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I I I I I I I I I I I I I I I I I I I show the amounts and the locations where precipitation will tend to fall given specific wind trajectories and topography. Another objective of the study was to provide a spatial representation of the convection/orographic ratio for the mountain areas. In this study the convection component was held constant. The spatial distribution of the orographic component is shown as isohyet contours for equal precipitation values mapped from a 10 by 10 kilometer grid for eight points of the compass. Model Components The Rhea model simplifies atmospheric dynamics and lumps preCIpitation causes into three categories. The first is large-scale vertical motion (large-scale low pressure lifting or high pressure subsidence), the second is convective motion and the third is the orographic component (lifting and descent associated with air passing over mountains and plateaus). This study held the large-scale vertical motion and convective motion parameters constant. The orographic component was evalu- ated by varying wind directions and computing resulting precipitation patterns and point precipitation amounts. Model Initialization To evaluate the effect of topography on atmospheric motion, topography was set up on a 10 kilometer grid having 51 rows and 48 columns. The grid center was at 40 degrees north Latitude and 111.5 degrees west Longitude, near Thistle, Utah. Terrain values (elevations) were derived from a digital terrain data set. A plot of 1,000 foot contour intelVals is presented in Figure 11. The overlay found on the back cover may be used for orientation. The projection is a tangential plane with radial distances determined by great circle navigation. Upper air data were based on the upper air sounding station at Salt Lake City, Utah. Other stations located at Lander, Wyoming; Denver, Colorado; Desert Rock, Nevada; and Winslow, Arizona, were considered but not used because of their distance and direction from the Basin. Problems The Rhea model does have some inherent problems. Some of the problems we dealt with in the model resulted from the use of the 10 kilometer grid spacing. The large grid spacing tended to smooth areas in mountainous terrain that would have been better selVed by a more dense grid. It is our understanding that the model will operate with a 2.5 kilometer grid spacing. At this time we are not, however, equipped to operate a grid of that density. There were four major problems encountered in using the Rhea model for this study. Underestima- tion of point data in a low area surrounded by higher ridges is probably the result of the sparse gdd. Underestimation of calculated precipitation data for isolated peaks and terrain funneling are also probably associated the grid density. Underestimation of precipitation for ridges aligned with wind vectors mayor may not be associated with the grid density. Underestimation of precipitation data when using observed winds that were light and variable was also a weakness of the model. 2U