A Modeling Study of the Effects of Vertical Wind Shear on the Raindrop Size Distribution in Typhoon Nida (2016)

In this study, the effects of vertical wind shear (VWS) on the raindrop size distribution in tropical cyclone (TC) are investigated, based on the theoretical analyses that intense VWS, which commonly appears in the lower layers of TCs, can enhance the collisional breakup of raindrops. This is achieved by comparing the numerical sensitivity experiments of Typhoon Nida (2016) using Weather Research and Forecasting (WRF) model against the polarimetric radar and disdrometer observations. In the control (CTRL) run with the default Morrison microphysics, unrealistic large raindrops are produced with excessively large differential reflectivity and heavier precipitation. An obvious decrease of raindrop size is present when the constant value of cloud droplet number concentration reduces from 250 to 30 (NC30), leading to more comparable microphysics characteristics with the observations due to the enhancement of autoconversion rate from cloud droplets to raindrops; however, some unrealistic large-sized raindrops still exist. A semi-empirical raindrop collection/breakup parameterization is further proposed in NC30_WS run by modifying the threshold diameter of raindrops at which breakup occurs as a function of VWS. The certain improvements of simulating raindrop size distribution and precipitation in Typhoon Nida are present in NC30_WS run, owing to the efficient collisional breakup and the induced stronger raindrop evaporation cooling. Our results indicate that the combined effects of reasonable cloud droplet numbers as well as reliable raindrop breakup parameterization are pronounced and highlight the impacts of VWS on raindrop size distribution, which should be involved in current microphysical schemes to forecast the TCs more accurately.

Figure. Time-height cross-sections of area-averaged source and sink terms (kg^-1s^-1) of raindrop number concentration within a radius of 150-km including (a-c) auto-conversion of cloud droplets to raindrops, (d-f) evaporation of raindrops, (g-i) breakup of raindrops, (j-l) self-collection of raindrops and (m-o) melting of snow/graupel particles. The upper, middle, and lower rows are from the CTRL, NC30 and NC30_WS runs, respectively.