Influence of Grid Size and Terrain Resolution on Wind Field Predictions from an Operational Mesoscale Model

Abstract

One of the activities of the National Oceanic and Atmospheric Administration's Air Resources Laboratory is to predict the consequences of atmospheric releases of radioactivity and other potentially harmful materials. This paper describes the application of the Regional Atmospheric Modeling System (RAMS) to support air quality forecasting. The utility of using RAMS for real time prediction of local-scale flows and for detailed postevent analysis is examined for a Nuclear Regulatory Commission exercise at the Susquehanna nuclear power plant in Pennsylvania. During the exercise (10 December 1992) a strong East Coast low pressure system created complex interactions between the regional-scale and local topographical features of the Susquehanna River valley. Results from a series of sensitivity experiments indicated significant topographical forcing and vertical de-coupling although the synoptic forcing was quite strong in this relatively wide and shallow valley. The best agreement between the RAMS predictions and observations was obtained with horizontal and vertical resolutions of 2.5 km and at 12 m above ground level for the first vertical wind level, respectively. Therefore, it would have been very difficult to configure RAMS to predict the local circulations in real time, given the very high resolution requirements. The vertical resolution needed to properly resolve terrain forcings in the Susquehanna Valley was similar to vertical resolution used by other researchers over steeper and narrower valleys. However, the horizontal resolution requirements were not as critical: about 10 times coarser than in more complex terrain. The degree of topographical smoothing was also found to have a significant effect upon the predictions. Experiments performed by assimilating all available surface-level winds in the model domain with various degrees of nudging slightly improved the simulation of the low-level winds. Subsequent analyses indicated that pressure-driven channeling and downward momentum mixing were the primary physical mechanisms for this case.