NAM News


2b. Mesoscale Modeling Branch:
Geoff Dimego reported lots of work on solving problems with the NAM forecasts. The NAMX parallel is focusing on damping the external mode to improve balance and reduce incorrect adjustment away from the analysis state. The NAMY parallel is testing changes in shallow and deep convection, divergence damping, and diffusion. Work is ongoing to reformulate the horizontal diffusion to be effectively calculated on isobaric surfaces instead of sigma surfaces so that restricting its application in areas of steep terrain slope would not be necessary – a physically more sound and more universal fix than some of the summer crisis changes. Work is also being done to adopt the global model formulation for form drag and momentum drag, which are absent in the NMM. The Eta had form drag based on subgrid-scale terrain variance and wind direction, but NMM only had a simple dependence on terrain variance which was ultimately removed in the NAM WRF-NMM because it was causing near-surface winds to be too light over mountainous terrain. It is hoped that the global model treatment will improve trough speed and depth because mountain wave momentum drag had that effect in the global model. Also, new datasets need to be tested for implementation, including AIRS and MODIS data, along with an increase in the vertical resolution of the radiative transfer solver inside the assimilation.