A Three-Dimensional Numerical Simulation of a Great Plains Dryline

A three-dimensional, nonhydrostatic, nested grid version of the Colorado State University Regional Atmospheric Modeling System (RAMS) was used to perform simulations of an actual dryline that was observed as part of the COPS-91 field experiment on 15 May 1991. A control run designed to reproduce the observed conditions as accurately as possible was generated and verified against standard National Weather Service observations, PAM-II observations, M-CLASS soundings, and vertical cross-sectional analyses obtained from the NOAA P-3 aircraft. A representative heterogeneous soil moisture field for use in the control simulation was generated using an antecedent precipitation index (API). Representative vegetation coverage based on the USGS normalized difference vegetation index (NDVI) dataset was input into the model. An additional simulation using a homogeneous soil moisture field is compared to the control run. Results of study indicate that the use of realistic heterogeneous soil moisture and vegetation may be extremely important for accurate prediction of dryline formation and morphology. The effect of variable 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.

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