SIMULATION OF CLIMATE-CHANGE OVER EUROPE USING A NESTED REGIONAL-CLIMATE MODEL .1. ASSESSMENT OF CONTROL CLIMATE, INCLUDING SENSITIVITY TO LOCATION OF LATERAL BOUNDARIES

Present-day climate simulations for Europe are presented, based on a 50 km regional-climate model (RCM) driven by output from a global general-circulation model (GCM) using a one-way nesting approach. Both models are components of the Meteorological office Unified Forecast/Climate Model and use the same subgrid-scale physics. The relationship between the RCM circulation and that of the driving GCM was assessed in seasonal RCM integrations using domains of different sizes. In the larger domains, both the mean flow and the day-to-day variability in the RCM diverge from that of the GCM on the synoptic scale, rendering the RCM solution physically inconsistent with the GCM solution external to the RCM domain. At the grid-point scale the RCM freely generates its own features, even in the smaller domains-only at points adjacent to the boundary buffer zone is there evidence of significant distortion by the lateral boundary forcing from the GCM. Using one of the smaller domains, a 10-year RCM simulation was carried out, driven by a coupled atmosphere/mixed-layer-ocean version of the GCM. Over the region of interest the general circulation and daily synoptic variability is realistically simulated by the GCM and, therefore, also by the RCM (see above). Stronger vertical motions in the RCM lead to a general increase in dynamical precipitation relative to the GCM, and thus a drier and warmer troposphere and reduced convective cloud and precipitation. Layer-cloud cover is also reduced in the RCM, due to a time-step dependence in the treatment of the dissipation of ice cloud. Significant changes occur in the surface heat balance. The spatial patterns of surface air temperature and precipitation over Europe are well simulated by both the GCM and the RCM on scales resolved by the former. Ar finer scales the RCM contains a strong signal which is related to orographic height. Validation against a detailed observed climatology for Great Britain demonstrates that this signal contains considerable skill.