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

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 (H2O) 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 situ-and 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 seasons-implying higher probabilities of cirrus around the tropopause being of convective origin. Averaged over 3 years, we find that at least similar to 30% of cirrus in the TTL are definitely of convective origin.