EXPERIMENTS WITH A STRATOSPHERIC GENERAL CIRCULATION MODEL III. LARGE-SCALE DIFFUSION OF OZONE INCLUDING PHOTOCHEMISTRY

Abstract

As a continuation of the previous experiments in this series, the 18-vertical-level general circulation model was used to investigate the large-scale diffusion of a further two tracers in the stratosphere. In the present experiment both tracers were initiated as photochemical ozone distributions, one distribution being based on photochemistry in an oxygen-only atmosphere, the other on photochemistry in an oxygen-hydrogen atmosphere. Photochemistry was included in the subsequent diffusion so that a joint photochemical-dynamical study resulted. The diffusion process was followed for 185 model days when the experiment was terminated, even though the tracer distributions had not attained a steady state. Despite this, substantial qualitative agreement was obtained with observation, particularly as regards the accumulation of ozone in the lower stratosphere at extratropical latitudes. The results also suggest that photochemistry for an oxygen-hydrogen atmosphere may be the more applicable to the actual atmosphere. Both the meridional circulation and the eddies were important in the diffusion of the tracers, although they did not constitute as interactive a system as in the previous experiment. The direct meridional circulation at low latitudes was found to be the prime mover in the polewards transport of the ozone, the eddies continuing this transport polewards from the subtropics. Owing to photochemica1 dampening of the ozone concentration gradient, the eddies were of minor importance at low latitudes. A schematic diagram summarizing the principal features of the large-scale diffusion mechanism is given.