Downscaling Regional Circulation Model rainfall to gauge sites using recorrelation and circulation pattern dependent quantile–quantile transforms for quantifying climate change

This paper maps out a methodology developed for downscaling Regional Circulation Model (RCM) estimates of rainfall to networks of gauge sites. We have produced, from an RCM, a set of gauge records which (i) captures the changes modelled by the RCM and (ii) has the statistics of the observations in the control period after suitable transformation. These statistics include the marginal distributions, the wet/dry sequences and the spatial dependence structure of rainfall. We have been careful to eliminate the RCM’s inherent bias, yet preserve its message of the past and possible future. The key issue is the ability of the method to transfer information from the RCM scale to the gauge scale (in networks), to serve planning, engineering and agricultural practitioners who are comfortable with such a product, in contrast to the raw RCM precipitation output. We exploit atmospheric Circulation Patterns (CPs) to condition the downscaling. Our main goal is to address the following issues which are germane to the downscaling process: • Scale: there is a considerable difference between frequency distribution functions (fdfs) and cross correlation coefficients (cccs) at the gauge and RCM grid-block scale; we show that precipitation distributions, as well as spatial correlations, are not reproduced well by the RCMs. • Information: A picture, or a take on the ‘future’, is available from RCMs. The problem is: how do we go about transferring the model signal to the gauge scale, concomitantly removing the bias? • CPs: We recorrelate the spatial dependence of the downscaled estimates at the gauge scale then we correct the fdfs at the gauge scale conditioned on CPs. • Check: We devise methods of transferring the RCM signal (fdfs and cccs) to gauge networks, both in a historical ten year period for verifying that the procedure is effective and importantly in a second nine year period for validating the procedure on unused data. The downscaling of RCM rainfall to gauge sites was achieved in five regions in South Africa. These gauge rainfall estimates may be used directly in design and planning scenarios, even under Climate Change, using this methodology.