THE SENSITIVITY OF A GENERAL-CIRCULATION MODEL CLIMATE TO THE MOISTURE TRANSPORT FORMULATION

The sensitivity of the climate of the NCAR Community Climate Model to the numerical method used to transport water vapor is described. We compare two versions of the model, one of which uses a spectral method and the other a shape-preserving semi-Lagrangian method for the water vapor transport. These methods are very different in terms of their computational properties and produce very different climatologies in model simulations. These differences in climate result primarily from the redistribution of water vapor in the model. The redistribution carries with it differences in convection and cloud distributions and the associated radiative heating, which in turn affects the model climate. We interpret the differences using a sensitivity study perspective. Some of the changes produced by changing from spectral to semi-Lagrangian transport are beneficial (that is, the model climate is closer to observations), and some are detrimental. We believe that many of the detrimental changes may be explained in terms of prior tuning of the physical parameterizations to the "spectral climatology" and may also be due to inadequate or missing physics. We examine many of the pertinent feedback loops and diagnose the processes associated with the changed climate.