A Retrieval Method for Ice Particle Size Using PARASOL Observations

Ice clouds play an important role in regulating the energy balance of the earth but their microphysical properties are still poorly understood, including the distribution of ice particle size and habit. This study employs a retrieval algorithm utilizing a large database of ice particle bulk scattering properties built by using calculated size distributions which encompass the full range of variation in size measured during various field campaigns. Using the database of scattering properties, the full Stokes vector at the top of the atmosphere (TOA) for a range of cloud optical thicknesses and different ice particle habits is simulated using a vector adding-doubling model including polarization. Measured polarization data from the PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) satellite is used for comparison to simulated data because it is able to measure the first 3 components of the Stokes vector for reflected sunlight from an ice cloudy pixel. The retrieval algorithm minimizes the difference between the simulated polarization quantities and the measured polarization from the PARASOL satellite. Ice particle size and cloud optical thickness are retrieved simultaneously and the results for several representative PARASOL pixels are discussed.