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SUMMARYPresent-day climate simulations for Europe are presented, based on a SO 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. At 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.

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Results are assessed from a 10‐year simulation of the equilibrium response to doubled carbon dioxide (CO2) over Europe made with a nested high‐resolution regional‐climate model (RCM). the simulated changes are compared against those produced by the driving global general‐circulation model (GCM). the domain‐averaged increases in temperature and moisture content are similar in both models. Because of a stronger hydrological cycle the increases in precipitation and evaporation are larger in the RCM than in the GCM, whereas the reductions in lower and middle tropospheric relative humidity and cloud cover are smaller. The frequency of intense precipitation events increases substantially in both models; however, the fractional changes are significantly smaller in the RCM. the proportion of precipitation associated with convection also increases in both models; however, much of the increase in intense events is explained simply by increased atmospheric moisture concentrations, especially in the RCM. The time‐averaged precipitation changes in the RCM contain a substantial mesoscale component on scales not resolved by the GCM. Attempts are made to reconstruct this component from the changes in the large‐scale atmospheric circulation using empirical relationships calibrated from the 1 x CO2 integration. These are largely unsuccessful, indicating that simple downscaling schemes to generate high‐resolution scenarios of climate change from coarse‐grid GCM data may be of only limited applicability. Further statistical calculations suggest that longer integrations (∼ 30 years) are needed to reduce the sampling uncertainty associated with the simulated mesoscale component to an acceptable level. The large‐scale patterns of change of surface temperature and precipitation reveal significant regional contrasts which are influenced both by changes in atmospheric circulation and regional physical feedbacks. the RCM changes are similar to those of the driving GCM except in summer. the differences in the summer changes are traced to differences between the 1 x CO2 integrations, in which the influence of the lateral boundary forcing on the RCM simulation is found to be anomalously weak. It is argued that any development of significant divergence between the RCM and GCM solutions on scales resolved by the latter may imply the need to refine or replace the one‐way nesting technique currently used in many regional modelling experiments.

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