Unsaturated Downdraft Thermodynamics in Cumulonimbus

Abstract

The thermodynamic structure of unsaturated downdrafts driven by the evaporation of falling rain can be parameterized using an evaporation pressure scale related to downdraft speed, rain rate and raindrop population. The larger this evaporation scale, which can be inferred diagnostically from bulk downdraft data, the more unsaturated the downdraft. Simple solutions exist for constant evaporation scale in which the downdraft temperature asymptotically approaches a wet adiabat, and for buoyancy driven downdrafts where the downdraft thermodynamic paths and speed can be related to the environmental stratification. The pressure difference to the lifting condensation level for the downdraft outflow may be an observable measure of the evaporation pressure scale.