Sensitivity of Hurricane WRF model track and intensity forecasts to microphysics and radiation parameterizations

The Hurricane Weather Research and Forecasting (HWRF) model provides operational guidance to forecasters at the NHC and uses 27/9/3 km horizontal grid spacing along with ocean coupling, hybrid ensemble-variational data assimilation, and vortex initialization. It is important that the HWRF model employ the best-performing physics schemes, so that it can provide superior representation of the large-scale environment and of the storm's inner core. The operational HWRF physics suite is comprised of the Ferrier microphysics parameterization and the Geophysical Fluid Dynamics Laboratory (GFDL) radiation scheme along with other physical parameterizations specifically designed for hurricane conditions.

The latest public release of HWRF, version 3.5a, provides the experimental capability to use the Thompson et al (2008) microphysics parameterization. In contrast to the Ferrier microphysics scheme that explicitly predicts only a single total condensate field and diagnoses separate water and ice species, the Thompson scheme explicitly predicts mass and number concentrations of multiple water and ice species. In addition, HWRF v3.5a contains a revised version of the Rapid Radiative Transfer Model (RRTM) parameterization that explicitly couples to the Thompson microphysics through direct calculation of the effective radius of cloud water, cloud ice, and snow and subsequently directly uses them in the computation of cloud optical depth. The non-coupled version of RRTMG uses look-up table effective radii irrespective of what the cloud physics scheme utilizes.

The Developmental Testbed Center (DTC) and EMC are conducting extensive tests of HWRF with Thompson microphysics and RRTMG radiation, and comparing the results with those obtained using the operational physics suite. Initial results from Hurricane Sandy and Earl show that the alternate physics suite leads to improved forecasts. Detailed diagnostic evaluation of these results will be presented at the conference.