Air flow influences on local climate: Comparison of simulations from a regional climate model with observations over the United Kingdom

The relationships between synoptic-scale air flow variability and surface temperature and precipitation simulated by the Hadley Centre’s HadRM2 regional climate model (RCM; horizontal resolution 50 km) were compared with observed relationships for the UK. The aims of the work were to test how well HadRM2 replicated observed relationships between atmospheric circulation and surface climate at a daily scale and to compare the regional model’s performance with the global climate model (GCM) within which the HadRM2 is nested (the HadCM2 GCM). Synoptic-scale air flow variability over the UK was measured on a daily time scale by 3 indices: geostrophic flow strength, vorticity and direction. The frequency distribution of these 3 UK air flow indices is well simulated by HadRM2 in most seasons, with significant differences most noticeable in winter vorticity (the model has a bias to cyclonic vorticity) and in spring flow direction. Although HadCM2 also produced a reasonable simulation of the major frequency distribution features, HadRM2 represents a significant improvement; in 8 of the 12 distributions the simulations were not significantly different from the observations. The relationship between temperature and air flow indices was very well simulated by HadRM2, representing a marked improvement over HadCM2. However, similar relationships for precipitation were slightly less accurate in the RCM than the GCM, the main feature being overestimation of precipitation totals by between 20 and 50% in all seasons in the 2 regions studied. This is possibly due to propagation of GCM biases through the RCM boundary conditions, and may be overcome by running HadRM2 using observed rather than GCM data for the boundary conditions. Overall, the results show that more confidence could be placed in future UK climate change scenarios generated using HadRM2 rather than HadCM2 alone