The role of cloud microphysical processes in the development and intensification of hurricane Dennis (2005): WRF simulations and observations

We are assessing the role of microphysical processes in tropical cyclone evolution through simulations of Hurricane Dennis (2005) and comparison to observations from the Tropical Cloud Systems and Processes (TCSP) field campaign. The Weather Research and Forecasting model (WRF) is run with a 3 km innermost nested grid and two microphysical parameterizations: the WRF single moment 6-class (WSM6) and Thompson (2006) schemes. The Thompson scheme prognoses cloud ice number concentration and diagnoses variable intercept parameters based on mass mixing ratio, features not present in WSM6. The minimum center pressure Pmin and the track varied by 14 hPa and 60 km between the 2 simulations respectively. To assess how varying spatial and temporal distributions of clouds and latent heating contribute to these differences, statistical distributions of microphysical parameters were compared for the 2 simulations and against retrievals of such parameters from the Advanced Microwave Precipitation Radiometer (AMPR) and the ER-2 Doppler radar (EDOP) on board the NASA ER-2 during TCSP. Observations over Dennis are ideal for such an investigation because they covered the entire life cycle from pre-genesis to post-landfall (flights were on 7/5/05 when Dennis was a tropical depression, 7/6/05 when it was a tropical storm, and 7/9/05 just prior to rapid intensification after leaving Cuba). Important differences were noted in comparison of contoured frequency by altitude diagrams (CFADs) of model-derived brightness temperatures Tb for the 2 simulations versus those observed by AMPR, and of equivalent reflectivity for the simulations and observed by EDOP. Evidence for an overprediction of the frequency of large graupel mixing ratios and heavy rain rates noted in previous simulations of other hurricanes was seen in the WSM6 simulation but not in the Thompson simulation. The dependence of these findings on the intercept parameter of the assumed exponential cloud distribution functions is discussed. In general, the representations of microphysics had significant impacts on cloud hydrometeor distributions and on the spatial distributions and temporal evolution of diabatic heating rates. Impacts of these findings on updraft and downdraft statistics will be discussed.