Evolution of warm-rain microphysical properties in Typhoon Usagi(2013): A numerical modeling study

The understandings about the properties of warm-rain microphysics in tropical cyclones (TCs) including the raindrop size distribution (RSD), source-sink of raindrop concentration and water vapor budget, especially in inner-core and outer region during different developing stages are quite limited. In this study, the 2-km grid spacing simulation with the Morrison microphysics is employed to investigate the microphysical characteristics of Typhoon Usagi (2013) over the ocean. The model reasonably reproduced the track, intensity, and overall structure of Usagi. The simulated RSD shows a rapid increase in small-size raindrops concentration but a decrease in large-size ones with the TC development. Two levels (5.25 and 1.25 km) with the maximum number concentration of raindrops exist. The ice-related microphysics at higher level starts prior to the pure warm-rain parts at lower level. Larger raindrops formed by self-aggregation in inner-core will suffer from significant breakup, and raindrops outside the eyewall are without evident breakup but with the similar evaporation rate as inner-core. Results indicate that the dominant water vapor terms are the horizontal moisture flux transport (HFC) and condensation (Cond). The evaporation from ocean surface (PBL) is small in inner-core area, but accounts for ~ 40% of HFC in outer region during the intensification and decaying periods, highlights the importance of PBL at the special position and special time of TC. The self-circulation of moisture supply happens only in the outer region where water vapor can be equally obtained through the local evaporation (Evap) and inward transport from farther ocean. Moreover, water vapor change is closely related to the thermodynamic field of vortex. A priority starting of local cloud microphysics exists in inner-core during the intensification, while the continuous weakening of condensation in outer region implies the beginning of typhoon decay.