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.

This dissertation focuses on the global investigation of optically thin cirrus cloud optical thickness (tau) and microphysical properties, such as, effective particle size (D_(eff)) and ice crystal habits (shapes), based on the global satellite observations and fast radiative transfer models (RTMs). In the first part, we develop two computationally efficient RTMs simulating satellite observations under cloudy-sky conditions in the visible/shortwave infrared (VIS/SWIR) and thermal inferred (IR) spectral regions, respectively. To mitigate the computational burden associated with absorption, thermal emission and multiple scattering, we generate pre-computed lookup tables (LUTs) using two rigorous models, i.e., the line-by-line radiative transfer model (LBLRTM) and the discrete ordinates radiative transfer model (DISORT). The second part introduces two methods (i.e., VIS/SWIR- and IR-based methods) to retrieve tau and D_(eff) from satellite observations in corresponding spectral regions of the two RTMs. We discuss the advantages and weakness of the two methods by estimating the impacts from different error sources on the retrievals through sensitivity studies. Finally, we develop a new method to infer the scattering phase functions of optically thin cirrus clouds in a water vapor absorption channel (1.38-µm). We estimate the ice crystal habits and surface structures by comparing the inferred scattering phase functions and numerically simulated phase functions calculated using idealized habits.