A study of the fraction of warm rain in a pre-summer rainfall event over South China

The precipitation is divided into the warm-rain and cold-rain processes according to the microphysical mechanism, but which processes are more important in a heavy precipitation even? An approach that quantitatively separates the warm-rain from total-rain processes is proposed by adding a set of new variables in microphysics schemes of the Weather Research and Forecasting model. The fraction of rainfall that formed by warm-rain processes and the related microphysical characteristic are investigated by a heavy precipitation event in southern China on 9 May 2016. The simulation using the Thompson microphysics reasonably reproduces the spatial distribution of precipitation and temporal evolution of rain bands. It is found that when the composite reflectivity is between 25 and 35 dBZ, the lowest percentage of warm-rain fraction occurs (median of 20–30%). While in the strong precipitation area, the contribution of warm-rain steadily increases with the median of ~50% due to the continuous moisture supply. The similar characteristics by two other microphysics schemes (Morrison and CAMS) further verify the results. In addition, abundant supercooled water exists above the 0 °C level due to the high condensation rate. The strong updrafts in lower-middle layers are closely associated with the areas of water vapor condensation, implying that the phase-change processes should be responsible for the small-scale buoyancy production. The budget of rain water shows that the warm-rain processes play a leading role in the initiating stage of convection, and the weak advection of rain water indicates that the transport of cloud body from surroundings to precipitation area is quite limited.