Influence of Initial Cloud Droplet Number Concentration on Warm-Sector Rainstorm in the Sichuan Basin

Abstract

Warm-sector rainstorms (WSR) are among the main weather events that cause significant casualties in the Sichuan Basin (SCB). These events are challenging to predict accurately using numerical models, partly due to the locally high air pollution that complicates WSR microphysical and precipitation processes. Aerosols affect the initial cloud droplet number concentration (CDNC) directly, and the CDNC is a key parameter in microphysical schemes that directly influences precipitation prediction. However, how and to what extent the CDNC affects WSR predictions in the SCB remains unclear. In this study, sensitivity experiments were conducted using a cloud-resolving model to investigate how the CDNC affects WSRs in the SCB. The study showed that when the CDNC is high, warm rainfall is reduced, while the cold rainfall is increased, which changes with convection development. First, a higher initial CDNC inhibits warm rainfall during the initial stage of convection. Second, during convection development, a higher initial CDNC accelerates graupel growth and its transformation into rainwater. The cold rainfall process plays a dominant role in this process, leading to an increase in rainfall intensity. Finally, during the convection mature stage, the promoting effect of the CDNC on the cold rainfall process weakens, leading to a decreased rainfall intensity in the higher initial CDNC. In the “initial-development-mature” stage, a higher initial CDNC distinctly affects the precipitation intensity in the form of "suppression-promotion-suppression." The findings of this study contribute to the ability to anticipate the development of WSRs based on pollution conditions in the SCB.