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  • Hewitson, B.C., and R.G. Crane, 2006: Consensus between GCM climate change projections with empirical downscaling: Precipitation downscaling over South Africa. International Journal of Climatology. In press.

This paper discusses issues that surround the development of empirical downscaling techniques as context for presenting a new approach based on Self-Organizing Maps (SOMs). The technique is applied to the downscaling of daily precipitation over South Africa. SOMs are used to characterize the state of the atmosphere on a localized domain surrounding each target location, based on NCEP 6-hourly reanalysis data from 1979-2002, and using surface and 700 hPa u and v wind vectors and specific and relative humidities, and surface temperature. Each unique atmospheric state is associated with an observed precipitation probability distribution function (PDF). Future climate states are derived from three GCMs: HadAM3, ECHAM4.5, CSIRO Mk2. In each case, the GCM data are mapped to the NCEP SOMs for each target location and a precipitation value is drawn at random from the associated precipitation PDF. The downscaling approach combines the advantages of a direct transfer function and a stochastic weather generator, and provides an indication of the strength of the regional versus stochastic forcing, as well as a measure of stationarity in the atmosphere-precipitation relationship.

The methodology is applied to South Africa. The downscaling reveals a similarity in the projected climate change between the models. Each GCM is projecting similar changes in atmospheric state and they converge on a downscaled solution that points to increased summer rainfall in the interior and eastern part of the country, and a decrease in winter rainfall in the Western Cape. The actual GCM precipitation projections from the three models show large areas of inter-model disagreement, suggesting that the model differences may be due to their precipitation parameterization schemes, rather than to basic disagreements in their projections of the changing atmospheric state over South Africa.

Last Updated: 2006-03-28

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