A 3-DIMENSIONAL NUMERICAL-SIMULATION OF A GREAT-PLAINS DRYLINE

A three-dimensional, nonhydrostatic, nested grid version of the Colora do State University Regional Atmospheric Modeling System (RAMS) was us ed to perform simulations of an actual dryline that was observed as pa rt of the COPS-91 field experiment on 15 May 1991. A control run desig ned to reproduce the observed conditions as accurately as possible was generated and verified against standard National Weather Service obse rvations, PAM-II observations, M-CLASS soundings, and vertical cross-s ectional analyses obtained from the NOAA P-3 aircraft. A representativ e heterogeneous soil moisture field for use in the control simulation was generated using an antecedent precipitation index (API). Represent ative vegetation coverage based on the USGS normalized difference vege tation index (NDVI) dataset was input into the model. An additional si mulation using a homogeneous soil moisture field is compared to the co ntrol run. Results of study indicate that the use of realistic heterog eneous soil moisture and vegetation may be extremely important for acc urate prediction of dryline formation and morphology. The effect of va riable soil moisture appears to be first order, with large impacts on the strength of the thermal and moisture gradients along the dryline, as well as its position, structure, and movement.