A series of 90-day 18 ka BP and O ka BP simulations were conducted to allow estimates to be made of the general climate and of the effect of feedbacks between the lakes and the atmosphere. Initial and lateral boundary conditions (SSTs and vertical profiles of temperature, humidity, wind velocity) for the simulations were obtained from 90 days of output from the paleosimulations conducted by Kutzbach and Guetter (1986) with the NCAR Community Climate Model (CCM0). The surface of RegCM included estimated distributions of 18 ka BP vegetation, montane glaciers, the continental ice sheet, and the area and depth of Lakes Bonneville and Lahontan. SSTs were fixed at the values used by Kutzbach and Guetter.
Results indicate that over the west 18 ka BP January air temperatures were about 2oK colder than the 0 ka BP control, whereas 18 ka BP July temperatures were >5oK colder than the July control. January precipitation over the region was substantially greater at 18 ka BP; however, 18 ka BP July precipitation was not much different than 0 ka BP, except around Lakes Bonneville and Lahontan.
Analyses indicate that the hydrologic "signal" associated with the position and strength of the 18 ka BP jet stream dominate the hydrologic budgets of the Bonneville and Lahontan basins. Lake-atmosphere feedbacks isolated over the Bonneville basin indicate that lake-effect precipitation is a substantial component of the hydrologic budget in both January and July; however, such feedbacks are minor components of the hydrologic budget of Lake Lahontan. These results may help to explain the relative difference in the sizes of the lakes at 18 ka BP.
The modeling system used in this simulation of 18 ka BP climate can readily be applied to studies of pre-Pleistocene climates to assess, for example, the effects of uplift of the Rocky mountains and Colorado Plateau or the growth and influence of large paleolakes such as "Lake Idaho". Application of the model to the Pliocene depends on our ability to provide meaningful boundary conditions (e.g., sea level and SSTs, topography, inland water, atmospheric composition). Global modeling now being conducted at the Goddard Institute for Space Science and NCAR ultimately will result in a "consensus" Pliocene climate simulation that could be used to provide initial and lateral boundary atmospheric conditions for the regional model. Because estimates of Pliocene features such as topography and the extent of inland water are likely to remain uncertain, the appropriate use of the regional model would be to conduct a series of sensitivity tests by varying, for example, model topography to provide a range of output with which to compare with geologic evidence. Comparisons with the data would allow further refinements of the boundary conditions in the model, and thus allow some degree of convergence between the geologic data and simulated climate. In this way, it may be possible to establish plausible Pliocene climatic conditions at a regional scale.