Tropopause-level thin cirrus coverage revealed by ICESat/Geoscience Laser Altimeter System

We analyze the distribution of thin (optical depth < 0.40) cirrus in the tropics at potential temperatures of 360, 370, 377.5, and 400 K, which are levels that bracket the tropical tropopause. The observations were obtained between 29 September and 17 November 2003 by the Geoscience Laser Altimeter System (GLAS), carried on board the Ice, Cloud, and land Elevation Satellite (ICESat). The GLAS data show that these thin, near-tropopause cirrus (TNTC) occur over broad regions of the latitude range 20 degrees S to 30 degrees N, with distinct maxima collocated with regions of intense convection, and that TNTC occurrence frequency decreases strongly with increasing altitude. At 377.5 K, approximately the level of the tropical tropopause, TNTC frequency over convection is two to six times larger than in the so-called "cold pool,'' the climatological temperature minimum located over the equatorial western Pacific where it has been suggested that dehydration of air entering the stratosphere is occurring. Comparisons between assimilated temperatures, outgoing longwave radiation (a proxy for deep convection), and TNTC frequency show that TNTC can be found where assimilated temperatures are low and deep convection is occurring. We find that assimilated temperatures, by themselves, are incomplete predictors of TNTC locations.