Regional Climate Models' Dynamic Downscaling Ability and Major Factors that Affect this Ability

Despite increases in computer power, most atmospheric general circulation models' (GCM) simulations still use coarse resolutions, and they are aiming to produce large- and synoptic-scale atmospheric features. Because of this limitation, regional climate models (RCM) have been developed and applied for dynamically downscaling GCM simulations or reanalyses at regional or local scales. There are a number of issues concerning the use of the dynamic downscaling method (DDM). The most important issue is whether, and if so, under what conditions the DDM is really capable of adding more information at different scales compared to the GCM or reanalysis that imposes lateral boundary condition (LBC) to the RCMs. Pielke and Castro (2007) have proposed four types of DDM. There are controversies regarding to the downscaling ability. In this presentation we will review the RCM studies using the ETA, WRF/ARW, WRF/NMM, and MM5 over three different regions, among which, North America and East Asia are the midlatitude region and North Africa is the tropical region. It has been found that RCM has downscaling ability in some aspects but only under certain conditions. We will present the results showing the downscaling ability with Pielke's second, third, and fourth types of downscaling methods (i.e., with reanalysis, AGCM, and AOGCM LBCs). The LBCs impose a severe constrains on the DDM's ability to make substantial improvements in some aspects. While the RCMs rarely show the improvement in the upper atmospheric circulation, it does show adding values in some aspects of near surface and surface conditions. The dynamic downscaling ability very much depends on the dynamic and physical representations in the RCMs as well as the setting of model configuration. It has been identified that major factors that affect the DDM ability comprises of land surface processes, including the PBL/surface coupling, convective schemes, lateral boundary positions, LBC quality and coupling, and horizontal resolutions. Any significant weaknesses in one of these aspects would cause a RCM losing its dynamic downscaling ability.