ANALYSIS OF VARIABILITY AND DIURNAL RANGE OF DAILY TEMPERATURE IN A NESTED REGIONAL CLIMATE MODEL - COMPARISON WITH OBSERVATIONS AND DOUBLED CO2 RESULTS

Analysis of daily variability of temperature in climate model experime nts is important as a model diagnostic and for determination of how su ch variability may change under perturbed climate conditions. The latt er could be important from a climate impacts perspective. We analyze d aily mean, diurnal range and variability of surface air temperature in two continuous 3 1/2 year long climate simulations over the continent al USA, one for present day conditions and one for conditions under do ubled carbon dioxide concentration, conducted with a regional climate model (RegCM), on a 60 km grid, nested in a general circulation model (GCM). Model output is compared with a 30-year daily observational dat a set for various regions of the USA. In comparison with observations the diurnal range in the model control run is somewhat too low althoug h the daily temperature mean is often well reproduced. The daily varia bility of temperature is underestimated by the model in all areas, but particularly when and where the observed variability is relatively hi gh. Causes for these underestimations are traced to deficiencies in th e general circulation of the driving GCM. With: doubled CO2, both maxi mum and minimum temperatures increase, but the change in the diurnal t emperature range (DTR) varies spatially and seasonally. On an annual a verage over the land domain, the DTR decreases by 0.25 degrees C. Chan ges in DTR are most strongly correlated with changes in absorbed short wave radiation at the surface, which explains 72% of the variance in D TR on an annual basis. Change in evaporation was factor affecting DTR only in the summer when it explained 52% of the variance. The most sig nificant findings with CO2 doubling are substantial decreases in daily variability in winter over large portions of the domain, and localize d increases in summer. Causes for these changes are traced to fluctuat ions in the intensity and position of the jet stream.