PNNL

Abstract

This study presents, to our knowledge, the first intercomparison and evaluation of three state-of-the-art mesoscale numerical models, MM5, RAMS, and Meso Eta at horizontal resolution finer than 1 km. Simulations were carried out for both weak and strong synoptic forcing cases during the Vertical Transport and MiXing (VTMX) field campaign conducted in the Salt Lake Valley in October 2000. Both upper air and surface observations at high spatial and temporal resolution were used to evaluate the simulations with a focus on boundary layer structures and thermally driven circulations that developed in the valley. Despite differences in the coordinate systems, numerical algorithms, and physical parameterizations used by the three models, the nature of the forecast errors was surprisingly similar. The common errors in predicted valley temperature structure include a cold bias extending from the surface to the top of the valley atmosphere, lower than observed mixed layer depths when the observed mixed layers were relatively high, and much weaker nocturnal inversion strengths over the valley floor. The developments of valley, slope, and canyon flows and their diurnal reversals under weak synoptic forcing were better captured by RAMS and MM5 than by Meso Eta. The latter model consistently under-predicted the strengths of these terrain-induced circulations and the associated convergence and divergence over the valley floor. As operational mesoscale models move towards sub-kilometer resolution in the near future, more detailed forecasts of the circulation patterns and boundary layer structure can be produced for local-scale applications. However, this study shows that relatively large forecast errors can still exist even with sufficiently fine spatial resolution, indicating that the future for accurate local forecasting still lies in improved model parameterization.