General Circulation of Air Parcels and Transport Characteristics Derived from a Hemispheric GCM

[摘要] A Lagrangian-type description of transport mechanisms of tracers in the stratosphere and troposphere is presented on the basis of trajectory analysis of a large number of marked air parcels using a hemispheric GCM. It is demonstrated that the transport of the lower stratospheric air is due both to slow advective general circulation and to large-scale quasihorizontal eddy diffusion, while the transports within the troposphere are highly diffusive both in the horizontal and vertical directions. The present simulation shows that the tropospheric air parcels are well mixed within one month throughout the troposphere by quasihorizontal mixing due to cyclones and vertical mixing due to small-scale convection. It is also shown that the stratospheric air can be supplied mainly from the tropical tropopause. Only the bottom layer (below∼100mb) of the extratropical stratosphere may be intruded by tropospheric air parcels through the subtropical tropopause gap. However, such intrusion can hardly affect the levels above the bottom layer.In this paper, the age of the stratospheric air is analyzed; the age of an air parcel is defined as the length of time elapsed since it entered the stratosphere. The results show that the tropical stratosphere consists of 'new' air parcels, which is thought to retain properties of the tropospheric air, while the polar stratosphere involves relatively 'old' air parcels which have long-term residence in the stratosphere. This interpretation may be useful to understand major features of distributions of tracers originating either the surface or the stratosphere.

[学科分类] 大气科学