Production of Dry Air by Isentropic Mixing

Abstract

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 sum...