Co-occurrence statistics of tropical tropopause layer cirrus with lower cloud layers as derived from CloudSat and CALIPSO data

Radar and lidar data collected by instruments aboard the CloudSat and CALIPSO satellites are used to demonstrate statistics of the spatial variability of tropical tropopause layer (TTL) cirrus, along with their tendency to occur with or without lower clouds, during the period July 2006 through June 2007. This time period included a mild El Niño event. TTL cirrus occurrence patterns contain maxima over the tropical convective areas of South America, of Africa, and of the west Pacific. It is demonstrated from co‐occurrence statistics that TTL cirrus events and general lower cloud events are nominally independent in a zonal average; however, tendencies toward co‐occurrence between TTL cirrus and lower clouds that appear in specific base height and geometrical thickness bins do exist. Nominally independent of clouds based below 3 km, TTL cirrus become increasingly less likely to occur with lower cloud layers of increasing base height than they are to occur generally. The independence of TTL cirrus from clouds based in the boundary layer is due to the ubiquitous, unconnected co‐occurrence of TTL cirrus with thin trade cumulus clouds. However, we find that TTL cirrus are more likely to occur with low‐based clouds of moderate depth (3–10 km) than they are generally and less likely to occur with deep (thicker than 10 km), low‐based clouds than they are generally. Results are used in commenting generally on the TTL cirrus maintenance and formation hypotheses of Hartman et al. (2001) and of Garret et al. (2006).