Air flow influences on local climate: observed and simulated mean relationships for the United Kingdom

Synoptic-scale air flow variability over the United Kingdom is measured on a daily time scale by following previous work to define 3 indices: geostrop hic flow strength, vorticity and direction. Comparing the observed distribu tion of air flow index values with those determined from a simulation with the Hadley Centre's global climate model (HadCM2) identifies some minor sys tematic biases in the model's synoptic circulation but demonstrates that th e major features are well simulated. The relationship between temperature a nd precipitation from parts of the United Kingdom and these air flow indice s (either singly or in pairs) is found to be very similar in both the obser vations and model output; indeed the simulated and observed precipitation r elationships are found to be almost interchangeable in a quantitative sense . These encouraging results imply that some reliability can be assumed for single grid-box and regional output from this climate model; this applies o nly to those grid boxes evaluated here (which do not have high or complex o rography), only to the portion of variability that is controlled by synopti c air flow variations, and only to those surface variables considered here (temperature and precipitation).