Evaluation of upper tropospheric water vapor in the NCAR Community Climate Model (CCM3) using modeled and observed HIRS radiances

Abstract

Upper tropospheric water vapor (UTWV) simulated by the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3) is evaluated by comparing modeled, clear-sky, brightness temperatures (BTs) to those observed from space by the High-Resolution Infrared Radiation Sounder (HIRS). The climate model was modified to utilize a highly accurate longwave radiation model (RRTM) and a separate radiance module, which contains physics consistent with RRTM, to calculate BTs in the National Oceanic and Atmospheric Administration (NOAA-7) HIRS 6.7 μm water vapor channel (CH12) and the 14.2 μm temperature channel (CH04). The CCM3 simulations follow the Atmospheric Model Intercomparison Project (AMIP) protocol for the period 1982-1984 to provide a basis for evaluating the water vapor distribution over a wide range of atmospheric conditions. BT differences of 2 K or less in CH04 indicate that CH12 differences are primarily due to water vapor rather than cloud effects or temperature variations. Regionally, CCM3 exhibits considerable positive and negative differences of 5-10 K in CH12 that are apparent in both convective and dry subtropical areas. This suggests that significant moist and dry discrepancies in UTWV amount of 50% or more are present in CCM3. These biases are also shown to persist through several annual cycles and at shorter timescales, and they are only slightly influenced by the improved longwave radiation model, which suggests a dynamical cause. Comparison to HIRS radiances provides a considerably more effective and comprehensive means of globally evaluating general circulation model (GCM)-simulated UTWV on various timescales than surface-based water vapor measurements.