Recent Improvements in Subgrid-Scale Cloud Paramaterizations in the Rapid Refresh and High-Resolution Rapid Refresh

The 13-km Rapid Refresh (RAP) and 3-km convection-allowing High-Resolution Rapid Refresh (HRRR) are hourly-updating forecast models that support short-range forecasting interests within the contiguous United States. Experimental versions of these models have shown forecast performance gains over predecessor versions, in part, due to enhancements to the Mellor–Yamada–Nakanishi–Niino (MYNN) planetary boundary layer (PBL) scheme and the Grell–Freitas–Olson (GFO) shallow-convection (ShCu) scheme. These enhancements have provided an improved representation of subgrid-scale shallow cumulus clouds atop unstable boundary layers, along with better coupling of this information with the radiation parameterization.

Recent work has sought to further improve the treatment of subgrid-scale clouds by incorporating probability density function (PDF) components into the PBL and ShCu schemes. These PDFs aim to predict the distributions of cloud properties (for stratus and cumulus) and the “activation” of shallow cumulus within each grid cell and time step. Compared to the subgrid cloud treatment in the operational versions of the RAP and HRRR, where subgrid cloud information is contributed by disparate physical parameterizations, a subgrid cloud scheme with PDF components may be advantageous by functioning as a unified eddy diffusivity/mass-flux (EDMF)-type scheme (currently in development) for all subgrid clouds, capable of accounting for shallow cumulus and stratiform clouds.

Results from single simulations and retrospective testing periods using this improved subgrid cloud scheme will be shown. Forecast verification metrics of shortwave flux and cloud ceiling, which have particular importance for renewable energy and aviation applications, will also be presented. Finally, case study results from testing the scheme in a 750-m HRRR nest will be discussed.