Constraining Rain Evaporation from Shallow-Clouds in the Trades using an Observation-Based Superdroplet Model

Abstract. Rain evaporation influences subcloud moisture and energy budgets as well as boundary layer buoyancy, making it important for shallow cumulus organization. This study constrains the amount of, and controls on, rain evaporation in the subcloud layer of trade-wind shallow cumuli. We combine observations from the EUREC⁴A field campaign with a 1D rainshaft using superdroplet model (SDM) microphysics. With SDM we explicitly resolve how the observed droplet size distributions (DSDs) evolve beneath cloud-base, and explore, for the first time, the potential impact of raindrop collisions. Constraining the amount of evaporation, we find mean column integrated evaporative cooling of 150 ± 170 W m⁻², and evaporation fraction of 30 ± 20%. The dominant controls on rain evaporation are the subcloud relative humidity and the cloud-base DSD. Subcloud relative humidity determines the slope of the evaporation profiles, and cloud-base rain water content (RWC) controls the column integrated cooling. In contrast, evaporation fraction can vary substantially between clouds, even with comparable RWC, because of small differences to the cloud-base DSD. For narrow DSDs, the evaporation fraction is well captured by an analytical approximation, which also highlights the importance of ventilation effects on evaporation. We find collisions have negligible impact on rain evaporation, however, beyond rain rates of ≈ 3 mm h⁻¹, differences emerge which suggest that collisions, in particular those causing severe droplet breakup, need to be considered for heavier precipitation. Our results stress the importance of accurately measuring and modeling DSDs for rain evaporation, but justify the use of simpler microphysics models that only represent condensation/evaporation for lightly precipitating regimes as observed during EUREC⁴A.