Environmental moisture and stability controls on raindrop size distribution within convective clouds under sheared conditions

Abstract. Environmental conditions significantly influence the intensity of organised convective systems. However, variations in factors such as temperature, humidity, and vertical shear suggest that non-linear relationships may exist between these systems and their environments. To address gaps in these variations, it is crucial to understand the cloud microphysical processes inherent to convective systems. In this study, we conducted numerical experiments using the Weather Research and Forecasting model in idealised setups with a spectral-bin microphysical scheme that explicitly handles drop-size distributions. The initial conditions included variations in humidity, temperature lapse rates, and vertical wind shear that reflect observed relationships. The results showed that larger temperature lapse rates produced larger raindrops, resulting in stronger rainfall intensity. Increased moisture in the lower troposphere led to stronger rainfall intensity, driven by a larger concentration of smaller raindrops and increased liquid water content. The magnitude of vertical shear generally causes fluctuations in the drop size distribution within convective clouds. Particularly, weaker vertical shear tends to inhibit the propagation of convective clouds, reduce vertical variations in the drop-size distribution within them, and may reduce the raindrop evaporation rate, thereby increasing rainfall amounts. These findings provide valuable insights that could enhance operational quantitative precipitation estimation and improve the microphysical schemes used in numerical weather models.