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EXPERIMENTS WITH A STRATOSPHERIC GENERAL CIRCULATION MODEL

II. LARGE-SCALE DIFFUSION OF TRACERS IN THE STRATOSPHERE

BARRIE G. HUNT and SYUKURO MANABE

Abstract

The 18-level primitive equation, general circulation model described in Part I was used to study the diffusion of two idealized tracers in the stratosphere. One tracer was designed to simulate broadly the behavior of the radioactive tungsten which escaped into the stratosphere following nuclear tests in the Tropics, the other was taken as a photochemical ozone distribution. Both the meridional circulation and the large-scale eddies were found to be important in the diffusion of the tracers, and for quasi-steady state conditions they formed a highly interrelated system in which their actions were mutually canceling. The large-scale eddies were of primary importance for the polewards transport of the tracers in middle and high latitudes, but the supply of tracer for these eddies was principally maintained from the higher levels by the downward branches of the meridional circulation. Two meridional cells were found to occur in the stratosphere, a tropical direct cell and a higher latitude indirect cell, and these provided a natural explanation for many of the observed features of the tracer distributions in the actual atmosphere. The only major tropospheric-stratospheric exchange took place in the subtropics through the tropopause gap, the vertical eddies and the meridional circulation being of comparable magnitude for this exchange.

The synoptic situation in the atmosphere was found to be of fundamental importance for the large-scale diffusion of the tracers in middle latitudes, and the downgradient transport of tracers in the lower stratosphere was primarily accomplished by the upper level troughs of the planetary scale wave system.

Although the model used in this investigation was based on radiative conditions corresponding to annual mean insolation it appeared to be representative of winter conditions, and was in agreement with many observational features.

Schematic diagrams illustrating the principal features of the large-scale diffusion of the two tracers are given in figures 12 and 24.

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SYUKURO MANABE and BARRIE G. HUNT

Abstract

An 18-vertical level primitive equation general circulation model was developed from previous models of the Geophysical Fluid Dynamics Laboratory in order to study the 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 was used and the model was 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 3-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, as is the basic cause of the difference in height of the tropopause at 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.

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