## 2 Material and Methods  
### MODIS Data  
NASA's MODIS instrument, ([[https://modis.gsfc.nasa.gov/](https://modis.gsfc.nasa.gov/)]([https://modis.gsfc.nasa.gov/](https://modis.gsfc.nasa.gov/)), last access 12 September 2020) is borne by two satellites, namely, Terra (since 2000) and Aqua (since 2002). As Aqua flies in the same satellite constellation as AMSR2's platform Global Change Observation Mission-Water (GCOM-W1) with a 4-minute time lag, we exclusively use MODIS Aqua data. MODIS operates in the visible and thermal infrared spectrum. It provides measurements in 36 wavelength bands centred at wavelengths between 645 nm and 14.235 um. The MYD29 IST dataset which we use has been developed at Goddard Space Flight Center [9] and is provided by the National Snow and Ice Data Center (NSIDC) at [https://n5 eil01u.ecs.nsidc.org/MOSA/MYD29.006/](https://n5 eil01u.ecs.nsidc.org/MOSA/MYD29.006/) (last access 12 September 2020). The brightness temperatures of band 31 (centred at 11.03 um) and band 32 (centred at 12.02 um) are used to derive the IST by applying a split-window technique [10]. The authors of [10] apply a conservative cloud mask by masking only those pixels which the MODIS cloud mask (MYD35_L2, [11] [[https://ladsweb.modaps.eosdis.nasa.gov/archive/allData/61/MYD35_L2/](https://ladsweb.modaps.eosdis.nasa.gov/archive/allData/61/MYD35_L2/)]([https://ladsweb.modaps.eosdis.nasa.gov/archive/allData/61/MYD35_L2/](https://ladsweb.modaps.eosdis.nasa.gov/archive/allData/61/MYD35_L2/)), last access 12 September 2020) classifies as \"confident cloudy\". We apply a stricter cloud mask by also discarding pixels which are labelled as \"probably cloudy\" and \"probably clear\", thus only tolerating pixels which are labelled \"confident clear\". Additionally, pixels over land, inland water and the open ocean are masked out by both cloud masks. The MYD29 data are sliced into granules which cover \(\sim\)5 min. Each granule has a spatial dimension of 2030 km by 1354 km. The grid spacing is 1 km. Further details on the MYD29 dataset are given in [10], at [[https://nsidc.org/data/MOD29/versions/6](https://nsidc.org/data/MOD29/versions/6)]([https://nsidc.org/data/MOD29/versions/6](https://nsidc.org/data/MOD29/versions/6)) (last access 12 September 2020) and in the Algorithm Theoretical Baseline Document at [[https://modis.gsfc.nasa.gov/data/atbd/atbd_mod10.pdf](https://modis.gsfc.nasa.gov/data/atbd/atbd_mod10.pdf)]([https://modis.gsfc.nasa.gov/data/atbd/atbd_mod10.pdf](https://modis.gsfc.nasa.gov/data/atbd/atbd_mod10.pdf)) (last access 12 September 2020). For each Sentinel-2 scene, we downloaded all MODIS granules which intersected with the scene on the same day (997 granules in total). For the assessment of the MODIS ice tie-point (Section 3.1) and the uncertainty estimates (Section 3.3), all of these MODIS granules are used. For the intercomparison with the Sentinel-2 SIC (Section 3.2), only the MODIS granules pertaining to the swath with the smallest time lag towards the Sentinel-2 scene are used. If the time lag exceeds 120 min, no intercomparison is done. For the merging, the granules are gridded to a north polar stereographic grid with the latitude of true scale at 70\({}^{\circ}\)N, also known and hereafter referred to as the NSIDC grid ([[https://nsidc.org/data/polar-stereo/ps_grids.html](https://nsidc.org/data/polar-stereo/ps_grids.html)]([https://nsidc.org/data/polar-stereo/ps_grids.html](https://nsidc.org/data/polar-stereo/ps_grids.html)), last access 12 September 2020). For the intercomparison with the Sentinel-2 SIC, they are resampled to a Transverse Mercator projection for consistency with the Sentinel-2 data.