Effect Of Soil Moisture and Vegetation Heterogeneity on a Great Plains Dryline: ANumerical Study

摘要

The impact of heterogeneous soil moisture and vegetation fields on a Great Plainsdryline are examined through the use of a mesoscale numerical model. A three dimensional, non-hydrostatic, nested grid version of the Colorado State University Regional Atmospheric Modeling System (RAMS) was used to perform five simulations of an actual dryline which was observed as part of the COPS-91 field experiment on 15 May 1991. A control run which was 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 section 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). Variable vegetation was input using the Biosphere-Atmosphere Transfer Scheme (BATS) and included 18 categories of land surface type derived from the USGS NDVI dataset. Because the standard RAMS code modifies the surface fluxes by the Leaf Area Index (LAI), the LAI was limited to a value of 3.0 or less in the flux calculations for all vegetation types although the BATS scheme assigns values of LAI as high as 6.0. Justification for this limitation is presented.