LES simulation of synoptic, mechanic-forcing, and thermally-driven flow interaction of Granite Mountain, UT

The NCAR-ATEC (National Center for Atmospheric Research and Army test and Evaluation Command) RTFDDA-LES (real-time four-dimensional data assimilation and LES simulation) model was employed to study multi-scale flow interactions at Granite Mountain and its surrounding areas. Granite Mountain is a locally erected mountain peak with an area of ~6x10 km2 located in the US Army Dugway Providing Ground, UT. The mountain sits in the midwest inter-mountains, but is surrounded by relatively flat terrain nearby. The Granite Peak is about 700m above the surrounding flat terrain. It significantly affects the flows in the DPG test area and often greatly impacts on the DPG test activities. The area has been selected by mountain terrain atmospheric modeling and observations (MATERHORN) program as a test bed for improving Meteorological Modeling in Mountain Terrain. In this paper, RTFDDA-LES was employed to study the multiple-scale flow interaction of synoptic, mechanic forcing, and thermally driven flows of Granite Mountain. Six nested-grid domains with grid sizes of 8100, 2700, 900, 300, 100, and 33m, respectively, were configured and a 48h simulation was carried out simultaneously on the six nested grids for a two-day period during Spring 2012. The data assimilation of RTFDDA was turned on for the mesoscale domains (1, 2 and 3), while the LES domains (4, 5 and 6) were run with 'free forecasting'. The mesoscale data assimilation on the coarse meshes provide realistic mesoscale forcing for the LES simulation, so that the model outputs of the LES domains can be reasonably verified using high-resolution (every 1 - 5 minutes) measurements of DPG surface stations and multi-level met-tower in the vicinity of Granite Mountain. The model successfully simulates the overall flow evolutions during the two-day period and also many features of microscale flows for different time periods of the day (with different thermally-forcing and boundary layer stability) and under varying larger-scale driven weather (with different wind speed, direction, and vertical shear). The encouraging verification results show that RTFDDA-LES model is capable for future microscale NWP operation. A real time RTFDDA-VLES modeling system has been implemented at DPG for assessment and experimental operational use. Comparison of the model results on different domains apparently indicates the advantage and greater value of the simulation on the intermediate domains (Dom 4 and 5, at very large eddy simulation (VLES) scales) over the coarse-resolution domains (1, 2 and 3).