Some Properties of a Low-order Model of Atmospheric Circulation

Numerical experiments are performed with a highly simplified model of the general circulation of the atmosphere in an attempt to gain insight into the physical causes of the surface zonal wind distribution (and associated momentum convergences) observed in the atmosphere. With the zonal wind constrained to be of the form =i2 A 5 +3r2,1jr s (O. 5'Ti) capable of non-linear interactions with a single wave in the x (zonal) direction in a two layer quasi-geostrophic model, numerical integrations are performed for various magnitudes of the thermal forcing (of the form rf. 'Ccos i ) and the rotation rate. Four broad categories of flow are observed in the results: a Hadley circulation for sufficiently low rotation or sufficiently high or low thermal forcing; a west to east motion of the wave without change of shape; a "vacillation" showing periodic changes in the shape of the wave; and an irregular flow. Further subdivision of the results is made on the basis of whether the surface frictionally influenced lower layer shows an "earthlike" character in the zonal wind i.e. mid-latitude westerlies flanked by easterly winds, or the reverse. The introduction of the " i effect" into the dynamics of the wave alters the pattern of the locations of the different flow types on the thermal forcing-rotation rate plane and alters the qualitative appearance of the flows moderately, mainly by inducing a greater concentration of zonally averaged momentum in the central regions of the flow. For those regions of the thermal forcing-rotation plane where the flow is not changed from one broad category to another the induced changes are not very great in magnitude, however. The momentum convergence structure of those flows which do not exhibit severe violation of the quasi-geostrophic assumption, generally those for moderate and low thermal forcing and moderate and high rotation rates, are examined in some detail. It is found that many of the salient characteristics of the atmospheric mean state, i.e. eddy momentum transports into the central regions of the flow balanced by divergence due to a thermally indirect mean meridional cell there, a central maximum in the westerly flow in the upper layer and prevailing westerlies flanked by -easterly flow in the frictionally influenced lower layer, are reproduced in the model flows but that no single case captures all of these characteristics at once. Some of the flows do show a majority of the characteristics and show the highly simplified model to be capable of a reasonably good representation of the general circulation within its self imposed limitations. From these results certain conclusions are drawn on the nature of atmospheric flow; in particular a single baroclinic wave by its nature incorporates sufficient dynamics to induce the zonal wind structure seen in the atmosphere, or that the dynamic, effect of the many waves of the atmosphere upon the large scale zonal structure may be considered as equivalent to the effect of a single wave. Thesis Superviser: Prof. Edward N. Lorenz Title: Professor of Meteorology