Production of Dry Air by Isentropic Mixing

Abstract

The authors have explored the factors governing upper-tropospheric relative humidity with a simple model based on isentropic mixing and condensation. Our analysis has focused on the Northern Hemisphere winter season and on the 315-K (dry) isentropic surface. The advection?condensation model yields the following results. In the absence of moisture resupply, about half of the mass of water is lost from the isentropic surface after only 10 days, with the main brake on drying being the weak mixing between Tropics and extratropics. The moist plumes escaping from the Tropics take the form of filamentary structures, which are more numerous and space filling in the summer/Southern Hemisphere than in the winter/Northern Hemisphere. These moist plumes are accompanied by substantial importation of extratropical dry air into the Tropics. The probability distributions of midlatitude relative humidity are bimodal, with a prominent dry peak having a lognormal tail and a spike representing saturated air; the summer hemisphere has generally higher relative humidity than the winter hemisphere. When moisture is maintained by periodically resaturating the Tropics, the resulting cloud and moisture fields exhibit a fractal character, with a tendency to become less space filling with distance from the Tropics. Some tentative comparisons with data are made, which tend to confirm the advective control of relative humidity patterns outside the Tropics. There are indications, however, that the advection-condensation model with moisture resupply only from the Tropics yields an upper troposphere that is far too dry. The authors suggest that the missing moisture is supplied from the 295-K isentropic surface via diabatic mixing arising in ascending, convecting saturated trajectories near that surface. As found in earlier passive tracer studies, the permeable mixing barrier between the Tropics and extratropics has the potential to exert a controlling influence on the global climate.