DOWNSCALING GENERAL-CIRCULATION MODEL OUTPUT - A REVIEW OF METHODS AND LIMITATIONS

General circulation models (GCMs) suggest that rising concentrations o f greenhouse gases may have significant consequences for the global cl imate. What is less clear is the extent to which local (subgrid) scale meteorological processes will be affected. So-called 'downscaling' te chniques have subsequently emerged as a means of bridging the gap betw een what climate modellers are currently able to provide and what impa ct assessors require. This article reviews the present generation of d ownscaling tools under four main headings: regression methods; weather pattern (circulation)-based approaches; stochastic weather generators ; and limited-area climate models. The penultimate section summarizes the results of an international experiment to intercompare several pre cipitation models used for downscaling. It shows that circulation-base d downscaling methods perform well in simulating present observed and model-generated daily precipitation characteristics, but are able to c apture only part of the daily precipitation variability changes associ ated with model-derived changes in climate. The final section examines a number of ongoing challenges to the future development of climate d ownscaling.