Development of a Subgrid-Scale Orographic Drag Parameterization Suite for the NOAA Rap/HRRR Atmospheric Model

The operational 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. We are currently developing a suite of subgrid-scale parameterizations for the models that represent drag forces imparted to the atmosphere by unresolved topography. Such parameterizations are commonly used in coarse-grid models, and we are evaluating their benefits at the finer grid scales of the RAP/HRRR models. The physical processes represented by the parameterizations are gravity wave breaking in the free atmosphere from waves launched by large-scale (>5km) horizontal topographic variations, low-level orographic blocking, gravity wave breaking in stable boundary layers by small-scale (~1km) topographic variations, and turbulent form drag. The parameterizations for the first two processes are adopted from work by colleagues at the National Center for Environmental Prediction (NCEP) within the National Oceanic and Atmospheric Administration (NOAA). They represent processes that are subgrid-scale at grid sizes of 12 km or greater and are only partially subgrid-scale between 12 and 4 km. The other two sources of drag, which result from smaller scale processes, are fully active at coarse resolutions and can be justifiably applied to higher resolutions as well. The goal of our work is to improve low-level wind speeds and related fields within the RAP/HRRR forecasts. We will show preliminary forecast results with the new drag parameterizations and demonstrate their ability to vary in strength with varying grid resolution.