A diabatic circulation experiment in a two‐dimensional photochemical model

Abstract

A two-dimensional photochemical model based on diabatic circulation has been used to simulate the behavior of N20, CFC13 (F-11), and CF2Cl• (F-12). The circulation is based on estimates of net heating from the ground to 60 kin. Eddy diffusion has been reduced with respect to other model studies with Kzz --2 x 103 cm • s-• everywhere above 100 mbar. Resulting tracer profiles show reasonable agreement with measured profiles in the tropics and fall off much more sharply with altitude than those produced by models using larger values of Kzz. The agreement obtained is at least as good as that obtained with adjustable, eddy diffusion parameters. The diabatic circulation treatment is more closely related to real physical processes and thus more easily interpreted. Diffusive mixing appears to be more important in determining the details of the tracer distributions than the basic morphology.

On the distributions of long‐lived tracers and chlorine species in the middle atmosphere

“…The differences between calculations and observations for these species are somewhat larger than those shown above for methane and nitrous oxide, particularly in the tropics. It has been noted [e.g., Gidel et al, 1983;Guthrie et al, 1984] that this may indicate that the photolysis rates for these chlorocarbons are underestimated in present chemical models.…”

“…Woodman and Rastogi [1984] have derived similar small vertical diffusivities using radar techniques. Guthrie et al [1984] and M. K. W. Ko et al (unpublished manuscript, 1984) have shown that the distribution of N20 calculated in diabatic and isentropic circulation models using these eddy coefficients as crude parameterizations of mixing processes is in good agreement with observations.…”

“…In this paper we use the residual Eulerian model described by Garcia and Solomon [1983] and Solomon and Garcia [1983] to explore the effects of using small eddy mixing coefficients in the stratosphere (103 cm 2 s-1 in the vertical and 3 x 109 cm 2 s-• in the horizontal). Like Guthrie et al [1984] and M. K. W. Ko et al (unpublished manuscript, 1984), we find that the distributions of chemical tracers such as N20, CF3C1, CF2C12, and CH,• are in much better agreement with observations than those obtained in the previous version of our model, which made use of substantially larger vertical diffusion coefficients for the middle and upper stratosphere. In our previous studies we employed a Kyy of 1 x 109 cm 2 s-•, but our K:: increased from 4 x 103 cm 2 s-• at 100 mbar to about 1 x 105 near the stratopause.…”

A diabatic circulation two‐dimensional model with photochemistry: Simulations of ozone and long‐lived tracers with surface sources

“…lation models included high diffusion [Prabhakara, 1963;Pyle and Rogers, 1980a;Miller et al, 1981], adopting diffusion coefficients similar to those usually used in the Eulerian models [Reed and German, 1965;Luther, 1973]. More recent diabatic models [Ko et al, 1985;Solomon and Garcia, 1984;Guthrie et al, 1984] have successfully simulated the distributions of ground-released long-lived constituents, e.g., N20, CH,•, and CF2C12 using slow diffusion, Kyy • 1-3 x 10 9 cm 2 s -1 and K:: • 10 3 cm 2 s -1. These model estimates indicate that the transport of long-lived tracers with sources at the ground can be described by the diabatic circulation combined with relatively small eddy diffusion.…”

On the sensitivity of a residual circulation model to differences in input temperature data

“…The RMC is then obtained as in the diabatic case but with the differenced temperature tendency subtracted from the heating rate. The temperature and circulations are used to drive a limited-chemistry model [Guthrie et al, 1984] in which N20 is used as a tracer. The cases were each run for ten model years and achieved repeating annual cycles in the tracer distributions.…”