Recent Improvements in Rapid Refresh and High-Resolution Rapid Refresh Forecasts of Low Ceilings

The 13-km Rapid Refresh (RAP) and 3-km convection-allowing High-Resolution Rapid Refresh (HRRR) are hourly-updating forecast models that support operational aviation weather forecasting and other short-range forecasting interests within the contiguous United States. This talk assesses recent warm- and cold-season performance of the RAP and HRRR in forecasting low cloud ceilings (i.e., < 3000 ft AGL) over land. Experimental modifications to the Mellor–Yamada–Nakanishi–Niino (MYNN) planetary boundary layer (PBL) scheme and the Grell–Freitas–Olson (GFO) shallow-convection scheme are shown to contribute to increased skill in forecasting low ceilings during the warm and cold seasons. This increased skill of the RAP and HRRR is attributed to (1) a more realistic representation of subgrid-scale clouds within the model parameterization schemes, and (2) the inclusion of double-moment cloud hydrometeor mixing within the MYNN scheme.

Verification results from retrospective simulations and selected case studies will be shown to illustrate these performance improvements in the RAP and HRRR for low cloud ceilings. Finally, ongoing physical parameterization work at ESRL will be summarized.