Statistical downscaling of monthly forecasts

Canonical correlation analysis (CCA) is used to downscale large-scale circu lation forecasts by the Centre for Ocean-Land-Atmosphere studies (COLA) T30 general circulation model (GCM) statistically to regional rainfall in Sout h Africa. Monthly GCM ensemble forecasts available from 1979 to 1995 have b een generated using NCEP reanalysis data as initial input and globally obse rved sea-surface temperature (SST) data at the lower boundary. Altogether, 51 30-day cases of CCM simulations, spanning 17 years, within the target se ason of December-February (DJF), are produced. This period is very importan t for agriculture and maize, in particular. A model output statistics (MOS) procedure is used to downscale GCM forecast sea-level pressure and 500 hPa height fields to regional rainfall for 30-day periods over South Africa. T he CCA model is trained on the first 31 cases (up to February 1989) and for ecasts are subsequently made for the remaining 20 cases. These retro-active real-time forecasts have a high potential (correlations > 0.5) over most o f the interior of South Africa and, furthermore, the prediction of extreme events seems feasible. CCA diagnostics of the GCM-output against rainfall r eveal that favourable rainfall over most of the interior is associated with low pressure systems at the surface over the west coast, with an associate d ridging high. This is supported by other observational studies. Copyright (C) 2000 Royal Meteorological Society.