex air which has mixed with midlatitude air. At the end of June there is excellent agreement in the position of air with anomalous F11-N 2 O tracer correlation and ozone-depleted air from the polar vortex. Transport of ozone-depleted air on the breakup of the stratospheric polar vortex in spring/summer 2000,
Atmospheric Chemistry and Physics, 2008
Strong perturbations of the Arctic stratosphere during the winter 2002/2003 by planetary waves led to enhanced stretching and folding of the vortex. On two occasions the vortex in the lower stratosphere split into two secondary vortices that re-merged after some days. As a result of these strong disturbances the role of transport in and 5 15 obtained from the Geophysica flights show in general good agreement.
Journal of Geophysical Research, 2007
Meridional transport from the tropics redistributes ozone and water vapor at middle and high latitudes. In situ measurements of water vapor, CH 4 , and N 2 O, acquired aboard the NASA ER-2 aircraft during JanuaryÀMarch 2000 in a campaign to survey the Arctic vortex, are used to examine transport into the lowermost stratosphere in the context of middle-and high-latitude ozone declines observed over the last several decades. Analysis of tracer-tracer correlations of H 2 O + 2 * CH 4 and N 2 O indicates that rapid, poleward isentropic transport from the lower tropical stratosphere coupled with diabatic descent between the subtropical and polar jet streams delivers very young air to the high-latitude lowermost stratosphere during winter, while descent of older air from the vortex and subsequent transport to lower latitudes is very limited. From middle to late winter, mixing ratios of H 2 O + 2 * CH 4 decrease by about 1 ppmv immediately outside the vortex, consistent with rapid transport of the winter phase of the seasonal cycle in water vapor to high latitudes from the lower tropical stratosphere. No evidence of isentropic mixing from the upper tropical troposphere survives in the high-latitude lowermost stratosphere except below 350 K, where markedly higher water vapor mixing ratios indicate mixing from the extratropical troposphere. All of these transport processes pose dynamical and chemical consequences for ozone. Transport from the lower tropical stratosphere (1) exports ozone-poor air to midlatitudes and the subvortex region and (2) distributes elevated water vapor to high latitudes, potentially enhancing halogen-catalyzed ozone destruction through heterogeneous processing in the polar vortex.
Characteristics of stratosphere-troposphere exchange in a general circulation model
Journal of Geophysical Research, 1994
Air and trace gases are exchanged between the stratosphere and the troposphere on a variety of scales; but general circulation models (GCMs) are tinable 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 exanfine 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 seasonality 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 extratropical stratosphere, even for tracers with large vertical gradients. The quest for tropopause-level temperatures "sufficiently cold to explain observed mixing ratios," known as the "cold trap," has guided much research on STE. The very dry lower stratosphere, by inference, sharply limits the location and season of mass transfer from troposphere to stratosphere ]. Thus although annual mean, zonal mean tropical tropopause temperatures are too warm to explain observed lower stratospheric water vapor mixing ratios, the cold trap condition is met at some times and locations. Newell and , in an analysis of tropical radiosonde 100 hPa data, identified northern hemisphere winter and the western Pacific as the most probable time and location for troposphere-stratosphere mass transfer to occur; they termed this the "stratospheric fountain." Robinson and Atticks Schoen [1987] compiled statistics using radiosonde measurements of saturation mixing ratios at 100 hPa and at the profile minimum temperature. The observations occurred in intensive observing periods during the FGGE year (fall 1978 to summer 1979) between 20øS and 20øN. Their results essentially confirmed those of Newell and Gould-Stewart but also indicated the degree of variability on short time and spatial scales and showed that the minimum saturation vapor pressure often occurred well above 100 hPa. in the troposphere the tropical rising motion implied by the Brewer-Dobson circulation does not take the form of slow ascent over a broad area, as this would cause widespread cirrus cloud, which is not observed. Instead, most ascent takes place as convection.
Journal of Geophysical Research: Atmospheres, 1999
We use ozonesondes launched from Samoa (14øS) during the Pacific Exploratory Mission (PEM) Tropics A to show that O3 mixing ratios usually start increasing toward stratospheric values near 14 km. This is well below the tropical tropopause (as defined either in terms of lapse rate or cold point), which usually occurs between 16 and 17 km. We argue that the main reason for this discrepancy in height between the chemopause and tropopause is that there is very little convective detrainment of ozone-depleted marine boundary layer air above 14 km. We conjecture that the top of the Hadley circulation occurs at roughly 14 km, that convective penetration above this altitude is rare, and that air that is injected above this height subsequently participates in a slow vertical ascent into the stratosphere. The observed dependence of ozone on potential temperature in the transitional zone between the 14-km chemopause and the tropical tropopause is consistent with what would be expected from this hypothesis given calculated clear-sky heating rates and typical in situ ozone production rates in this region. An observed anticorrelation between ozone and equivalent potential temperature below 14 km is consistent with what would be expected from an overturning Hadley circulation, with some transport of high O3/low 0• air from midlatitudes. We also argue that the positive correlations between 03 and N20 in the transitional zone obtained during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft) (ASHOE/MAESA) campaign support the notion that air in this region does have trace elements of stratospheric air (as conjectured previously), so that some of the ozone in the transitional zone does originate from the stratosphere rather than being entirely produced in situ.
