A growing body of literature is increasingly establishing that high resolution regional climate models driven by time-dependent forecast lateral boundary conditions from a coupled GCM can be used to successfully downscale climate simulations generated from relatively coarse resolution global models. To examine seasonal climate predictability using these regional models, in this study we developed and tested a high resolution Regional Climate Model (RCM). The model was based on the NCEP operational Eta model (as of 24 July, 2001, and also the version in the NCEP 25-year Regional Reanalysis), with changes made to make the model run over a longer time period and to update the Sea Surface Temperature (SST), sea ice, greenness fraction, and albedo fields on the daily basis. The model was run on the same large domain as does the operational Eta model with a resolution of 32 km and 45 levels, as used in the Regional Reanalysis. Presently, the model can be executed off of the NCEP Global Reanalysis I and II.

To test the skill of the Eta RCM in predicting warm season precipitation anomalies, two summertime cases were chosen, as recommended by the North American Monsoon Experiment (NAME), representative of both wet and dry years in the northwest Mexico region. Most previous studies of RCM seasonal simulation driven by analysis lateral boundary conditions and observed SST employed only "1-member" executions from one single initial condition date. In contrast, we executed 5 members whose initial conditions vary by one day (May 27, 28, 29, 30, 31). The study period is from June to September and the executions were started from late May and continued to early October. In both cases, ensemble mean of total precipitation for each month and 500 and 200 mb heights were examined, where the monthly mean precipitation was compared to the CPC unified daily precipitation data, and the 200 and 500 mb geopotential heights were compared to the NCEP Reanalysis data, respectively. As part of this study, we also test the impact of initial land states on seasonal precipitation predictions. To do this, we tested a suite of combinations of initial land states and boundary forcing from the NCEP global reanalysis I with II. The impact resulted from using different land states was examined.

The resulting ensemble mean shows that the Eta RCM successfully simulates the dry bias in the total precipitation over the northwest Mexico region in 1991 and has substantial member-to-member variability of seasonal precipitation. This suggests that previous RCM studies that employed only one member may be misleading by failing to represent the inherent internal variability. Comparison of results obtained from using different sources of land states indicates that the Eta RCM is sensitive to the initial land surface conditions and shows a great variability in the simulated monthly and total precipitation, suggesting that a careful initialization of land states is important to seasonal predictions.