A 3D SIMULATION OF THE EARLY WINTER DISTRIBUTION OF REACTIVE CHLORINEIN THE NORTH POLAR VORTEX

Early in December 1991, high values of ClO are seen by the Microwave L imb Sounder (MLS) on the Upper Atmosphere Research Satellite at latitu des south of areas of temperatures cold enough to form polar stratosph eric clouds (PSC's). A three dimensional simulation shows that the het erogeneous conversion of chlorine reservoirs to reactive chlorine on t he surfaces of PSC's (processing) takes place at high latitudes. Often the ''processed'' air must be transported to lower latitudes, where t he reactive chlorine is photochemically converted to ClO, to be observ ed by MLS. In this simulation, one incidence of cold temperatures is a ssociated with an anticyclone, and a second with a cyclone. The transp ort of processed air associated with the anticyclone is marked by shea ring; a decrease in the maximum of the processed air is accompanied by growth of the area influenced by the processing. In contrast, the air processed in the cyclonic event spreads more slowly. This shows that transport and shearing is a crucial element to the evolution of reacti ve chlorine associated with a processing event. In particular, transpo rt and shearing, as well as photochemical processes, can cause variati ons in observed ClO.