Possible Influence of Evaporation Below Cloud Base on Rain Enhancement

Abstract

The possibility is discussed that evaporation in the subcloud layer might be a major factor controlling the efficiency of rain enhancement by seeding, under the simplistic assumption that seeded clouds produce larger proportions of small raindrops than unseeded clouds. The concept of an evaporation parameter, E = ?0H?Tdh, is introduced, and it is shown that this quantity is a measure of the amount of water evaporated from a raindrop falling through an air column of height H and with a wet-bulb depression profile ?T(h). Based on the evaporation parameter a simple numerical model is constructed, which models the evaporation process as a function of air mass conditions and raindrop-size distributions. This makes it possible to estimate what changes in rainfall at the ground might occur for given changes in rainfall at cloud base. Numerical results are obtained for ambient conditions of the Swiss hail suppression project Grossversuch III. These results establish the potential importance of evaporation in attempts at rain enhancement and suggest the types of observations required for further study. The need of careful measurements of the drop-size distribution beneath cloud base is especially stressed.