MESOSCALE MODELING AND 4-DIMENSIONAL DATA ASSIMILATION IN AREAS OF HIGHLY COMPLEX TERRAIN

A multiscale four-dimensional data assimilation (FDDA) technique, base d on Newtonian relaxation, is incorporated into a mesoscale model and evaluated using meteorological and tracer data collected during the At mospheric Studies in Complex Terrain (ASCOT) field experiment in the w inter of 1991. The mesoscale model is used to predict the synoptically driven flows and small-scale circulations influenced by terrain along the Front Range in Colorado in the vicinity of Rocky Flats Plant for four nocturnal periods during the ASCOT field experiment. Data assimil ation is used to create dynamically consistent analysis fields based o n the mesoscale forecasts and the special asynoptic data taken during this experiment. Observations from towers, minisodars, airsondes, teth ersondes, rawinsondes, and profilers near the Rocky Flats Plant, as we ll as observations from surface stations throughout Colorado, are inco rporated into the high-resolution analysis fields. The wind and turbul ence quantities produced by the mesoscale model are then used to deter mine the dispersion of tracer released from the Rocky Flats Plant for each evening. A subjective and statistical evaluation of the meteorolo gical and dispersion results is performed to examine the Effect of FDD A on the nocturnal circulations and tracer transport. The mesoscale mo del is able to qualitatively predict the mesobeta-scale drainage flows from the Front Range into the South Platte River basin; however, the largest wind forecast errors occurred in a region immediately adjacent to the foothills. As expected, the current FDDA technique reduced the overall errors in the atmospheric and dispersion calculations, while the model generated realistic small-scale circulations not resolved by the data. Still, the model did not capture the shallow surface draina ge flows just east of the Rocky Flats Plant for two of the evenings du ring the field experiment. When the model was initialized with the hig h-resolution analysis fields generated by FDDA and left to forecast on its own, little improvement in the forecasts were seen two hours afte r the initialization time. This may be due to the fact that only the o bserved horizontal wind components were assimilated into the analyses generated by FDDA; assimilation of temperature observations was not in cluded in this study.