Title: Cloud droplet size distributions from the Research Scanning Polarimeter 
(RSP) measurements

Abstract:
The Research Scanning Polarimeter (RSP) is an airborne prototype for the
Aerosol Polarimetry Sensor (APS), which is due to be launched in 2010 as
part of the NASA Glory Project. The RSP measures both polarized and
total reflectances in 9 spectral channels with center wavelengths of
410, 470, 555, 670, 865, 960, 1590, 1880 and 2250 nm. For our cloud
droplet size retrievals we utilize the dependences on the scattering
angle of the polarized reflectances in 410, 865, and 2250 nm spectral
channels. Our technique is based on the fact that the polarized
reflectances of clouds within the scattering angle range between 140 and
170 degrees exhibit sharply defined structure ("rainbow"), which is
determined mainly by single scattering properties of the cloud
particles. The latter observation significantly simplifies both forward
modeling and inversions, while also eliminating uncertainties from
unknown aerosol load and possible presence of undetected clouds nearby.
We present retrievals from two recent field campaigns: the Coastal
Stratocumulus Imposed Perturbation Experiment (CSTRIPE, 2003) and the
Routine AVP CLOWD (Clouds with Low Optical Water Depths) Optical
Radiative Observations (RACORO, 2009). The latter campaign was
coordinated by the Atmospheric Radiation Measurement (ARM) Aerial
Vehicles Program (AVP). Our retrievals were compared with the
correlative in situ measurements of cloud droplet size performed during
these two field campaigns.