The Effect of Cumulus Friction on the Simulation of the January Hadley Circulation by the GLAS Model of the General Circulation

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Abstract
A simple scheme to parameterize the vertical mixing of horizontal momentum by cumulus convection has been added to the GLAS model of the general circulation. This cumulus friction term had little effect on the simulation of the general circulation, however, until deep, “hot tower” clouds were added to the model's cumulus cloud spectrum. Therefore, two January simulations have been run with the new, extended cumulus spectrum: an experiment with cumulus friction and a control without cumulus friction. Cumulus friction has strengthened the winter Hadley circulation in the experiment by 14% and has smoothed the mean meridional wind field. The zonally averaged, mean westerly shear of the tropics has decreased, and the tropical “bulge” of the jet stream has diminished. Contrary to the results of previous simulations by the GLAS model, which have included a significant amount of vertical momentum mixing, eddy kinetic energy has actually increased slightly in the present experiment. The results of this experiment support the idea that the intensity of the mean meridional circulation is regulated by the atmosphere's angular momentum budget. Changes in the zonally-averaged, mean Coriolis force correlate well with the new cumulus friction term. The intensification of the winter Hadley circulation is a direct response of the mean meridional flow field to the downward cumulus flux of relative angular momentum in the winter hemisphere. The figure obtained for the strengthening of the Hadley cell by cumulus convection is in line with Schneider's (1977) results. The stronger Hadley circulation and most of the structural changes of the mean meridional wind field render the GLAS simulation closer to Oort and Rasmusson's (1970, 1971) observations. A more realistic simulation of the mean meridional wind field, however, will require a more realistic mean zonal wind field as well as an improved cumulus parameterization scheme which realistically predicts very deep clouds and which makes full use of the available vertical resolution of the GLAS model.

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