Janna Elisabeth Rückert, Philip Rostosky, Marcus Huntemann, David Clemens-Sewall, Kerstin Ebell, Lars Kaleschke, Juha Lemmetyinen, Amy R. Macfarlane, Reza Naderpour, Julienne C. Stroeve, Andreas Walbröl, Gunnar Spreen 

Warm air intrusions over Arctic sea ice can rapidly change the snow and ice surface
conditions and can alter sea ice concentration (SIC) estimates derived from satellite-
based microwave radiometry without altering the true SIC.
Here we focus on two warm moist air intrusions that produced surface glazing during
the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)
expedition that reached the research vessel Polarstern in mid-April 2020. After the
events, we observe increased SIC deviations between different satellite products,
including climate data records, and especially an underestimation of SIC for algorithms
based on polarization difference.
To examine the causes of this underestimation, we use the extensive MOSAiC snow
and ice measurements to computationally model the brightness temperatures of the
surface on a local scale. We further investigate the brightness temperatures observed
by ground-based radiometers at frequencies 6.9 GHz, 19 GHz and 89 GHz.
We show that the drop in the retrieved sea ice concentration of some satellite products
can be attributed to large-scale surface glazing, i.e., the formation of a thin ice crust at
the top of the snowpack, caused by the warming events.
Another mechanism affecting satellite products which are mainly based on gradient
ratios of brightness temperatures, is the interplay of the changed temperature gradient
in the snow and snow metamorphism.
From the two analyzed climate data record products, one is less affected by the
warming events.
The low frequency channels at 6.9 GHz were less sensitive to these snow surface
changes, which could be exploited in future retrievals of sea ice concentration.