Comparison of optical-equivalent snow grain size estimates under Arctic low Sun conditions during PAMARCMiP 2018

Abstract. 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 (r_opt) 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 – 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 nm wavelength. This reduced the effect of atmospheric masking and improved the sensitivity on r_opt. From this approach we achieved a significantly improved uncertainty (< 25 %) compared to the previous method. Ground-based in situ measurements indicated an increase of r_opt of 15 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 %. Satellite-based and airborne retrieval methods showed higher and more variable r_opt-values over sea ice (< 300 µm) than over land surfaces (< 100 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.