Seminar

Abstract

The exceptionally low temperatures around the tropical tropopause strongly constrain the mean amount of water entering the stratosphere (H2Oentry). In addition, variations of H2Oentry on both annual and interannual timescales are strongly correlated with zonal mean tropopause temperatures. However, dehydration occurs during periods and in locations of temperature minima rather than (time or area) average temperatures, and the dehydration efficiency (i.e. fraction of condensate effectively removed) depends – in particular for the thin cirrus clouds in the tropical tropopause layer (TTL) – fairly strongly on cloud microphysical properties. In this talk, I will present an overview of efforts to constrain processes that control H2Oentry to higher accuracy than a correlation with tropopause temperatures. Using reanalysis data from the European Centre for Medium-range Weather Forecast (ECMWF) and a wide range of currently available TTL temperature datasets, we compare model predictions based on trajectory calculations with observations. Dehydration along the trajectories is estimated with a cloud box model. These calculations show that the cloud microphysical aspects of the problem primarily affect the average H2Oentry, unless one assumes trends in, e.g., aerosol affecting ice number densities. To first order, the amplitudes of mean annual cycle and interannual variability scale with the average H2Oentry as expected from Clausius-Clapeyron. Observational uncertainties in these amplitudes, however, prevent definite conclusions whether the model calculations capture all relevant processes. This is also true for trends over the past three decades, with observational uncertainties in both temperature and H2Oentry being particularly large in the 1980's. The model calculations allow to derive statistics of the locations where air entering the stratosphere is last dehydrated. It is shown that even when reducing cloud microphysical aspects to the bare minimum (assuming complete removal of condensate upon reaching saturation), these statistics are remarkably complex and have so far withstood attempts to formulate a theory with predictive skill. Finally, we discuss the dynamical processes that control the variations in H2Oentry in the observational record, with a focus on the Pinatubo period, and the period before and after October 2000, when a sudden drop in H2Oentry was observed.