TCD The Cryosphere Discussions TCD The Cryosphere Discuss. 1994-0440 Göttingen, Germany 10.5194/tc-2021-14 Comparison of optical-equivalent snow grain size estimates under Arctic low Sun conditions during PAMARCMiP 2018 Jäkel Evelyn 1 Carlsen Tim https://orcid.org/0000-0003-0695-9047 1 2 Ehrlich André https://orcid.org/0000-0003-0860-8216 1 Wendisch Manfred https://orcid.org/0000-0002-4652-5561 1 Schäfer Michael https://orcid.org/0000-0003-1896-1574 1 Rosenburg Sophie 1 Nakoudi Konstantina https://orcid.org/0000-0001-9639-3109 3 Zanatta Marco https://orcid.org/0000-0002-7711-6808 3 4 Birnbaum Gerit 5 Helm Veit https://orcid.org/0000-0001-7788-9328 5 Herber Andreas 5 Istomina Larysa https://orcid.org/0000-0002-0946-1386 5 6 Mei Linlu 6 Rohde Anika https://orcid.org/0000-0002-6450-9960 7 Leipzig Institute for Meteorology (LIM), University of Leipzig, Germany Department of Geosciences, University of Oslo, Oslo, Norway Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), Potsdam, Germany LISA (Laboratoire Interuniversitaire des Systèmes Atmosphériques), UMR CNRS 7583, Université Paris-Est-Créteil, Université de Paris, Institut Pierre Simon Laplace (IPSL), Créteil, France Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), Bremerhaven, Germany Institute of Environmental Physics, University of Bremen, Bremen, Germany Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany 29 01 2021 2021 1 36 Copyright: © 2021 Evelyn Jäkel et al. 2021 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://tc.copernicus.org/preprints/tc-2021-14/ The full text article is available as a PDF file from https://tc.copernicus.org/preprints/tc-2021-14/tc-2021-14.pdf

<p>The size and shape of snow grains directly impacts the reflection by a snowpack. In this article, different approaches to retrieve the optical-equivalent snow grain size (<i>r</i><sub>opt</sub>) or, alternatively, the specific surface area (SSA) using satellite, airborne, and ground-based observations are compared and used to evaluate ICON-ART (ICOsahedral Non-hydrostatic &ndash; Aerosols and Reactive Trace gases) simulations. The study is focused on low Sun and partly rough surface conditions encountered during a three-week campaign conducted North of Greenland in March/April 2018 within the framework of the PAMARCMiP (Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project) project. Further, we propose an adjusted airborne retrieval method by using the albedo at 1700&thinsp;nm wavelength. This reduced the effect of atmospheric masking and improved the sensitivity on <i>r</i><sub>opt</sub>. From this approach we achieved a significantly improved uncertainty (<&thinsp;25&thinsp;%) compared to the previous method. Ground-based in situ measurements indicated an increase of <i>r</i><sub>opt</sub> of 15&thinsp;µm within a five-day period after a snowfall event, which is small compared to previous observations under similar temperature regimes. This low growth rate is well represented by a parametrization of snow metamorphism, when the vertical temperature gradient effect is suppressed. ICON-ART captured the observed change of ropt during snow fall events, but systematically overestimated the snow grain growth by about 100&thinsp;%. Satellite-based and airborne retrieval methods showed higher and more variable <i>r</i><sub>opt</sub>-values over sea ice (<&thinsp;300&thinsp;µm) than over land surfaces (<&thinsp;100&thinsp;µm), which could partly be attributed to the impact of surface roughness on the retrieval. Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals revealed a large spread within a series of subsequent individual overpasses, indicating their limitations in observing the snow grain size evolution in early spring conditions with low Sun.</p>

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