Analysis of cirrus in the tropical tropopause layer from CALIPSO and MLS data: A water perspective

Abstract

Two mechanisms are thought to be primarily responsible for the formation of cirrus in the Tropical Tropopause Layer (TTL): detrainment from deep convective anvils and in situ initiation. By analyzing water vapor (H 2 O) measurements from the Aura Microwave Limb Sounder (MLS) and ice water content (IWC) measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), we identify TTL cirrus that contain too much ice to have been formed in situand therefore must be of convective origin. Analyzing 3 years of CALIPSO measurements (2008-2010), we found three maxima in the occurrence of convective cirrus: equatorial Africa, the tropical western Pacific, and South America. Over the entire tropics, we found that convective cirrus occur more frequently during boreal winter-spring and less frequently during boreal summer-fall. The convective fractions of cirrus also increase until the cold point tropopause is reached in most seasonsimplying higher probabilities of cirrus around the tropopause being of convective origin. Averaged over 3 years, we find that at least ∼30% of cirrus in the TTL are definitely of convective origin. Copyright 2012 by the American Geophysical Union.