GENERAL CIRCULATION OF THE LOWER STRATOSPHERE 397 and 30 mb. The results indicated a three-cell structure with equatorward drift at middle latitudes, but the values were too large to satisfy the momentum budget. Dickinson” has considered the mean meridional motions as being a forced circulation produced by the overall imbalances of other terms in the momentum and the energy budgets. He has estimated the mean meridional motions from the equation for the conservation of momentum alone and finds a three-cell structure similar to that found by Oort. Reproduction of such large values as those observed by Oort required a coefficient of eddy friction in the vertical direction of 10° cm?/sec at 50 mb; this value is certainly too large. It will be recalled that values of 10* to 10°, depending on latitude and season, have been put forward for the lower stratosphere (Friend et al.)** based on the spread of '®5w. Such values represent the vertical diffusion produced by all scales of motion combined, and it is possible that the large-scale motions produce essentially all the diffusion, in which case the appropriate value remaining to be ascribed to friction is zero. Another of our colleagues, P. Gilman, has also given much thought to the problems of mean meridional motion’’~?* considered as forced by heat and momentum sources and sinks and has showntheoretically that a three-cell circulation is a necessary consequenceof conditions in the lowest 20 km of the atmosphere. For present purposes let us consider diabatic heating by radiation and by convergence of horizontal-eddy heat fluxes as the only important heat-forcing functions and convergence of horizontal momentum eddy fluxes as the only important momentum-forcing function. Thusfriction, vertical flux of heat by eddies, and vertical flux of momentum by eddies are ignored. Clark*®® has demonstrated that the latter is the case for the stratosphere (100 to 30 mb). From our data the heating due to convergence of vertical-eddy heat fluxes is also small compared with the diabatic heating in the lower stratosphere. Estimates of the vertical-eddy heat flux in the troposphere are not very reliable. There are no real observations of friction. Thus the approximations made are partly a matter of expediency. With these assumptions the equa- tions for the mean meridional motion given by Gilman and others simplify to 1 TG “Tet TZ aap 9u/ap T =— H=& 1 8 pa * Te ye + [V*T*]) a cos @ ag (lV’T’] cos —