Abstract
Arctic rain-on-snow (ROS) deposits liquid water onto existing snowpacks. Upon refreezing, this can form icy crusts at the surface or within the snowpack. By altering radar backscatter and microwave emissivity, ROS over sea ice can influence the accuracy of sea ice variables retrieved from satellite radar altimetry, scatterometers, and passive microwave radiometers. During the Arctic Ocean MOSAiC Expedition, there was an unprecedented opportunity to observe a ROS event using in situ active and passive microwave instruments similar to those deployed on satellite platforms. During liquid water accumulation in the snowpack, there was a four-fold decrease in radar energy returned at Ku- and Ka-bands. After the snowpack refroze and ice layers formed, this decrease was followed by a six-fold increase in returned energy. Besides altering the radar backscatter, analysis of the returned waveforms shows the waveform shape changed in response to rain and refreezing. Microwave emissivity at 19 and 89 GHz increased with increasing liquid water content and decreased as the snowpack refroze, yet subsequent ice layers altered the polarization difference. Corresponding analysis of CryoSat-2 waveform shape and backscatter as well as AMSR2 brightness temperatures further shows the rain/refreeze was significant enough to impact satellite returns. Our analysis provides the first detailed in situ analysis of the impacts of ROS and subsequent refreezing on both active and passive microwave observations, providing important baseline knowledge for detecting ROS over sea ice and assessing their impacts on satellite-derived sea ice variables.
Highlights
Over the past 50 years, the Arctic has warmed three times faster than the planet as a whole [AMAP, 2021]
A rain-on-snow event towards the end of the year440 long MOSAiC expedition in September 2020 provided a unique opportunity to utilize these assets to study the impact of ROS on active and passive microwave radiation, providing information germane to developing ROS detection over sea ice and for understanding ROS impacts on derived snow depth, sea ice thickness and ice concentration
While the geometries of ground-based systems are different from satellite altimeters, we show that ROS can alter waveform shape, with impacts on discrimination of sea ice from leads, and that shifting in the range of the peak can lead to an 445 apparent increase in sea ice freeboard, and thickness retrievals from CryoSat-2/AltiKa
Summary
Over the past 50 years, the Arctic has warmed three times faster than the planet as a whole [AMAP, 2021]. Given that currently all CryoSat-2 radar altimeter derived sea ice thickness data products assume the dominant scattering surface is the snow/ice interface, the presence of liquid water, changes to snow structure and/or ice layers resulting from a cold-season ROS event could bias thickness 55 retrievals. Without accounting for this effect, derived snow depth obtained using a combination of SARAL/AltiKa (Ka-band) and CryoSat-2 (Ku-band) radar freeboards [e.g. Guerreiro et al, 2016], or CryoSat-2 radar freeboards with ICESat-2 (laser altimeter) snow freeboards [e.g. Kwok et al, 2020] would be biased. Details of the remote sensing site setup can be found in Nicolaus et al [2021]
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