The Nebraska Sand Hills are one of the largest grass-stabilized dune areas in the world. Even though the Sand Hills are generally perceived as a dry environment, they play an important role in the regional hydrology as a significant source of both groundwater recharge for the High Plains aquifer and surface flow in the Missouri Basin. With a climate varying from subhumid in the east to semiarid in its western portion, the Sand Hills support a unique mix of vegetation that changes across its breadth. Furthermore, the sandy soils of the Sand Hills, with their high infiltration rates, form a clear contrast to the soils in the cropped areas to the south. In these regions, clay soils with low hydraulic conductivity and slow infiltration rate predominate. The resulting spatial heterogeneity, both within the Sand Hills and with its surroundings, presents a challenge to understanding the role of land-atmosphere interactions and their seasonal variations on the region's weather and its hydrological cycle.

Preliminary investigations at the Omaha/Valley WFO have shown that the Sand Hills act 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, a physical mechanism for this phenomenon was not determined. 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 could cause rising motion and create a mesoscale pressure gradient towards the Sand Hills.

In order to allow analysis of the full range of the hydrologic cycle, representative wet, dry and average May, June and July (MJJ) periods were identified and three individual precipitation events were selected from each period. These events were chosen to represent 1) a synoptic event, 2) a convective event, and 3) a null event for the Sand Hills. This latter category is best described as a situation for which there was extensive precipitation in the region around the Sand Hills, but there was little or no precipitation over most of the Sand Hills.

The most recent version of MM5 includes a detailed land-surface model that represents a significant improvement in how MM5 computes surface fluxes and allows 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. Land-surface characteristics (e.g., topography, vegetation cover, soil type) are varied individually during successive simulations. Modeled surface fluxes and near surface temperature and moisture fields are compared between control runs and runs with altered surface properties. These sensitivity runs yield estimates of the influence of the Sand Hills' surface characteristics on the atmosphere.