Aircraft observations of thin cirrus clouds near the tropical tropopause

This work describes aircraft-based lidar observations of thin cirrus clouds at the tropical tropopause in the central Pacific obtained during the Tropical Ozone Transport Experiment/Vortex Ozone Transport Experiment (TOTE/VOTE) in December 1995 and February 1996, Thin cirrus clouds were found at the tropopause on each of the four flights which penetrated within 15 degrees of the equator at 200-210 east longitude. South of 15 degreesN, thin cirrus were detected above the aircraft about 65% of the time that data were available. The altitudes of these clouds exceeded 18 km at times. The cirrus observations could be divided into two basic types: thin quasi-laminar wisps and thicker, more textured structures. On the basis of trajectory analyses and temperature histories, these two types were usually formed respectively by (1) in situ cooling on both a synoptic scale and mesoscale and (2) recent (a few days) outflow from convection. There is evidence from one case that the thicker clouds can also be formed by in situ cooling. The actual presence or absence of thin cirrus clouds was also consistent with the temperature and convective histories derived from back trajectory calculations, Notably, at any given time, only a relatively small portion (at most 25%) of the west central tropical Pacific has been influenced by convection within the previous 10 days. The structures of some of the thin cirrus clouds formed in situ strongly resembled long-wavelength (500-1000 km) gravity waves observed nearly simultaneously by the ER-2 on one of the flights. Comparison with in situ water vapor profiles made by the NASA ER-2 aircraft provide some observational support for the hypothesis that thin cirrus clouds play an important role in dehydrating tropospheric air as it enters the stratosphere.