Overshooting of clean tropospheric air in the tropical lower stratosphere as seen by the CALIPSO lidar

The evolution of aerosols in the tropical upper troposphere/lower stratosphere between June 2006 and October 2009 is examined using the observations of the space borne CALIOP lidar aboard the CALIPSO satellite. Superimposed on several volcanic plumes and soot from an extreme biomass-burning event in 2009, the measurements reveal the existence of fast-cleansing episodes in the lower stratosphere to altitudes as high as 20 km. The cleansing of the layer, which extends from 14 to 20 km, takes place within 1 to 4 months during the southern tropics convective season that transports aerosol-poor tropospheric air into the lower stratosphere. In contrast, the convective season of the Northern Hemisphere summer shows an increase in the particle load at the tropopause consistent with a lofting of air rich with aerosols. These aerosols can consist of surface-derived material such as mineral dust and soot as well as liquid sulfate and organic particles. The flux of tropospheric air during the Southern Hemisphere convective season derived from CALIOP observations is shown to be 5 times at 16 km and 20 times at 19 km larger, respectively, than that associated with flux caused by slow ascent through radiative heating. These results suggest that convective overshooting is a major contributor to troposphere-to-stratosphere transport with concomitant implications for the Tropical Tropopause Layer top height, the humidity, the photochemistry and the thermal structure of the layer.