Assessment of the nested grid model estimates for driving regional visibility models in the southwestern United States

The Nested Grid Model (NGM) is a primitive-equation meteorological model th at is routinely exercised over North America for forecasting purposes by th e National Meteorological Center. While prognostic meteorological models ar e being increasingly used to drive air quality models, their use in conduct ing annual simulations requires significant resources. NGM estimates of win d fields and other meteorological variables provide an attractive alternati ve since they are typically archived and readily available for an entire ye ar. Preliminary evaluation of NGM winds during the summer of 1992 for appli cation to the region surrounding the Grand Canyon National Park showed seri ous shortcomings. The NGM winds along the borders between California, Arizo na and Mexico tend to be northwesterly with a speed of about 6 m/sec, while the observed flow is predominantly southerly at about 2-5 m/sec. The mesos cale effect of a thermal low pressure area over the highly heated Southern California and western Arizona deserts does not appear to be represented by the NGM because of its coarse resolution and the use of sparse observation s in that region. Tracer simulations and statistical evaluation against spe cial high resolution observations of winds in the southwest United States c learly demonstrate the northwest bias in NGM winds and its adverse effect o n predictions of an air quality model. The "enhanced" NGM winds, in which s elected wind observations are incorporated in the NGM winds using a diagnos tic meteorological model provide additional confirmation on the primary cau se of the northwest bias. This study has demonstrated that in situations wh ere limited resources prevent the use of prognostic meteorological models, previously archived coarse resolution wind fields in which additional obser vations are incorporated to correct known biases provide an attractive opti on.