Co-Is: Mikhail Alexandrov, Columbia University; Igor Geogdzhayev, NASA Goddard Institute for Space Studies; Daniel Miller, University of Maryland Baltimore County (UMBC); Zhibo Zhang, UMBC
In addition to the Ocean Color Instrument (OCI), the PACE mission will also carry two polarimeters, namely the Spectro-polarimeter for Planetary Exploration (SPEXone) and The Hyper Angular Research Polarimeter (HARP-2). Both of these polarimeters provide multi-angle polarimetric measurements that contain unique information on cloud properties that compliment OCI-based retrievals, especially in challenging conditions such as broken cloud cover and mixed-phase tops. Here we propose to develop and evaluate cloud products inferred from the measurements of SPEXone and HARP-2.
Cloud properties that we propose to infer from SPEXone and HARP-2 measurements are cloud top thermodynamic phase, cloud droplet size distribution, ice crystal shape and ice scattering properties, cloud top height, cloud fraction and cloud physical thickness. Methods for droplet size retrievals have substantial heritage from their operational applications to, e.g., POLDER and the airborne Research Scanning Polarimeter (RSP). These approaches are expected to be readily applicable to HARP-2, which includes one channel with sufficiently high angular sampling. For their application to SPEXone, we propose using its broad and continuous wavelength range to partly compensate for its lower angular resolution. Existing ice cloud retrieval approaches that can be readily adapted to both HARP-2 and SPEXone and yield unique and relevant microphysical information on cloud top ice crystals, in addition to allowing OCI retrievals of ice cloud optical thickness and effective radius to be corrected for biases in assumed asymmetry parameter. In addition to applying these heritage approaches to PACE, we propose to explore other retrieval approaches of cloud properties utilizing the strengths of the distinct measurements of SPEXone. Specifically, we propose to infer cloud top height and layer physical thickness from polarimetric measurements in the Oxygen A band made by SPEXone. Furthermore, we propose to explore the potential to infer cloud fraction from the polarimetric measurements in the UV provided by SPEXone. We aim to develop a flexible suite of algorithms to infer consistent cloud properties from either SPEXone or HARP-2, or both. For the evaluation of the proposed methods, we will use available 1-D and 3-D radiative transfer simulators and measurements of airborne versions of SPEXone and HARP-2, in addition to RSP observations. Such simulations will allow quantification of effects caused by, e.g., sub-pixel inhomogeneity, mixed- phase conditions and errors associate with the collation of angular measurements with respect to the surface rather than cloud top. Given the specific sampling of scattering angle ranges that are required for each of the proposed products, their availability will depend on solar and viewing geometries and thus on season and location. We propose to map the expected availability of each product from SPEXone and HARP-2 given, e.g., day of year, latitude and orbital specifics. We aim for delivering implementable approaches to the PACE science team before launch. In this project, we will leverage many algorithms, tools and knowledge acquired over past years using NASA funding. Our proposed work yields to potential for continuity products using not only the PACE polarimeters, but also past POLDER measurements, and other future polarimeters, such as 3MI and potential polarimeters on NASA's A&CCP mission concept.