LAND-SURFACE TEMPERATURE AND RADIATIVE FLUXES RESPONSE OF THE NCAR CCM2 BIOSPHERE-ATMOSPHERE TRANSFER SCHEME TO MODIFICATIONS IN THE PROPERTIES OF CLOUDS

Climate simulations of the National Center for Atmospheric Research (N CAR) community climate model version 2 (CCM2) are compared with severa l data sets. These data sets are a multiyear climatology of the Earth Radiation Budget Experiment (ERBE) top-of-the-atmosphere (TOA) radiati ve fluxes, the International Satellite Cloud Climatology Project (ISCC P) cloudiness, and the Surface Radiation Budget (SRB) Project surface insolation. The comparison focuses on global and regional spatial scal es and the seasonal timescale. In the control simulation, the overall pattern appears reasonable, but some serious discrepancies are found. In particular, in the midlatitude summer, the model reflects too littl e solar radiation according to ERBE data (by as much as 50-100 W m(-2) ), has surface solar fluxes that are too large according to SRB data ( by as much as 40-80 W m(-2)), and is low in fractional cloudiness acco rding to ISCCP data (by as much as 10%). These biases are evidenced at the land surface through too warm (by up to 15 degrees C) summer hemi sphere continents. A revised computational scheme of the cloud optical properties is introduced in the solar and longwave radiative transfer parameterizations of CCM2. A new simulation with these revisions is i n much closer agreement with the ERBE and ISCCP data and with midlatit ude summer surface temperatures. This study demonstrates a large sensi tivity of summer surface temperatures to cloud optical properties.