The Impacts of Microphysics and Terminal Velocities of Graupel/Hail on the Rainfall of Typhoon Fitow (2013) as Seen From the WRF Model Simulations With Several Microphysics Schemes

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摘要

This study investigated cloud microphysical processes, and the upward and downward movement of precipitating particles with the Weather Research and Forecasting model during Typhoon Fitow (2013). The Purdue-Lin single-moment scheme and the Milbrandt-Yau and National Severe Storms Laboratory (NSSL) double-moment microphysics schemes were utilized. The area-mean rainfall simulated by the three schemes shared similar magnitudes and variations. The area-mean rain rates were insensitive to microphysics schemes, as they were mainly determined by large-scale water vapor convergence. However, the local rainfall intensity was sensitive to the schemes and graupel and hail parameterizations. This indicates great impacts of microphysics and terminal velocities of graupel/hail on the local rainfall intensity. Upward-moving graupel had a stronger net source compared to the downward-moving graupel over the spiral rainband in the Milbrandt-Yau simulation due to the low terminal velocity; whereas the net source was much weaker for upward-moving graupel than for downward-moving graupel over the eyewall in the NSSL simulation due to the large terminal velocity. Decreasing the terminal velocities of graupel and hail (if hail exists) in the mid and lower troposphere significantly reduced the maximum local rainfall intensity via increasing the sources of upward-moving graupel. This is because the upward-moving graupel is prone to remaining in the clouds being advected out of the updraft region instead of directly converting to raindrops as the sinks of graupel. The microphysics and the terminal velocities were together responsible for the local rainfall intensity.