Investigation of Marine Stratocumulus Cloud-top Droplet Size Retrievals from Observations of Supernumerary Bows and Glories in Polarized Light

Abstract:

Polarimetry provides valuable constraints on atmospheric particle optical and microphysical properties. We have been using imagery acquired by the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) over liquid water clouds to retrieve cloud-top droplet size distributions from multiangular observations in polarized light. AirMSPI is an 8-band ultraviolet-visible-near infrared pushbroom camera, measuring polarization at 470, 660, and 865 nm. It is mounted on a gimbal to acquire multiangular observations over a ±67° along-track range from the NASA ER-2 high-altitude aircraft. Droplet size retrievals from AirMSPI and other multiangular polarimeters typically apply single scattering theory to the spectral and angular structure of polarized radiance in the supernumerary bows (interference fringes from internally reflected and transmitted waves that appear at scattering angles between ~140° and ~165°), using a two-parameter gamma distribution governed by droplet effective radius and variance to model the clouds. However, different values of these parameters are inferred from surface wave-involved interference fringes in polarized radiance observed within the glory region (scattering angles exceeding ~170°). Although multiple scattering can affect the magnitude of such oscillations, it does not affect their angular positions, suggesting the need to account more rigorously for details of the size distribution, vertical stratification, or other effects. Theoretical analysis of AirMSPI supernumerary bow and glory observations in polarized light is being undertaken in an attempt to reconcile the measurements in both angular domains, with the objective of retrieving more details about the cloud tops than can be inferred from supernumerary bow polarimetry alone.