The ansatz behind the dynamical downscaling technique is that a Regional Climate Model (RCM), driven by large-scale atmospheric fields at its lateral boundary, generates fine scales that are dynamically consistent with these. Hence RCM are expected to act as a kind of magnifying glass that reveals details that could not be resolved on a coarse mesh. The small scales represent the main potential added value of high-resolution RCM.

Several issues remain with respect to nested RCM: Are the large scales perfectly replicated, degraded or improved by an RCM? Is lateral-boundary nesting sufficient to control the RCM simulation? What domain size and location should be used for a given application? For a given set of lateral boundary conditions, is the course of an RCM simulation uniquely defined? Almost 20 years after the inception of RCMs, and despite recognition that RCM results are sensitive to the choice of domain and driving technique, these questions have still not been fully answered.

A series of methodical investigations spread over the course of several years have been performed to address these issues in an unambiguous manner, following a strict experimental protocol: the Big-Brother Experiment. The results to date point to the advantage of using rather large domains to permit the full spin-up of small scales, acknowledging however that such configuration permits the intermittent occurrence of large internal variability and divergence in phase space of RCM simulations. Alternative nesting techniques to the traditional imposition of lateral boundary conditions, which allow forcing the large scales throughout the domain, appear to offer certain advantages.