Spatial distribution of cloud droplet size properties from Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) measurements

Author/Creator ORCID





Citation of Original Publication

McBride, B. A., Martins, J. V., Barbosa, H. M. J., Birmingham, W., and Remer, L. A.: Spatial distribution of cloud droplet size properties from Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) measurements, Atmos. Meas. Tech., 13, 1777–1796,, 2020.


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The global variability of clouds and their interactions with aerosol and radiation make them one of our largest uncertainties related to global radiative forcing. The droplet size distribution (DSD) of clouds is an excellent proxy that connects cloud microphysical properties with radiative impacts on our climate. However, traditional radiometric instruments are information-limited in their DSD retrievals. Radiometric sensors can infer droplet effective radius directly, but not the distribution width, which is an important parameter tied to the growth of a cloud field and to the onset of precipitation. DSD heterogeneity hidden inside large pixels, lack of angular information and the absence of polarization limits the amount of information these retrievals can provide. Next-generation instruments that can measure at narrow resolutions, multiple view angles on the same pixel, with a broad swath, and sensitivity to intensity and polarization of light are best situated to retrieve DSDs at the pixel-level and over a wide spatial field. The Airborne Hyper-Angular Rainbow Polarimeter (HARP) is a wide field-of-view imaging polarimeter instrument designed by the University of Maryland, Baltimore County (UMBC) for retrievals of cloud droplet size distribution properties over a wide swath, narrow resolution, and at up to 60 unique, co-located view zenith angles in the 670 nm channel. Cloud droplet effective radius (CDR) and variance (CDV) of a unimodal gamma size distribution are inferred simultaneously by matching measurement to Mie polarized phase functions. For all targets with appropriate geometry, a retrieval is possible, and unprecedented spatial maps of CDR and CDV are made for cloud fields that stretch both across the swath and along the entirety of a flight observation. During the NASA Lake Michigan Ozone Study (LMOS) aircraft campaign from May–June 2017, the Airborne HARP (AirHARP) instrument observed a heterogeneous stratocumulus cloud field along the solar principal plane. Our retrievals from this dataset show that cloud DSD heterogeneity can occur at the 200 m scale, much smaller than the 1–2 km resolution of most spaceborne sensors. This heterogeneity at the subpixel level can create artificial broadening of the DSD in retrievals made at resolutions on the order of 0.5 to 1 km. This AirHARP study demonstrates the viability of the HARP concept to make cloud measurements at scales of individual clouds with global coverage, and all in a low-cost, compact CubeSat-size payload.