Dehydration of the stratosphere (original) (raw)
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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.
Simulation of stratospheric water vapor and trends using three reanalyses
The domain-filling, forward trajectory calculation model developed by Schoeberl and Dessler (2011) is extended to the 1979–2010 period. We compare results from NASA's MERRA, NCEP's CFSR, and ECMWF's ERAi reanalyses with HALOE, MLS, and balloon observations. The CFSR based simulation produces a wetter stratosphere than MERRA, and ERAi produces a drier stratosphere than MERRA. We find that ERAi 100 hPa temperatures are cold biased compared to Singapore sondes and MERRA, which explains the ERAi result, and the CFSR grid does not resolve the cold point tropopause, which explains its relatively higher water vapor concentration. The pattern of dehydration locations is also different among the three reanalyses. ERAi dehydration pattern stretches across the Pacific while CFSR and MERRA concentrate dehydration activity in the West Pacific. CSFR and ERAi also show less dehydration activity in the West Pacific Southern Hemisphere than MERRA. The trajectory models' lower northern high latitude stratosphere tends to be dry because too little methane-derived water descends from the middle stratosphere. Using the MLS tropical tape recorder signal, we find that MERRA vertical ascent is 15 % too weak while ERAi is 30 % too strong. The trajectory model reproduces the observed reduction in the amplitude of the 100-hPa annual cycle in zonal mean water vapor as it propagates to middle latitudes. Finally, consistent with the observations, the models show less than 0.2 ppm decade −1 trend in water vapor both at mid-latitudes and in the tropics.
Stratospheric dryness: model simulations and satellite observations
Atmospheric Chemistry and Physics, 2007
The mechanisms responsible for the extreme dryness of the stratosphere have been debated for decades. A key difficulty has been the lack of comprehensive models which are able to reproduce the observations. Here we examine results from the coupled lower-middle atmosphere chemistry general circulation model ECHAM5/MESSy1 together with satellite observations. Our model results match observed temperatures in the tropical lower stratosphere and realistically represent the seasonal and inter-annual variability of water vapor. The model reproduces the very low water vapor mixing ratios (below 2 ppmv) periodically observed at the tropical tropopause near 100 hPa, as well as the characteristic tape recorder signal up to about 10 hPa, providing evidence that the dehydration mechanism is well-captured. Our results confirm that the entry of tropospheric air into the tropical stratosphere is forced by large-scale wave dynamics, whereas radiative cooling regionally decelerates upwelling and can even cause downwelling. Thin cirrus forms in the cold air above cumulonimbus clouds, and the associated sedimentation of ice particles between 100 and 200 hPa reduces water mass fluxes by nearly two orders of magnitude compared to air mass fluxes. Transport into the stratosphere is supported by regional net radiative heating, to a large extent in the outer tropics. During summer very deep monsoon convection over Southeast Asia, centered over Tibet, moistens the stratosphere.
Modeling upper tropospheric and lower stratospheric water vapor anomalies
Atmospheric Chemistry and Physics, 2013
The domain-filling, forward trajectory calculation model developed by Schoeberl and Dessler (2011) is used to further investigate processes that produce upper tropospheric and lower stratospheric water vapor anomalies. We examine the pathways parcels take from the base of the tropical tropopause layer (TTL) to the lower stratosphere. Most parcels found in the lower stratosphere arise from East Asia, the Tropical West Pacific (TWP) and Central/South America. The belt of TTL parcel origins is very wide compared to the final dehydration zones near the top of the TTL. This is due to the convergence of rising air due to the stronger diabatic heating near the tropopause relative to levels above and below. The observed water vapor anomalies-both wet and drycorrespond to regions where parcels have minimal displacement from their initialization. These minimum displacement regions include the winter TWP and the Asian and American monsoons. To better understand the stratospheric water vapor concentration we introduce the water vapor spectrum and investigate the source of the wettest and driest components of the spectrum. We find that the driest air parcels originate below the TWP, moving upward to dehydrate in the TWP cold upper troposphere. The wettest air parcels originate at the edges of the TWP as well as in the summer American and Asian monsoons. The wet air parcels are important since they skew the mean stratospheric water vapor distribution toward higher values. Both TWP cold temperatures that produce dry parcels as well as extra-TWP processes that control the wet parcels determine stratospheric water vapor.
