Retrieving Liquid Cloud Droplet Size Distribution from the Geometric Parameters of Polarized Cloudbow: Preliminary Demonstrations

Clouds are key regulators of Earth’s radiation budget, reflecting incoming short-wave radiation back to space and trapping long-wave radiation close to Earth’s surface. Understanding the balance between these two processes is key to quantifying the net effect clouds have on Earth’s energy budget as the planet warms. The net cloud radiative effect is dependent upon cloud macrophysical and microphysical properties such as cloud optical thickness and cloud droplet size distribution (DSD). The liquid cloud DSD is often represented by a gamma distribution formed by two parameters, cloud droplet effective radius (CDR) and cloud droplet effective variance (CDV). CDV is effectively the width of the DSD and as such, it contains information on cloud evolution processes and how cloud microphysical properties affect radiative forcing. Polarimetric measurements are sensitive to CDR and CDV simultaneously. Previous works on polarimetric DSD retrievals have shown that CDR modulates the scattering angle of the primary and supernumerary cloudbow peaks and CDV varies the amplitude of the supernumerary bows. The geometric parameters of the polarized cloudbow are believed to contain the first-order cloud DSD information. In this work, we will introduce (LUT) retrieval method based on the geometric parameters of simulated cloudbow signals. The LUT allows for the efficient retrieval of liquid cloud DSD from polarimetric measurements such as those from the Hyper-Angular Rainbow Polarimeter (HARP) instruments. The Airborne Hyper-Angular Rainbow Polarimeter (AirHARP), designed and built at the University of Maryland, Baltimore County (UMBC) was the first in a series of wide field-of-view instruments capable of measuring polarized radiance at ~ 4 km resolution and up to 60 viewing angles in the 670 nm spectral band. The proposed retrieval method is also applicable to the HARP2 instrument onboard NASA’s Plankton, Aerosol, Cloud ocean Ecosystem (PACE) satellite which will be launched in January 2024.