WRF-HAILCAST Hail Forecasting System Improvements

The one-dimensional hail model developed as part of the HAILCAST algorithm was inserted into the Weather Research and Forecasting model (WRF) as part of the Air Force Weather Agency (AFWA) diagnostics package. This new hail forecasting system, termed WRF-HAILCAST, was distributed to the WRF user community as part of v3.6.1, and was also implemented operationally as part of the AFWA Mesoscale Ensemble Prediction System. WRF-HAILCAST provides 2D forecasts of hail diameter when WRF is run at convection-allowing resolution, as vertical profiles of updraft, temperature, and liquid and ice water content are passed from WRF to the 1D hail model. Object-based verification statistics of the WRF-HAILCAST forecasts produced during the 2014 NOAA Hazardous Weather Testbed (HWT) Spring Forecasting Experiment will be presented.

Several improvements have been made to WRF-HAILCAST since the 2014 HWT. Constant initial hailstone embryo sizes are now used instead of retrieving initial embryo sizes from the microphysical parameterization graupel size distribution, notably reducing the sensitivity of WRF-HAILCAST to the microphysical parameterization; this improvement was part of the WRF v3.6.1 release. The required processing time was also significantly reduced (included in the v3.7 release). Forecasts from the 2014 HWT during a selected time period were rerun using these WRF-HAILCAST improvements, and verification statistics from these forecasts will also be presented. Finally, results from sensitivity tests will be shown that evaluate the effects of perturbations in updraft strength on the forecast hail diameter.