BRIEF RESEARCH REPORT article
Front. Remote Sens.
Sec. Atmospheric Remote Sensing
This article is part of the Research TopicInstruments and Technologies for Earth Observation Satellite MissionsView all 5 articles
A new way to see the clouds: The Hyper-Angular Rainbow Polarimeter (HARP2) on the NASA PACE satellite mission
Provisionally accepted
Rachel E Smith1,2,3*
Brent McBride1,2,3
Xiaoguang Xu1,2,3
Anin Puthukkudy1,2,3
Noah Sienkiewicz1,2,3
J. Dominik Cieslak1,2,3
Lorraine Ann Remer1,2,3Roberto Fernandez-Borda1,2,3
J. Vanderlei Martins1,2,3- 1Goddard Earth Science Technology and Research (GESTAR) II, University of Maryland Baltimore County, Baltimore, MD, United States
- 2University of Maryland Baltimore County, Baltimore, United States
- 3Earth and Space Institute, Baltimore, United States
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The Hyper-Angular Rainbow Polarimeter 2 (HARP2) on the NASA Plankton Aerosol Cloud ocean Ecosystem (PACE) mission is a wide field of view imaging polarimeter instrument designed for highly accurate and resolved cloud observations. HARP2 is uniquely sensitive to the polarized cloudbow, a ring-like structure in polarized light that appears above liquid water clouds. The structure of the cloudbow encodes information about the droplet size distribution, which is a critical link between cloud microphysical and radiative properties. Matching a multi-angle measurement of the cloudbow to Mie scattering predictions allows for a retrieval of important cloud properties: droplet effective radius and variance. HARP2 is the first instrument of its kind suitable for this retrieval at 5 km spatial resolution. Its wide swath facilitates global coverage of polarimetric measurements in two days, making it a uniquely powerful tool for studying cloud microphysics. This paper briefly presents the HARP2 instrument, demonstrates its retrieval capabilities, and discusses future science that it makes possible.
Keywords: Clouds, liquid cloud microphysics, Polarimetry, remote sensing, Satellites
Received: 22 Sep 2025; Accepted: 08 Dec 2025.
Copyright: © 2025 Smith, McBride, Xu, Puthukkudy, Sienkiewicz, Cieslak, Remer, Fernandez-Borda and Martins. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Rachel E Smith
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