The Importance of Humidity in the Afternoon Local-Scale Precipitation Intensity over Eastern China and Its Impacts on the Aerosol Effects

Thermally driven local-scale precipitation (LSP) is an important type of summer precipitation over China, but the prestorm environmental conditions remain unclear. In order to investigate the major factors controlling the LSP intensity, the meteorological parameters preceding the occurrence of light and heavy afternoon LSP over Eastern China during 2018–2022 are examined using rain gauge, radiosonde sounding, and satellite observations. The temperature differences between heavy and light LSP events are relatively small, but heavy LSP events exhibit larger water vapor mixing ratios (Qv) below a 5 km altitude than light LSP. With an almost identical vertical temperature distribution, an increment in Qv increases the relative humidity (RH) in the lower troposphere. Furthermore, large eddy simulations with spectral bin microphysics are performed to investigate the impacts of humidity and aerosols on the LSP intensity. Increased low-level RH leads to larger mass concentrations of rain and graupel at the expense of cloud droplets due to enhanced drop collisions and the riming of ice particles, respectively, thereby reinforcing the LSP. However, an increased aerosol concentration leads to more cloud water but reduced rain water content, resulting mainly from suppressed drop collisions. The graupel mixing ratio exhibits a non-monotonic trend with aerosols, mostly contributed by riming. As a result, the LSP intensity first increases and then decreases with an increment in the aerosol concentration in both dry and humid air. Moreover, more aerosols lead to the humidification of the surrounding air due to the enhanced evaporation of cloud droplets, particularly under lower-RH conditions. These findings provide an enhanced understanding of the effects of covariations in humidity and aerosol concentrations on the afternoon LSP intensity over Eastern China.