A Comparison of GCM-simulated and Observed Mean January and July Surface Air Temperature

Abstract

January and July surface air temperature fields simulated by the GFDI, OSU, GISS, and UKMO general circulation models (GCMS) are compared to the global surface air temperature climatology compiled by Legates and Willmott. Legates and Willmott's climatology was selected as the verification standard because it provides better spatial and temporal coverage than its predecessors, such as the frequently employed RAND climatology compiled in the early 1970s. Difference maps between each GCM-simulated field and the Legates and Willmott climatology are presented and evaluated. Zonal averages by 10° latitudinal bands for each GCM as well as for the Legates and Willmott and RAND climatologies also are examined. Results indicate that surface air temperature simulations are greatly influenced by model representations of topography, sea level pressure, and precipitation. Inclusion of the diurnal cycle and the type of ocean model used also impact simulated surface air temperatures. Mean January and July surface air temperatures are well simulated by the GISS and UKMO models, whereas temperatures are overestimated by the OSU GCM and underestimated by the GFDL GCM. GISS and UKMO simulations seem even more accurate, on the average, than the data contained in the RAND observation-based climatology. Simulated equatorial air temperatures are slightly higher than observed, particularly in the Southern Hemisphere. Model simulated air temperatures between 30°S and 60°S are usually lower than observed, while air temperatures poleward of 60°S are overestimated. Northern Hemisphere temperatures are generally better simulated than their Southern Hemisphere counterparts.