Optimising predictor domains for spatially coherent precipitation downscaling

Statistical downscaling is widely used to over- come the scale gap between predictors from numerical weather prediction models or global circulation models and predictands like local precipitation, required for example for medium-term operational forecasts or climate change impact studies. The predictors are considered over a given spatial domain which is rarely optimised with respect to the target predictand location. In this study, an extended version of the growing rectangular domain algorithm is proposed to provide an ensemble of near-optimum predictor domains for a statis- tical downscaling method. This algorithm is applied to find five-member ensembles of near-optimum geopotential pre- dictor domains for an analogue downscaling method for 608 individual target zones covering France. Results first show that very similar downscaling performances based on the continuous ranked probability score (CRPS) can be achieved by different predictor domains for any specific target zone, demonstrating the need for considering alternative domains in this context of high equifinality. A second result is the large diversity of optimised predictor domains over the coun- try that questions the commonly made hypothesis of a com- mon predictor domain for large areas. The domain centres are mainly distributed following the geographical location of the target location, but there are apparent differences between the windward and the lee side of mountain ridges. Moreover, do- mains for target zones located in southeastern France are cen- tred more east and south than the ones for target locations on the same longitude. The size of the optimised domains tends to be larger in the southeastern part of the country, while do- mains with a very small meridional extent can be found in an east-west band around 47 N. Sensitivity experiments finally show that results are rather insensitive to the starting point of the optimisation algorithm except for zones located in the transition area north of this east-west band. Results also ap- pear generally robust with respect to the archive length con- sidered for the analogue method, except for zones with high interannual variability like in the Cevennes area. This study paves the way for defining regions with homogeneous geopo- tential predictor domains for precipitation downscaling over France, and therefore de facto ensuring the spatial coherence required for hydrological applications.