Rain Evaporation Below Shallow Tropical Trade-Wind Cumuli as Predicted by a New Super-Droplet Model

How much rain evaporation occurs below shallow cumuli is a crucial determinant of the organisation of such clouds, yet both modelling studies and observations which can robustly quantify the amount of evaporation are in short supply. In this study we combine observations of a diverse population of precipitating cumuli from the EUREC4A field campaign with a simple 1-D rain-shaft model, in order to predict the amount of rain evaporation in the sub-cloud layer and the relative influence on it of microphysical vs environmental controls. The rain-shaft uses superdroplet model microphysics so that our analysis benefits from its detailed and yet comprehensible depiction of the droplet size distribution and microphysical processes involved. Surprisingly, we find that evaporated fractions are usually low, below 20%, and that collisional processes between droplets have a very minor influence on rain evaporation, meanwhile dominant roles are played by the droplet size distribution at cloud base and the sub-cloud layer relative humidity profile. The evaporated mass fraction can vary substantially between clouds and even when clouds’ liquid water contents are comparable, evaporated fraction can be upto 50% larger because of small differences to mean cloud droplet radius. These results stress the importance or accurately measuring/modelling droplet size distributions rather than microphysical processes in order to study rain evaporation. We find too that the vertical profile of evaporation rate is above all controlled by the vertical relative humidity profile, which suggests the sub-cloud layer could be highly sensitive to the feedback between evaporation and relative humidity.