Warm Season Precipitation Prediction over North American using the Eta Regional Climate Model

To examine climate predictability on seasonal time scales (3-6 month forecasts) using regional climate models, in this study we continue our advancement and testing of a high resolution (32-km), Eta model-based Regional Climate Model (Eta RCM). The model is an adaptation of the NCEP operational Eta NWP model as of 24 July, 2001 (which is the Eta model version used in the NCEP 25-year Regional Reanalysis), with changes made to make the configuration of the model execution consistent with the longer time scales of seasonal forecasts, including daily updates to the fields of sea surface temperature (observed or predicted), sea ice cover (observed or predicted), green vegetation cover (climatology), and albedo (climatology). To test the skill of the Eta RCM in predicting warm-season anomalies of precipitation, the 2004 summer case was chosen, when the North American Monsoon (NAM) Field Experiment was held. In contrast to many previous RCM studies driven by analysis lateral boundary conditions and initialized from one single date (i.e., one member realization), we used an ensemble approach in both a simulation mode, using observed SSTs and analyzed boundary conditions from the NCEP Global Reanalysis II, and a seasonal prediction mode, using the predicted SSTs and lateral boundary conditions from the latest NCEP Coupled Forecast System (CFS). The Eta RCM was run from three different initial dates starting from early May. Our focus in this study is the seasonal predictability of summer season precipitation and to what extent the Eta RCM predictability does or does not show improved skill or value-added attributes via downscaling, relative to the driving CFS global model ensemble predictions.

We examine the resulting ensemble mean and individual members to demonstrate a) whether the Eta RCM successfully simulates/predicts total precipitation over the U.S. by using the regional climate modeling approach compared to the original CFS runs and NAMAP observations, and b) the predictability in terms of the choice of both SSTs and lateral boundary conditions. The results show that the Eta RCM has notable sensitivity to the choice of SSTs and lateral boundary conditions, with substantial member-to-member variability.