A numerical model of the zonally averaged dynamical and chemical structure of the middle atmosphere

Abstract

A two‐dimensional, time‐dependent model has been constructed to study the zonally averaged structure of the middle atmosphere (16–116 km) allowing interaction among dynamics, radiation and photochemistry. The zonally averaged dynamics are governed by a stream function equation for the residual Eulerian meridional circulation wherein the effects of wave transience and dissipation have been neglected. The resulting circulation is thus driven by diabatic heating and cooling, with Rayleigh friction introduced to balance the momentum budget. The temperature structure is computed from the zonal mean thermodynamic equation, while the appropriate continuity equations are solved to determine the distribution of the various chemical species and families. In the chemical continuity equations, two types of eddy transport processes are present in addition to the transport by the residual Eulerian circulation. Small‐scale disturbances are assumed to produce turbulent mixing which is modeled in terms of horizontal and vertical diffusion coefficients, while steady state planetary waves give rise to fluxes which are expressed in terms of observed wave structure and the photochemical lifetime of the species transported. The circulation, temperature structure, and distribution of chemical constituents obtained with the model are for the most part in satisfactory agreement with observations. In particular, the direct, equator‐to‐pole circulation computed for the stratosphere seems to be capable of explaining the major features of the distribution of trace constituents there, although the effect of the Fickian diffusion included in the model is important in determining the magnitude of gradients.

A diabatic circulation experiment in a two‐dimensional photochemical model

“…For the present study we have chosen to specify the 2-D transport fields at monthly intervals and to neglect feedback processes caused by changes in the distributions of radiatively active species. A number of attempts have been made to include such a coupling [e.g., Garcia and Solomon, 1983;Harwood and Pyle, 1975;Vupputuri, 1978], but the problem of incorporating the details of transport feedbacks in a two-dimensional approximation, especially when the eddy contributions become large in comparison to the mean circulation, has not been completely resolved.…”

Calculation of a residual mean meridional circulation for a zonal-mean tracer transport model: Revision 1

“…and is also found in many papers (Garcia and Solomon, 1983;Solomon et al, 1986;Brasseur et al, 1990, Garcia et al, 1992. Brasseur et al (1990) called ty2 as the "velocity" stream function contrast to the "mass" stream function of y,.…”

“…The four equations from (1) to (4) what are given by Garcia and Solomon (1983) who used the temperature instead of potential temperature in the thermodynamic energy equation.…”

“…Likewise we can test y2 by the same method, which gives us which becomes identical to (C7) only when y2 = 0. The third stream function y3 is identical to Ti,, used in Garcia and Solomon (1983) and Solomon et al (1986). Garcia and Solomon (1983) noted that "isopleths of Zzms coincide with the streamlines of the velocity field in the meridional plane".…”

Isentropic Diffusion Coefficient Derived from Chemical Constituent Data

“…The adiabatic component of the eddy diffusion coefficient, Kyy, reflects wave motions of the atmosphere since the eddy transport is almost parallel to the isentropic surface, particularly in the stratosphere. Kyy has also been used to parameterize the Rossby wave mixing in two-dimensional chemical transport models (CTMs, e.g., Garcia and Solomon 1983). Kyy is generally diagnosed using the potential vorticity (PV) flux (Newman et al 1988) since the PV acts as a conservative passive tracer for adiabatic and frictionless flow.…”