The bias correction of the outputs from RCMs: Comparison of advanced Delta change method with other approaches

As the outputs from regional climate models (RCMs) have to be adjusted and further downscaled to be used, e.g., in hydrology impact studies, the two fundamental approaches are usually implemented. These two approaches differ in how the bias of a climate output is dealt with. The first approach (delta change method) gets the information on the climate signal from a comparison of the model control and future periods. Such information (change factor) is then applied to modify the observational data. The second possibility is to get information on model bias by comparing observational data and model outputs for the control period. This approach is called bias correction, and it uses the information on the bias (correction factor) of the model output to adjust the outputs from the climate model for a future period. Both approaches are based on assumptions that cannot be verified: the stationarity of bias (bias correction) and/or independence of the changes on bias (delta change method).

Here, we present the results of the application of different types of Bias correction and Delta change methods on precipitation data outputs from the model ALADIN-CLIMATE/CZ (CNRM-ESM2-1) for the area of the domain of the model implemented in the project PERUN (roughly the area of the Czech Republic, PERUN domain) in daily time step. The grid of the PERUN domain contains more than 29K points with a spatial resolution of 2.3 km. The applied delta change method is the advanced delta change method. In contrast with the classical delta change method, which reflects only the changes in mean of observations and the model outputs, the advanced delta change method considers changes in means and variability. Along with the advanced delta change method, we tested the performance of several types of bias correction methods (methods based on statistical distribution, parametric transformations, and non-parametric empirical quantiles method). The climate model outputs differ significantly from the precipitation observations, mainly in the mountainous areas. All the tested methods bring the climate model outputs towards the precipitation observations. However, the use of a specific method depends on the specifics of the given impact study.