A globally balanced two-dimensional middle atmosphere model: Dynamical studies of mesopause meridional circulation and stratosphere-mesosphere exchange

A globally balanced two-dimensional middle atmosphere model has been developed. The dynamical fields are obtained by numerically solving two elliptic equations for the mass stream function of the residual circulation and the geopotential field. By comparison with exact solutions, it is demonstrated that this model is more accurate than two-dimensional models in which the nonlinear advection equations are solved numerically. General features of the dynamical fields from the model and a focused study of meridional circulation in the mesopause region are presented. The simulated zonal mean temperature and zonal wind are consistent with the CIRA (1986) model atmosphere and the direct wind measurements from the high-resolution Doppler imager (HRDI) and wind imaging interferometer (WINDII) instruments on board the Upper Atmosphere Research Satellite (UARS). The predicted meridional component of the residual circulation <(upsilon)over bar>* around 92 km is approximately 25 m s(-1) with maximum near the summer mesopause. In the mesopause region the salient features in the UARS data are peak zonal mean meridional velocities of 20-30 m s(-1) and substantial latitudinal gradients from equator to midlatitude maxima. The consistency between the model and the direct measurements supports the hypothesis that the mesospheric circulation is forced by gravity wave induced drag. The model tracer fields indicate the existence of a mesospheric ''depletion zone'' and ''tracer front'' similar to the ''surf zone'' and ''barrier'' in the lower stratosphere. The depletion zone and tracer front are generated by divergent and convergent <(upsilon)over bar>* fields analogous to tropospheric frontogenesis by a deformation field. In addition, the model shows that the most active region for the stratosphere-mesosphere mass exchange is at the summer polar stratopause. The active exchange is driven by a strong summer mesospheric drag force through the ''downward control'' principle.