**2.6 Arctic mean sea surface data**

The Mean Sea Surface in the Arctic region is one of the parameters used for the sea ice freeboard estimation. We use the UCL13 mean sea surface model produced by

**Figure 3.**

*Schematic of the Canadian Meteorological Centre snow thickness products in March and October 2018.*

**Figure 4.** *Schematic diagram of the March 2013 OIB flight path.*

University College London (UK) in 2013. Because previous mean sea surface models of the Arctic lacked satellite data north of 81.5°N, the UCL13 is more accurate over the entire Arctic region [23–25].
