Simulations were performed using MM5 with three two-way nested domains defined with grid spacing of 36, 12 and 4 km, respectively, with the inner domain centered on the Sand Hills. Model physics for all three domains include the MRF PBL scheme and explicit moisture with Reisner ice physics. The outer two domains used the Grell cumulus parameterization; no cumulus parameterization was used for the innermost domain. The OSU/Eta land surface model was used to predict surface energy fluxes and soil temperature and moisture fields. The model was initialized at 0000 UTC of the first day of each selected case using initial and boundary conditions derived from NCEP/NCAR Reanalysis data obtained from CDC. The model was run for 48 hours with new lateral boundary conditions supplied every six hours.
Two simulations were performed for each case using the configuration outlined above. The only difference between the runs was that for the second (experimental) run all grid points with sand soils were changed to a silt loam soil. This replacement was done for all three model domains, but only for those grid points located within the region defined by the innermost domain. Following a four-day spinup period for each simulation, soil moisture values clearly show the Sand Hills as a region of low soil moisture throughout the upper 2 meters in the control run, but when the sandy soils are replaced by a silt loam, soil moisture levels are more consistent with the surrounding region.
For the cases studied, both the control and modified simulations reproduce the observed 2-day total precipitation distribution and amounts quite well, with only small differences between the two simulations. However, analysis of the hourly results leads to several generalizations concerning the differences between the control and experimental run. First, the timing and strength of individual convective cells differs between runs. Second, there is some indication of weakening of cells entering the Sand Hills in the control cases and redevelopment of these cells as they exit the Sand Hills and travel over the surrounding moister surface. Finally, in several of the cases studied, there appears in the control cases to be preferred paths for convection along the Sand Hills' boundaries, where there is a significant soil moisture discontinuity.