Erosion and mixing of filaments in the arctic lower stratosphere revealed by airborne lidar measurements

Abstract

The extent of ozone depletion in the Arctic stratosphere is limited by the dynamical instability of the winter circulation. During January 1999, transport across the polar vortex boundary occurred with intrusions of midlatitude air into the vortex and via peeling of filaments off the vortex edge, followed by intensive stirring. Contour advection simulations follow the reduction of the scales. The structures are evident as heterogeneities in two‐dimensional (2‐D) airborne lidar sections of aerosol backscatter ratio, along flight paths inside the vortex during January/February 1999. Large gradients of the particle backscatter ratio indicate that microscopic mixing is slow in the presence of continuous intensive macroscopic stirring. The gradients persist several weeks after the vortex recovered and isentropic stirring started. The filament boundary layers typically extend over 100 m vertically and a few kilometers in the horizontal. Under typical stratospheric conditions, it takes about two weeks until 3‐D turbulence takes over and effectively mixes air masses at these scales. In late January 1999 only about 10–13% of the air probed during the flights inside the vortex are mixed microscopically, which, however, is a prerequisite for their chemical and microphysical interaction. As chemical transport models cannot distinguish subgrid‐scale stirring from molecular mixing, the incomplete mixing implies that homogenization would occur prematurely in a model run and interaction rates would be overestimated.