Effects of mountain representation and nonhydrostatic dynamics in a case of orographic precipitation

As the National Weather Service operational regional model moves down scale to 10 km resolution and beyond, sensitivity tests of terrain and dynamics formulation have begun. Parallel tests were made with the NCEP nonhydrostatic Meso model using the step-like mountains and the conventional sigma coordinate mountains in a case of heavy winter orographic precipitation in Northern California. Most of the precipitation was produced by the grid-scale precipitation scheme with the amount produced by the convection being insignificant. The precipitation forecasts were generally reasonable in both tests. However, significant differences were observed on smaller scales. In particular, in the sigma mode the precipitation was generally shifted further up the slopes of Sierra Nevada, which was in better agreement with the verification.

Tests were also made with higher resolution using hydrostatic and nonhydrostatic dynamics. At the resolutions of the order of 10 km the impact of the nonhydrostatic dynamics on the precipitation was generally small. However, noticeable local nonhydrostatic pressure gradients were observed even at that resolution. As expected, the impact of the nonhydrostatic dynamics on the precipitation becomes significant at the resolutions of the order of 1 km.