Developing the MYNN-Eddy Diffusivity/Mass-Flux PBL Scheme to Improve Non-Local Mixing Across the Greyzone

The 13-km Rapid Refresh (RAP) and 3-km convection-allowing High-Resolution Rapid Refresh (HRRR) are hourly-updating operational forecast models that support short-range forecasting interests within the contiguous United States. The experimental version of the RAP/HRRR uses a modified form of the Mellor–Yamada–Nakanishi–Niino (MYNN) planetary boundary layer (PBL) scheme, which has been extended to include non-local mixing associated with convective plumes and cloud-top cooling. The addition of these new components to the MYNN has improved the morning formation and structure of the daytime PBL, cloud-radiative interactions, and subcloud mixing in stratocumulus regimes.

The new MYNN Eddy Diffusivity/Mass-Flux (EDMF) is used to re-investigate the performance of the RAP/HRRR physics suite within the first Greyzone case selected from the CONSTRAIN cold-air outbreak case of 30-31 January 2010. Previous problems found in this case study with the original version of the RAP/HRRR physics suite included delayed break-up of the stratus (the transition region in the model was shifted too far south compared to observations) at coarse resolutions (dx = 8 or 16 km). At higher resolutions (dx = 1 or 2 km), the stratus was broken-up too early and the open-cellular convection observed in the visible satellite imagery was misrepresented as a popcorn cumulus structure in the model. Improvements made to the stratus, transition, and cumulus regions will be presented. Error reductions will be attributed to specific design features of the local mixing and/or mass-flux components. Lingering challenges will also be noted.