Transport of near-tropopause air into the lower midlatitude stratosphere
Quarterly Journal of the Royal Meteorological Society, 1998
During the last week of January 1992 ozonesonde observations over Europe revealed a layer of very low ozone concentrations in the stratosphere-below 100 parts per billion (lo9) by volume in the potential-temperature range 360-380 K. A coincident lidar observation revealed that the air was virtually free of volcanic aerosol, which filled the lower stratosphere at that time. The layer corresponded well with low potential vorticity (PV) in ECMWF analysis fields. Trajectory calculations confirmed a subtropical origin for the layer, and PV fields suggest that it was formed in a streamer of low-PV air drawn from the troposphere over North America as the subtropical jet stream turned northwards on 25 January. Ozone profiles on the equatorward flank of the subtropical jet stream contain similar mixing ratios to those seen in the layer over Europe in the same potential-temperature range. The mass of the layer at its maximum extent is estimated as 6 x lOI5 kg. Most of the air in the layer eventually returned to the subtropics after mixing with ambient midlatitude air.
Geophysical Research Letters, 1998
Observations of stratospheric water vapour, made by the Microwave Limb Sounder (MLS) during the 1992 and 1993 Arctic and 1992 Antarctic late winters have now been produced using version 4 of the retrieval software. These improved measurements are analysed as equivalent latitude zonal means. Major interhemispheric differences are revealed in the water vapour content of the vortex in the lower stratosphere. This technique emphasises mixing ratio gradients at the edges of both polar vortices, and a local maximum at the edge of the Antarctic vortex. There are some small interhemispheric differences in mixing ratios in mid-latitudes, but they are not strongly related to the dehydration of the Antarctic vortex. A mixing ratio gradient across the interior of the Antarctic vortex at 530K indicates it is not isentropically mixed. A strong local maximum in mixing ratio at the centre of the Antarctic vortex in the mid-stratosphere indicates it is not well mixed in the mid-stratosphere also. There is little evidence of significant structure inside the Arctic vortex.
Transport and mixing processes in the lower troposphere over the ocean
Journal of Geophysical Research, 1992
Aircraft observations during the summer over the eastern Pacific Ocean, _•400 km offshore, show that the free troposphere has a distinctly nonuniform, layered structure. Analysis of plots of ozone versus total water mixing ratio indicates that the layers typically consist of a mixture of air from several sources: (1) moist, ozone-depleted boundary layer air, (2) very dry air with high ozone content, which probably originated in the middle or upper troposphere, and (3) air with relatively low ozone and moderate, varying moisture contents, which may represent residues of convective clouds that had formed over the ocean upstream of the research area. On two research days we also observed what we infer to be continental air. On one day in the usual research area we encountered air with elevated concentrations of aerosols whose trajectories, traced backward in time, indicated that it had passed over the Alaskan continent. On another day, when soundings were made about 100 km off the California coast, we observed a layer just above the marine inversion containing unusually high amounts of ozone, aerosols, and moisture. This layer probably represents a direct intrusion of polluted air from the coast, which could have been the result of baroclinlc flow associated with boundary layer warming over land. Over the eastern Pacific Ocean in summer the subtropical high results in stably stratified, subsiding air, where discrete horizontal layers with differing flow velocities transport air over long distances with little vertical mixing. Here, localized large-scale flows (such as tropospheric folding, deep convection, or baroclinic flow from coastal areas) "inject" air of different properties at various levels in the lower troposphere. This air is then subject to buoyancy sorting and differential advection. Because the lower troposphere is very stably stratified, the mixing eddies are small compared to the air mass size, and small-scale turbulent diffusion is slow. Here, wind shear plays an important role through stretching and thinning the different air masses until small-scale diffusion completes the mixing process. 1. INTRODUCTION Understanding the transport and mixing of air masses throughout the troposphere is essential for predicting tropospheric responses to natural and anthropogenic perturbations of hiogeochemical cycles. Aside from sources and sinks at the surface or within the troposphere the chemical constituents of the troposphere can be altered by tropospheric folding events (as described, e.g., by Danielson [1968], Shapiro [1980], and Haagenson et al. [1981] that inject dry, ozone-rich stratospheric air into the troposphere. The situation is further complicated by pollution from urban sources and biomass burning, which produces elevated concentrations of aerosols, ozone, carbon monoxide, and other trace species. A number of observational studies have cused on the chemistry and micrometeorology of the boundary layer. Tropical rainforests and seasonal biomass burning have been studied extensively in the Amazon Boundary Layer Experiment during dry and wet seasons. (The results from this experiment have appeared in special issues of the Journal of Geophysical Research, 93, pp. 1351-1624, and 95, pp. 16,721-17,050.) From a global perspective, much new information on trace gas distributions has been gained from
Stratosphere-troposphere exchange in a midlatitude mesoscale convective complex: 1. Observations
Journal of Geophysical Research, 1996
On June 28, 1989, a severe thunderstorm over North Dakota developed into a squall line and then into a mesoscale convective complex (MCC) with overshooting tops as high as ---14 l•n and a cirrus anvil that covered more than 300,000 1,an 2. In this paper we describe the trace gas concentrations prior to, in, and around the storm; paper 2 presents numerical simulations. Observations of 0 3 and 0eq unaffected bv upstream convection for at least 3 days prior to the flights placed the undisturbed tropopaus• bctxveen 10.7 and 11 1,an.