Downscaling general circulation model output: a review of methods and limitations

Abstract

General circulation models (GCMs) suggest that rising concentrations of greenhouse gases may have significant consequences for the global climate. What is less clear is the extent to which local (subgrid) scale meteorological processes will be affected. So-called 'downscaling' techniques have subsequently emerged as a means of bridging the gap between what climate modellers are currently able to provide and what impact assessors require. This article reviews the present generation of downscaling 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 precipitation models used for downscaling. It shows that circulation-based downscaling methods perform well in simulating present observed and model-generated daily precipitation characteristics, but are able to capture only part of the daily precipitation variability changes associated with model-derived changes in climate. The final section examines a number of ongoing challenges to the future development of climate downscaling. I The rationale for downscalingEven if global climate models in the future are run at high resolution there will remain the need to 'downscale' the results from such models to individual sites or localities for impact studies. Downscaling methodologies are still under development and more work needs to be done in intercomparing these methodologies and quantifying the accuracy of such methods (DOE, 1996: 34).The present generation of general circulation models (GCMs) of the climate system are restricted in their usefulness for many subgrid scale applications by their coarse spatial Source: Modified after Hostetler (1994) and temporal resolution (Wigley et al., 1990;Carter et al., 1994). For example, hydrological models are frequently concerned with small, subcatchment (even hillslope) scale processes, occurring on spatial scales much smaller than those resolved in GCMs (see Figure 1). GCMs deal most proficiently with fluid dynamics at the continental scale and parameterize regional and smaller-scale processes. These scale-related sensitivities and mismatch problems are further exacerbated because they usually involve the most uncertain components of climate models, water vapour and cloud feedback effects (Rind et al., 1992). As Hostetler (1994) has observed, the greatest errors in the parameterizations of both GCMs and hydrological models occur on the scale(s) at which climate and terrestrial impact models interface. These mismatch problems, which affect both the temporal and spatial dimensions, have important implications for the credence of impact studies derived by the output of models of climate change, especially as research into potential human-induced modifications to hydrological and ecological cycles is assuming increasing significance. Downloaded from 532 change. A major focus of the BAHC (Biological Aspects of the Hydrol...

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