Land surface temperature and radiative fluxes response of the NCAR CCM2/Biosphere‐Atmosphere Transfer Scheme to modifications in the optical properties of clouds

Abstract

Climate simulations of the National Center for Atmospheric Research (NCAR) community climate model version 2 (CCM2) are compared with several data sets. These data sets are a multiyear climatology of the Earth Radiation Budget Experiment (ERBE) top‐of‐the‐atmosphere (TOA) radiative fluxes, the International Satellite Cloud Climatology Project (ISCCP) cloudiness, and the Surface Radiation Budget (SRB) Project surface insolation. The comparison focuses on global and regional spatial scales 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 little 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 according to ISCCP data (by as much as 10%). These biases are evidenced at the land surface through too warm (by up to 15°C) summer hemisphere 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 in much closer agreement with the ERBE and ISCCP data and with midlatitude summer surface temperatures. This study demonstrates a large sensitivity of summer surface temperatures to cloud optical properties.