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

Abstract

An advective motion in the lower stratosphere is determined calculating trajectories of a large number of air parcels in a quasi-equilibrium general circulation obtained from a long-term integration of a GCM over the hemispheric domain. A new method is developed for deducing the advective motion of air parcels, i.e., the non-divergent part of the Lagrangianmean motion; the method is to calculate the usual Lagrangian-mean motion of air parcels both forward and backward in time, and then to take the average of the two. It is clearly shown that the advective motions of air mass in the lower stratosphere form one cell with an upward branch at the tropics and a downward branch in high latitudes, being quite different from the usual zonal-mean meridional circulations having direct and indirect cells over the tropical and extratropical regions, respectively. The regions of the upward and downward motions coincide very well with those of the radiative heating and cooling, respectively. This suggests a balance between the adiabatic heating/cooling due to the vertical motion of air parcels and the diabatic heating/cooling. From the dispersion rate of a large number of air parcels initially located along a latitude circle at the same interval of longitude, eddy diffusion coefficients are estimated as -3×109cm2/s in the horizontal and -l×103cm2/s in the vertical. These values are one order or more smaller than those employed in most of 2-D transport models. The results seem to be reasonable in recent theories of Lagrangian dynamics of planetary waves. However, note that the horizontal diffusion as well as the horizontal advection plays an important role in poleward movements of air parcels in the lower stratosphere. Furthermore, a new formulation of a 2-D transport model based on the advective motion and diffusion of air parcels is briefly discussed.