Simulation Experiments with a 12-Layer Stratospheric Global Circulation Model. II. Momentum Balance and Energetics in the Stratosphere

Abstract

We discuss momentum balance and energetics in the stratosphere (18?36 km) based on the simulated mean January results described in Part I. The latitudinal distributions of component terms in the momentum budget equation clearly demonstrate the two-cell structure of the mean meridional circulation in the stratosphere as opposed to the three-cell structure in the troposphere. In contrast to the troposphere, the eddy and mean transports of momentum are equally important in the stratosphere in all latitudes with some tendency for the eddy transport to counteract the mean transport. The latitudinal distributions of component terms in the energy budget equations suggest the following mechanism for maintaining the zonal and eddy kinetic energies. The vertical flux of wave energy through the lower boundary of the stratosphere provides a major source of the eddy kinetic energy in the stratosphere. On the other hand, the zonal kinetic energy in the stratosphere appears to be maintained by energy conversion from the eddy kinetic energy against the energy loss by frictional dissipation. The present results of zonal and eddy kinetic energy budgets are compared with those based on real data of the Northern Hemisphere by various investigators and those of a numerical simulation experiment by Manabe and Hunt. The zonal internal energy budget shows that the zonal internal energy in the stratosphere is maintained by the supply of sensible heat from the troposphere and energy conversion from the eddy kinetic energy against the loss of energy by radiative cooling and the downward transport of internal energy by mean vertical motion. In addition, we find that interpretation of the energetics of the stratosphere depends upon how various energy terms are combined and energy conversion terms formulated. This special consideration is needed because the vertical transports of energy at the interface between the troposphere and the stratosphere play important roles in the energetics of the stratosphere.