Liquid Water Cloud Retrievals from HARP2 and AirHARP2 Measurements from the PACE-PAX Validation Campaign

The Hyper Angular Rainbow Polarimeter 2 (HARP2), developed at UMBC, is a state-of-the-art wide field-of-view polarimeter capable of measuring total and polarized radiances with fine angular resolution (≥2 degrees) and high polarization accuracy in four spectral channels (440, 550, 670, 870 nm). HARP2 was successfully launched in February 2024 aboard NASA’s Plankton Aerosol Cloud and ocean Ecosystem (PACE) satellite and has since been collecting critical science data on Earth’s atmospheric, oceanic, and surface properties. In September 2024, UMBC’s AirHARP2, an advanced airborne polarimeter closely resembling the orbital HARP2, participated in the PACE Postlaunch Airborne eXperiment (PAX). This campaign provides a unique opportunity to validate radiometric and polarimetric measurements and derived science products from the PACE satellite by conducting direct cross-platform comparisons using co-located scenes. This study focuses on the retrieval comparisons of liquid water cloud microphysical properties from HARP2 and AirHARP2 during PACE-PAX using a novel look-up-table retrieval algorithm that leverages the geometric features of the polarized cloudbow to infer the cloud droplet size distribution. The retrievals will be performed using a novel look-up-table retrieval algorithm that uses the geometric parameters of the polarized cloudbow to retrieve the cloud droplet size distribution. With AirHARP2’s Level-1C grid resolution (~120 m) approximately 42 times finer than HARP2’s (~5 km), we will also examine the impact of spatial resolution on retrieval performance. The results will be further validated by cross-comparisons with official cloud products from the Ocean Color Instrument, the primary instrument aboard PACE.