Stratospheric dryness: model simulations and satellite observations

[摘要] The mechanisms responsible for the extreme dryness of the stratosphere havebeen debated for decades. A key difficulty has been the lack ofcomprehensive models which are able to reproduce the observations. Here weexamine results from the coupled lower-middle atmosphere chemistry generalcirculation model ECHAM5/MESSy1 together with satellite observations. Ourmodel results match observed temperatures in the tropical lower stratosphereand realistically represent the seasonal and inter-annual variability ofwater vapor. The model reproduces the very low water vapor mixing ratios(below 2 ppmv) periodically observed at the tropical tropopause near 100hPa, as well as the characteristic tape recorder signal up to about 10 hPa,providing evidence that the dehydration mechanism is well-captured. Ourresults confirm that the entry of tropospheric air into the tropicalstratosphere is forced by large-scale wave dynamics, whereas radiativecooling regionally decelerates upwelling and can even cause downwelling.Thin cirrus forms in the cold air above cumulonimbus clouds, and theassociated sedimentation of ice particles between 100 and 200 hPa reduceswater mass fluxes by nearly two orders of magnitude compared to air massfluxes. Transport into the stratosphere is supported by regional netradiative heating, to a large extent in the outer tropics. During summervery deep monsoon convection over Southeast Asia, centered over Tibet,moistens the stratosphere.

[学科分类] 大气科学