The evaporation of frontal and other stratiform precipitation

Abstract

The evaporation of precipitation of differing types is considered by use of one-dimensional models. It is shown that the evaporation of snow and crystalline ice precipitation takes place in shorter times and much shallower depths than for rain, while for graupel it falls between the two. The main factors responsible for the differences in behaviour are bulk density and terminal velocity of particles, which combine to produce evaporation depths for snow less than one tenth those for rain. Further, the evaporation depth predicted for snow is shown to be rather insensitive to detailed characteristics of the snow particles within the observed range. Some of the implications of this difference are explored by means of a model describing the interaction of evaporating precipitation with its environment when subjected to atmospheric descent. The model, originally due to Kamburova and Ludlam (1966), has been extended to study the evolution of a variable particle spectrum. It is shown that for rainfall rates of the order of 1–10 mm h−1 the evaporation of snow is sufficient to maintain the atmosphere in a near-saturated condition despite descent rates of the order of 10–30 cms−1, typical of those observed on the mesoscale in stratiform environments like rainbands; rain exhibits no comparable behaviour. These results suggest that the evaporation of snow plays a major role in the dynamics of fronts and other mesoscale systems with stratiform cloud. This view is supported by comparison with dropsonde observations from the FRonts 87 experiment, which show a clear indication of enhanced mesoscale descent in the region of maximum precipitation.