POLAR RADIATION BUDGETS OF THE NCAR CCM3

Present-day Arctic and Antarctic radiation budgets of the National Cen ter for Atmospheric Research Community Climate Model version 3. (CCM3) are presented. The CCM3 simulation is from a prescribed and interannu ally varying sea surface temperature integration from January 1979 thr ough August 1993. Earth Radiation Budget Experiment (ERBE) data from 1 985 through 1989 are used for validation of top-of-atmosphere (TOA) ab sorbed shortwave radiation (ASR) and outgoing longwave radiation (OLR) . Summer ASR in both polar regions is less than the observations by ab out 20 W m(-2). While the annual mean OLR in both polar regions is onl y 2-3 W m(-2) less than the ERBE data, the seasonal amplitude in OLR o f 40 W m(-2) is smaller than the observed of 55-60 W m(-2). The annual polar TOA radiation balance is smaller than observations by 5-10 W m( -2). Compared to selected model and observational surface data, downwa rd shortwave (SW) is too small by 50-70 W m(-2) and downward longwave (LW) too large by 10-30 W m(-2). Surface downward LW in clear atmosphe res is too small by 10-20 W m(-2). The absence of sea-ice melt ponds r esults in 10-20 W m(-2) too much SW absorption during early summer and from 20 to 40 W m(-2) too little during late summer. Summer cloud cov ers are reasonably well simulated, but winter low cloud cover is too h igh by 0.5-0.7 compared to surface cloud observations. Comparison with limited satellite and in situ observations indicates cloud water path (CWP) is too high by about a factor of 2. While cloud particle sizes are approximately in the range of observed values, regional variation between maritime and continental droplet sizes is too strong over coas tlines. Despite several improvements in CCM3 radiation physics, the ac curacy of polar TOA annual radiation balance is degraded against the E RBE data compared to CCM2. Improvement in CCM3 polar radiation budgets will require improved simulation of CWP clear sky LW, and sea ice alb edo.