Abstract : 2E.19
Orographic parametrizations in HIRLAM

Laura Rontu
Finnish Meteorological Institute

Scale-dependent orography-related parametrizations are being introduced to the High Resolution Limited Area Model (HIRLAM). The parametrizations comprise schemes for handling the effects of mesoscale (MSO) and small-scale (SSO) orographic effects on the simulated flow, and a scheme of orographic effects on the surface-level radiation fluxes. We summarize the recent developments of these parametrizations. Main attention is paid to the parametrization of subgrid-scale momentum fluxes. Representation of orography, scale-dependencies of the simulated processes and interactions between the parametrized and resolved processes are discussed. From the high-resolution digital elevation data, orographic parameters are derived for the subgrid-scale momentum and radiation flux schemes and for the definition of the model's vertical coordinate.

Orography is always averaged, representing the surface elevation within the horizontal resolution of the model. In order to remove the smallest scales and steepest slopes, the continuous spectrum of orography is normally filtered (truncated) even more, typically beyond a few gridlengths of the model. This means that in the numerical weather prediction (NWP) models, there will always be subgrid-scale orography effects, which cannot be explicitly resolved by numerical integration of the basic equations, but require parametrization. In the subgrid-scale, different physical processes contribute in different scales. The parametrized processes interact with the resolved-scale processes and with each other. Advanced diagnostic tools are required for understanding these processes and interactions.