Evaluating and Modifying a Partially Double Moment Microphysics Scheme for Hurricane Simulations

Accurately parameterized microphysical processes are integral to accurate tropical cyclone (TC) numerical predictions given that latent heat release is an important source of energy for TCs. Brown et al (2016, GRL) showed that the aerosol-aware Thompson scheme as implemented in the Weather Research and Forecasting (WRF) model version 3.6.1 produced more accurate TC intensity forecasts (as measured by maximum 10m wind speed) than other bulk microphysical parameterizations tested; however, there were clearly regions where the Thompson scheme overpredicted simulated differential reflectivity. Differential reflectivity, the difference between the horizontal and vertical reflectivity factors measured by dual-polarized radar, is dependent on the median drop size of a population of liquid drops. Here, we show that the relatively large differential reflectivity produced by the Thompson scheme is generally located in the stratiform regions in simulations of hurricanes Arthur and Ana (both 2014). In addition to evaluating the simulations against ground-based NEXRAD radar observations, we provide drop size distributions (DSD) estimates from the dual-frequency precipitation radar onboard the Global Precipitation Measurement (GPM) satellite. The vertical structure of the simulated differential reflectivity from rain suggests that melting snow or graupel is the source of increased median drop size of the DSDs in this case. Joint probability frequency distributions in horizontal and differential reflectivity space make a comprehensive microphysical evaluation straightforward as we modify the melting processes in the Thompson scheme to reduce the median drop size of the DSD in targeted regions of the phase space. We find that the joint probability distribution more closely resembles that from the radar observations as we increase the number of liquid particles resulting from the melting of snow, a process which contains several tunable parameters as the Thompson scheme is double moment in rain, but single moment in snow.