Convection Parameterization, Tropical Pacific Double ITCZ, and Upper-Ocean Biases in the NCAR CCSM3. Part I: Climatology and Atmospheric Feedback

The role of convection parameterization in the formation of double ITCZ and associated upper-ocean biases in the NCAR Community Climate System Model, version 3 (CCSM3) is investigated by comparing the simulations using the original and revised Zhang-McFarlane (ZM) convection schemes. Ten-year model climatologies show that the simulation with the original ZM scheme produces a typical double ITCZ bias, whereas all biases related to the spurious double ITCZ and overly strong cold tongue in precipitation, sea surface temperature (SST), wind stress, ocean thermocline, upper-ocean currents, temperature, and salinity are dramatically reduced when the revised ZM scheme is used. These results demonstrate that convection parameterization plays a critical role in the formation of double ITCZ bias in the CCSM3. To understand the physical mechanisms through which the modifications of the convection scheme in the atmospheric model alleviate the double ITCZ bias in the CCSM3, the authors investigate the impacts of convection schemes on the atmospheric forcing and feedback in the uncoupled Community Atmospheric Model, version 3 (CAM3). It is shown that the CAM3 simulation with the original ZM scheme also produces a signature of double ITCZ bias in precipitation, whereas the simulation with the revised ZM scheme does not. Diagnostic analyses have identified three factors on the atmospheric side (i.e., the sensitivity of convection to SST, the convection-shortwave flux-SST feedback, and the convection-wind-evaporation-SST feedback) that may contribute to the differences in the coupled simulations.