Stratospheric dryness: model simulations and satellite observations J. Lelieveld1, C. Brühl1, P. Jöckel1, B. Steil1, P. J. Crutzen1,2, H. Fischer1, M. A. Giorgetta3, P. Hoor1, M. G. Lawrence1, R. Sausen4, and H. Tost1 1Max Planck Institute for Chemistry, J. J. Becherweg 27, 55128 Mainz, Germany 2Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0221, USA 3Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany 4DLR-Institut für Physik der Atmosphäre, Oberpfaffenhofen, 82234 Wessling, Germany
Abstract. 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.
Citation: Lelieveld, J., Brühl, C., Jöckel, P., Steil, B., Crutzen, P. J., Fischer, H., Giorgetta, M. A., Hoor, P., Lawrence, M. G., Sausen, R., and Tost, H.: Stratospheric dryness: model simulations and satellite observations, Atmos. Chem. Phys., 7, 1313-1332, doi:10.5194/acp-7-1313-2007, 2007.