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Show Steve Hostetler USGS 200 SW 35th St. Corvallis, OR 97333 MODELING THE PALEOCLIMATE AND PALEOHYDROLOGY OF THE GREAT BASIN The climate of the Great Basin displays spatially and temporally heterogeneous variations over all time scales. These variations are related to physiography and the variety of possible source regions from which air masses moving across the basin originate. In order to unravel the paleohydrology of the Great Basin, we thus need to gain an understanding of how past climatic controls affected the surface hydrologic budget, which means we must rely on models. Simulations of global paleoclimate are conducted with general circulation models of the atmosphere ( and ocean in climate system models). GCMs are used to simulate equilibrium paleoclimates in response to prescribed, large- scale boundary conditions including insolation, continental ice sheets, atmospheric composition, and possibly sea surface temperatures. The coarse grid spacing ( hundreds of kilometers) currently being used in GCMs, however, limits the detail with which regional- scale atmospheric features are simulated. This lack of detail means mesoscale circulations that occur at scales on the order of 10- 150 km, such as those induced by orographic lifting and blocking and by thermal interactions with lakes and glaciers, are not explicitly resolved by GCMs. As a result, mesoscale climates that may explain a large measure of the heterogeneity in geologic records are likely to be absent in GCM output. A mesoscale or regional climate model ( RCM) resloves mechanisms such as orographic lifting, blocking, and channeling which are primary controls of precipitation, air temperature and thus surface hydrology. These models are run over a limited area of interest at spatial resolutions on the order of tens of kilometers. Boundary conditions for an RCM simulation can be obtained directly from the output of a GCM simulation. In this manner, large- scale atmospheric features, such as the strength and location of the polar jet stream, are preserved and used to force high resolution simulations of climate within the domain of the RCM. Although the output from an RCM run at 50 or 60 km resolution still does not capture local ( i. e., order of 1 to 10 km) climate features that may be important for hydrologic systems, the higher resolution does yield climate parameters that reflect regional- scale influences. If finer detail climate is needed, the output from an RCM simulation conducted at 50 or 60 km can be used as boundary conditions for a yet higher resolution simulation over a more limited area. Hydrologic models can be coupled interactively with the RCM, or output from RCM simulations can be used in stand- alone models to simulate hydrologic processes. Physically based lake models, for example, implement equations of heat transport, including diffusion and mixing, and thereby simulate a set of |
