The Nebraska Sand Hills are a unique part of the Missouri River Basin that can be expected to exert an influence on the local and regional atmospheric conditions. There has been little research to date that has focused on land surface-atmosphere interactions in this environment. Preliminary investigations at the Omaha/Valley WFO have shown that the Sand Hills acts as an orographic uplift area to initiate afternoon convection in the summer (S. Byrd, pers. comm., 1999). This was determined by analyzing surface wind fields just before, during and after convective events. However, the physical mechanism for this phenomenon was not determined. Since the highly permeable soils of the top layer of the Sand Hills quickly dry following precipitation events, contrasts between the Sand Hills and the surrounding plains may initiate a regional circulation in this part of the Great Plains. In the warm season, higher daily temperatures over the Sand Hills can cause rising motion and create a mesoscale pressure gradient towards the Sand Hills. This rising motion draws air into the Sand Hills region from regions to the south. In summer months, the low level jet has a diverted branch flowing into central Nebraska, very likely because of this heating effect of the Sand Hills. This southerly and southeasterly low level inflow of moist air to the region may result in enhanced precipitation over and around the Sand Hills. This enhancement is suggested by frequent severe storms, including squall lines and tornadoes, at preferred locations along the southern boundary of the Sand Hills. Land surface-atmosphere interactions in the Sand Hills are investigated through the use of sensitivity analyses with MM5. These interactions are investigated by comparing modeled surface fluxes and near surface temperature and moisture fields between Sand Hills locations and the surrounding plains and between control runs and runs with altered surface properties.

The most recent version of MM5 includes a detailed land-surface model that represents a significant improvement in how MM5 computes surface fluxes and will allow more realistic investigations of the effects of spatial variations or temporal changes of land surface characteristics on the atmosphere. Specifically, soil hydrology is now included explicitly, which vastly improves the utility of MM5 for hydrologic modeling. Soil type in the Sand Hills was altered during successive simulations with initial and boundary conditions derived from selected warm season precipitation events. These sensitivity runs yield estimates of the influence of the Sand Hills' soil characteristics on the atmosphere.