Journal of Geophysical Research-Atmospheres, 2013
We compare global water vapor observations from Microwave Limb Sounder (MLS) and simulations with the Lagrangian chemical transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) to investigate the pathways of water vapor into the lower stratosphere during Northern Hemisphere (NH) summer. We find good agreement between the simulation and observations, with an effect of the satellite averaging kernel especially at high latitudes. The Asian and American monsoons emerge as regions of particularly high water vapor mixing ratios in the lower stratosphere during boreal summer. In NH midlatitudes and high latitudes, a clear anticorrelation between water vapor and ozone daily tendencies reveals a large region influenced by frequent horizontal transport from low latitudes, extending up to about 450 K during summer and fall. Analysis of the zonal mean tracer continuity equation shows that close to the subtropics, this horizontal transport is mainly caused by the residual circulation. In contrast, at higher latitudes, poleward of about 50 ı N, eddy mixing dominates the horizontal water vapor transport. Model simulations with transport barriers confirm that almost the entire annual cycle of water vapor in NH midlatitudes above about 360 K, with maximum mixing ratios during summer and fall, is caused by horizontal transport from low latitudes. In the model, highest water vapor mixing ratios in this region are clearly linked to horizontal transport from the subtropics.
Simulations of the Interannual Variability of Stratospheric Water Vapor
Journal of the Atmospheric Sciences, 2002
Observations and model results indicate that the quasi-biennial oscillation (QBO) modulation of stratospheric water vapor results from two causes. Dynamical redistribution of water vapor from the QBO-induced mean meridional circulation dominates the observed variability in the middle and upper stratosphere. In the lower stratosphere, the QBO water vapor variability is dominated by a ''tape recorder'' that results from the dehydration signal accompanying the QBO variation of the tropical cold point tropopause. It is suggested that another low frequency tape recorder exists due to ENSO modulations of the tropical tropopause, but insufficiently long observations of stratospheric water vapor exist to identify this in the observations.
Quarterly Journal of the Royal Meteorological Society, 1999
Stratospheric humidity analyses produced operationally by the European Centre for Medium-Range Weather Forecasts (ECMWF) are discussed for the period since late January 1996 when the practice of resetting the upperlevel specific humidity to a fixed value at each analysis time was abandoned. Near-tropopause analyses are in reasonable overall agreement with independent observations. Very low humidities occur in conjunction with deep convection and a particularly cold tropopause over the equatorial western Pacific during the northern winter. Drying occurs also in the cold core of the Antarctic polar-night vortex. The lower stratosphere is moistened in the outer tropics and subtropics in summer and autumn, predominantly in the northern hemisphere. Changes associated with the latest occurrence of El Niiio are illustrated.
Variations of stratospheric water vapor over the past three decades
We examine variations in water vapor in air entering the stratosphere through the 15 tropical tropopause layer (TTL) over the past three decades in satellite data and in a trajectory 16 model. Most of the variance can be explained by three processes that affect the TTL: the quasi-17 biennial oscillation, the strength of the Brewer-Dobson circulation, and the temperature of the 18 tropical troposphere. When these factors act in phase, significant variations in water entering the 19 stratosphere are possible. We also find that volcanic eruptions, which inject aerosol into the 20 TTL, affect the amount of water entering the stratosphere. While there is clear decadal 21 variability in the data and models, we find little evidence for a long-term trend in water entering 22 the stratosphere through the TTL over the past 3 decades. 23 24
The circulation response to idealized changes in stratospheric water vapor
Observations show that stratospheric water vapor (SWV) concentrations increased by ;30% between 1980 and 2000. SWV has also been projected to increase by up to a factor of 2 over the twenty-first century. Trends in SWV impact stratospheric temperatures, which may lead to changes in the stratospheric circulation. Perturbations in temperature and wind in the stratosphere have been shown to influence the extratropical tropospheric circulation. This study investigates the response to a uniform doubling in SWV from 3 to 6 ppmv in a comprehensive stratosphere-resolving atmospheric GCM. The increase in SWV causes stratospheric cooling with a maximum amplitude of 5-6 K in the polar lower stratosphere and 2-3 K in the tropical lower stratosphere. The zonal wind on the upper flanks of the subtropical jets is more westerly by up to ;5 m s 21 . Changes in resolved wave drag in the stratosphere result in an increase in the strength of tropical upwelling associated with the Brewer-Dobson circulation of ;10% throughout the year. In the troposphere, the increase in SWV causes significant meridional dipole changes in the midlatitude zonal-mean zonal wind of up to 2.8 m s 21 at 850 hPa, which are largest in boreal winter in both hemispheres. This suggests a more poleward storm track under uniformly increased stratospheric water vapor. The circulation changes in both the stratosphere and troposphere are almost entirely due to the increase in SWV at pressures greater than 50 hPa. The results show that long-term trends in SWV may impact stratospheric temperatures and wind, the strength of the Brewer-Dobson circulation, and extratropical surface climate.