How Effectively Can Freeze-Drying by Optically Thin, Laminar Cirrus Dehydrate Air Rising Slowly Across the Tropical Tropopause

摘要

Over the past 20 years, many theories have been proposed to explain the extreme dryness of air in the tropical lower stratosphere. Recent observations suggest that the flux of air into the stratosphere may be dominated by slow ascent across the tropopause throughout much of the tropics. In this study, cloud model simulations were used to show that laminar, optically thin cirrus clouds (frequently observed near the tropopause) can effectively freeze-dry air entering the tropical stratosphere. A detailed ice cloud microphysical model coupled to a large-eddy simulation dynamical model was used for these simulations. As shown in the top panel of the figure, if no cloud forms, the slow ascent across the tropopause will eventually increase the water vapor mixing ratio above 5 parts per million by volume (ppmv). These values are much higher than observed water vapor mixing ratios. However, if we include cloud formation, then the slow ascent drives adiabatic cooling and nucleation of a small number of ice crystals (<10/liter). These crystals grow rapidly and precipitate out within a few hours. The ice crystal nucleation and growth prevents the relative humidity (with respect to ice) from rising above the threshold of ice nucleation (130-160) and limits the water vapor mixing ratio above the tropopause to 3-4 ppmv (bottom panel of figure). The nucleation threshold depends upon the aerosol composition in the tropopause region, which is not well known. Simulations including gravity waves propagating through the model were also done. Temperature oscillations driven by the waves drive nucleation of larger ice number densities and more complete dehydration of the rising air.