DISPERSION PROPERTIES OF THE FLOW IN THE SOUTHERN STRATOSPHERE DURINGWINTER AND SPRING

The STRATEOLE project, organized by France's Centre National de la Rec herche Scientifique (CNRS), will release a large number of isopycnal b alloons to drift in the lower stratosphere of the southern hemisphere. In preparation for STRATEOLE this paper studies the structure and dis persion properties of the flow in the lower stratosphere at high south ern latitudes during winter and spring. The approach for investigation is based on computing trajectories of fluid parcels and isopycnal bal loons using the velocity field obtained on-line during simulations wit h a three-dimensional primitive-equation model of the stratosphere and mesosphere. A scheme for computation of isopycnal balloon trajectorie s is devised, and a method for estimating the location of the polar ni ght vortex edge is developed. It is found that trajectories of fluid p arcels initiated well inside the vortex remain within the vortex for a period of months. A small number of fluid parcels released near the v ortex edge cross this edge with about an equal number of crossings fro m inside to outside the vortex as from outside to inside. The isopycna l balloons show a stronger tendency to cross the vortex edge than do f luid parcels. The relative dispersion properties of the flow in the lo wer stratosphere are analyzed in the light of theories on two-dimensio nal turbulence. It is determined that the flow inside the polar vortex can be considered as quasi two-dimensional, isotropic, homogeneous, a nd stationary for a period of about 6 days. Outside the vortex the dif ferential advection by the strong westerly flow primarily determines t he balloon dispersion. The mean square separation between balloons rel eased 2 hours apart within the vortex (70 degrees S, 50 mbar) increase s in time, following approximately (1) the Kraichnan-Lin exponential l aw for small timescales and space scales (1.5-4 days, 90-150 km), (2) the Richardson-Obukhov t(3) law for intermediate timescales and space scales (7-18 days, 180-640 km), and (3) the asymptotic linear behavior found by Babiano et al. (1990), for large timescales and space scales (20-40 days, 700-1200 km).