Toward improved convection-allowing ensembles: Model physics sensitivities and optimizing probabilistic guidance with small ensemble membership

During the 2007 NOAA Hazardous Weather Testbed (HWT) Spring Experiment, the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma produced daily 10-member 4-km resolution ensemble forecasts. Each member used the WRF-ARW core, was initialized at 21 UTC, and ran for 33 hours over a domain covering approximately 3/4 of the continental United States. Different initial condition (IC), lateral boundary condition (LBC), and physics perturbations were introduced in four of the ten ensemble members, while the remaining six members used identical ICs and LBCs, differing only in terms of microphysics and PBL schemes. This study focuses on precipitation forecasts from the ensemble.

The ensemble forecasts revealed WRF-ARW sensitivity to microphysics and PBL schemes. For example, over the 7-week Experiment, the Mellor-Yamada-Janjic (MYJ) PBL and Ferrier microphysics parameterizations were associated with relatively high precipitation totals, while members configured with the Thompson microphysics or Yonsei University (YSU) PBL scheme were comparatively dry. Additionally, as the first known real-time application of a convection-allowing ensemble, the forecasts allowed us to explore different approaches for generating probabilistic severe weather guidance. In this regard, a “neighborhood” approach is described and shown to considerably enhance model forecast skill of severe events when combined with traditional techniques of producing ensemble probability fields.

These results have important implications for convection-allowing guidance in both deterministic and ensemble frameworks. The implications for severe convective storm forecasters will be discussed at the conference.