The frequency of tropopause-level thick and thin cirrus clouds as observed by CALIPSO and the relationship to relative humidity and outgoing longwave radiation

Thin cirrus clouds play an important radiative role in the earth's atmosphere and climate system, yet are one of the least understood components of the climate system. With the use of data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), thin cirrus and thick cloud distributions in the tropics are analyzed at 121, 100, and 82 hPa. Observations obtained between December 2006 and November 2007 show that thin cirrus between 30?N and 30?S occur in close proximity to regions of intense convection and are positively correlated with low values of outgoing longwave radiation (OLR). In conjunction with the CALIPSO data, water vapor data from the Earth Observing System (EOS) Microwave Limb Sounder (MLS), OLR data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/, and linearly interpolated NCEP reanalysis temperature data were used. These data were used to examine how thick and thin cirrus cloud fractions at 121-hPa and 100-hPa are related to relative humidity with respect to ice (RHI), temperature, and OLR. Our observations show that both RHI and convection play important roles in the development and maintenance of thick and thin cirrus clouds at the pressure levels of interest. The highest fractions of clouds are almost always seen within OLR values representative of convection and at relatively high values of RHI. However, when peaks in cloud fraction are found above the convective threshold, higher RHI values are needed than are needed when convection is responsible for the formation and maintenance of these clouds.