Stratospheric dryness

Abstract

The mechanisms responsible for the extreme dryness of the stratospherehave been debated for decades. A key difficulty has been the lackof comprehensive models which are able to reproduce the observations.Here we examine results from the coupled lower-middle atmospherechemistry general circulation model ECHAM5/MESSy1 together with satelliteobservations. Our model results match observed temperatures in thetropical lower stratosphere and realistically represent the seasonaland inter-annual variability of water vapor. The model reproducesthe very low water vapor mixing ratios (below 2 ppmv) periodicallyobserved at the tropical tropopause near 100 hPa, as well as thecharacteristic tape recorder signal up to about 10 hPa, providingevidence that the dehydration mechanism is well-captured. Our resultsconfirm that the entry of tropospheric air into the tropical stratosphereis forced by large-scale wave dynamics, whereas radiative coolingregionally decelerates upwelling and can even cause downwelling.Thin cirrus forms in the cold air above cumulonimbus clouds, andthe associated sedimentation of ice particles between 100 and 200hPa reduces water mass fluxes by nearly two orders of magnitude comparedto air mass fluxes. Transport into the stratosphere is supportedby regional net radiative heating, to a large extent in the outertropics. During summer very deep monsoon convection over SoutheastAsia, centered over Tibet, moistens the stratosphere.