How Flat is Flat? Investigating the spatial variability of snow surface temperature and roughness on landfast sea ice using UAVs in McMurdo Sound, Antarctica

Abstract. How do snow distribution patterns influence the surface temperature of snow on sea ice? Despite its crucial role in the sea-ice energy balance, snow on Antarctic sea ice remains under-sampled and poorly understood. To address this knowledge gap, we used an Uncrewed Aerial Vehicle (UAV) and ground measurements to produce a Digital Elevation Model (DEM) of the snow topography and a map of snow surface temperature over relatively uniform landfast sea ice (2.4 ± 0.04 m thick) in McMurdo Sound, Ross Sea, Antarctica during our field season in November-December 2022. A key methodological innovation in this study is an algorithm that corrects thermal drift caused by Non-Uniformity Correction (NUC) events in the DJI Matrice 30T thermal camera. The new algorithm minimizes temperature jumps in the imagery, ensuring consistent and accurate high-resolution (9 cm/px) snow surface temperature maps. Our airborne maps reveal a mean snow depth of 0.16 ± 0.06 m and a mean surface temperature of -14.7 ± 0.4 °C. As expected, the largest surface temperature anomalies were associated with visible sediment depositions on the snow surface, which were manually identified. We found that the small-scale topography on a seemingly flat snow field significantly influences the incoming solar radiation (irradiance) at the point scale. Using a model that accounts for topographical effects on irradiance, we found that assuming uniform irradiance over our study (200x200 m) area underestimated irradiance variability due to relatively small-scale surface topography. The modeled mean irradiance, which accounts for surface topography, is 592 ± 45 Wm−2 (1 Standard Deviation), whereas the mean measured irradiance at the point scale is 593 ± 20 Wm−2. This shows that assuming a flat surface fails to represent the full irradiance range and may impact non-linear energy balance processes. While we initially hypothesized that snow depth was a key driver of snow surface temperature, our results indicate that sediment deposition and irradiance exert a far greater influence, overriding the effect of snow depth for this test site. Our results improve our understanding of snow’s spatial distribution, how it influences snow surface temperatures and how it may influence the sea-ice energy balance.