Hyperangular Polarization Measurements of Aerosol and Cloud Properties from the Airborne PACS and the Future Spaceborne HARP imagers

The HARP (Hyper-Angular Rainbow Polarimeter) Satellite funded by the NASA EARTH Science Technology Office (ESTO) under the INVest Program is currently being built to be the first hyper-angular imaging polarimeter in space for Earth Science applications. HARP will be able to measure simultaneous images of three linear polarization states with high polarimetric accuracy from up to 60 independent viewing angles, with no moving parts. The HARP payload will have up to 4 wavelengths (440, 550, 670 and 870nm), 110degs cross track, and +/- 55degs along track FOV, with a nadir resolution better than 4 km. The spacecraft consists of a nadir pointing 3U Cubesat with 3-axis stabilization. HARP has an innovative sampling scheme that will allow for up to 20 viewing angles from all wavelengths and a hyper-angular channel with up to 60 wavelengths for a designated band, which is enough to fully characterize the cloudbow features. The HARP Cubesat mission is a joint effort between the University of Maryland Baltimore County (UMBC), Space Dynamics Laboratory – Utah State University (SDL), Science and Technology Corporation (STC), and NASA Goddard Space Flight Center (Greenbelt and Wallops facilities). HARP is challenging the main constrains of a CubeSat mission dedicated to Earth Science applications including very high data volume, excellent pointing and geolocation capabilities, and significant power requirements associated with a highly accurate and innovative instrument. HARP's hyperangular capability will allow for the angular sampling of cloud properties in higher angular and spatial resolution (better than 2 degrees and 4km respectively) than the current characteristics achieved by the POLDER instrument. HARP's concept will allow for a full cloudbow retrieval to be performed within its native 4km resolution. The biggest advantage of the cloudbow retrieval is the ability to provide two parameters of the droplet distribution: the droplet effective radius and the effective variance. This capability with the particular sampling scheme that is proposed for HARP has been demonstrated with the PACS VNIR instrument (with wavelengths at 470, 550, 675, 760 and 875nm) during the PODEX experiment in January/February 2013 aboard the NASA ER-2 aircraft in California. PACS is a prototype instrument designed to meet the requirements of the proposed ACE decadal survey mission. The PACS system consists of three wide field of view (110deg cross track) telescopes covering the UV, VNIR, and SWIR spectral ranges with angular coverage between +55 deg forward to -55deg backwards. The angular density can be selected to cover up to 100 different viewing angles at selected wavelengths. HARP (and PACS) also provides enough parameters for aerosol retrievals from its multi-angle, multi-wavelength polarization measurements. During the PODEX flights, PACS has collected data for aerosol and clouds over variable surface types including, water, vegetation, urban areas, and snow. The data is currently being calibrated, geolocated and prepared for the inversion of geophysical parameters including cloud droplet size distribution and aerosol microphysical parameters. The large density of angles in PACS allows for the characterization of cloudbow features in relatively high spatial resolution in a pixel to pixel basis, detailed observation of ice particles, surface characterization, and optimum selection of the number of angles desired for aerosol retrievals. The aerosol and cloud retrieval algorithms under development for the retrieval of particle microphysical properties from the PACS/HARP data will be discussed in this presentation including examples developed with the PACS data collected during the PODEX experiment.