Monthly Webher Review

An 18-vertical level primitive equation general circulation model was developed from previous models of the Geophysical Fluid Dynamics Laboratory in order to study t,he lower stratosphere in detail. The altitude range covered was from the surface to 4 mb. (37.5 km.), the vertical resolution being optimized in the tropopause region to permit a more accurate calculation of the vertical transport terms. A polar stereographic projection waa used and the model waa limited to a single hemisphere. The model now resolves two distinct jet streams, one in the troposphere and the other in the middle polar stratosphere. The wind systems produce a %cell meridional structure in the troposphere, which evolves into a 2-cell structure in the stratosphere. However, the wind structure and associated features of the model in the troposphere had a general equatorward shift compared with observation. A considerable improvement was also obtained in some features of the temperature distribution, in particular the local midlatitude temperature maximum in the lower stratosphere is well defined and shown to be dynamically maintained. The low temperature and sharpness of the equatorial tropopause temperature distribution are closely reproduced by the model, and these features are attributed to the action of the upwards branch of the direct meridional cell in the Tropics, aa is the basic cause of the difference in height of the tropopause a t low and high latitudes. The energy balance of the lower stratosphere in the present model agrees better with observation than previous models did, and confirms earlier work that this region is maintained from the troposphere by a vertical flux of energy. A similar flux of energy is also required to maintain the middle stratosphere, even though this region generates kinetic energy internally, and it is concluded that it is only marginally possible that this region may be baroclinically unstable. It appears that forcing from below extends to higher altitudes in winter than previously suspected.