A study on parametrization of orography-related momentum fluxes in a synoptic-scale NWP model

In a synoptic-scale atmospheric model, winds and subgrid-scale momentum fluxes are influenced by the properties of the underlying surface. Parametrization schemes have been developed to describe the interactions of the air flow with the rough surface and small- and mesoscale orography. The schemes interact with each other and with the model’s dynamical processes. Newparametrizations were introduced to the High Resolution Limited Area Model (HIRLAM), to account for the subgrid-scale orography effects. Needed orography parameters were derived from a high-resolution digital elevation data set. A detailed analysis of the momentum fluxes and kinetic energy budget helped to understand the interactions between parametrizations. Mean and root mean square error, averaged over all observations and forecasts, did not reveal significant differences between the updated and reference model results. However, more detailed diagnosis of the forecast-observation differences allowed to show that the new parametrizations of mesoscale and small-scale orography lead to more realistic low-level wind distribution.