Ensemble simulations of regional flood and drought climates

Ensemble simulations are performed for the 1993 drought and 1988 flood over the central U.S. using the RegCM2 nested regional climate model. Each ensemble consists of 20 simulations structured as a 4x5 matrix produced by varying two closure constants in the cumulus parameterization. Ensemble members are 60-day simulations over the continental U.S. driven at their lateral boundaries by data from the NCEP/NCAR reanalysis. Precipitation predictions from the ensembles are evaluated for a target region over the upper Mississippi River basin using a variety of statistics including decomposition of mean square error and relative operating characteristic (i.e., tradeoff between probability of detection versus false alarm ratio).

Results indicate that for both the drought and flood, mean square error is dominated by spatial position error. Domain-mean bias is only very a small contributor to mean square error, especially for the flood period. The relative operating characteristic shows that the ensemble has increased value compared with the control forecast.