# Remote Sensing of Environmental Changes in Cold Regions: Methods, Achievements and Challenges
## 2 Principles and Methods
### Remote Sensing of Ice
#### 2.1.2 Lake Ice Cover  
Ice cover plays an important role in lake-atmosphere interactions at high latitudes. The presence (or absence) and extent/concentration of ice cover on large lakes has a significant impact on regional weather and climate (e.g., lake-effect snowfull, thermal moderation effect) [64; 65; 66; 67; 68]. Ice cover (extent) and ice thickness have recently been identified as Essential Climate Variables by the Global Climate Observing System (GCOS) of the World Meteorological Organization [69]. Both ice extent, from which ice phenology (i.e., ice dates during freeze-up and break-up, and ice cover duration) can be determined, and ice thickness are sensitive indicators of climate change [70; 71]. Not identified by GCOS is the bedfast ice regime of shallow Arctic/sub-Arctic lakes (less than about 3-m). Such lakes are widespread across permafrost regions of Alaska, Northern Canada, and Siberia. Determining if and when entire lakes or lake sections become bedfast (i.e., ice cover is thick enough to reach lake bottoms) in winter has been shown to also be relevant for climate monitoring [72; 73]. Winters with a larger (smaller) fraction of bedfast ice are generally indicative of colder (warmer) air temperature and/or lower (higher) on-ice snow depth conditions which can lead to thicker (thinner) ice. Considering the sparse distribution of weather stations in northern high latitudes, whose temperature measurements are not representative for large areas, satellite remote sensing provides an alternative to measure regional ice cover extent (phenology), ice thickness, and bedfast ice as summarized below. Ice cover extent and phenology--Satellite remote sensing has assumed a greater role in lake ice observations in recent years due to the dramatic reduction in ground-based observational recordings and the availability of increasingly longer satellite time series, particularly from the 2000s onward [74]. Ice cover extent products are either generated manually, largely from visual interpretation of multi-source/frequency satellite imagery such as the National Oceanic and Atmospheric Administration (NOAA) National Ice Center Interactive Multisensor Snow and Ice Mapping System (IMS). The IMS products are produced manually through assimilation of various sources of data, including polar-orbit and geostationary satellite imagery and in situ data. In some cases, automated algorithms are applied to these data to facilitate analysis. The IMS products are available at various resolutions (1 km, 4 km, and 24 km) [75]. Ice phenology dates (freeze-up/ice-on and break-up/ice-off dates) and ice cover duration can also be derived from the IMS products [74; 76]. MODIS 500-m snow (MOD10A1/MYD10A1) and 250-m surface reflectance (MOD09 GQ) products have been used in a few recent studies, alone or in combination with each other and 1-km MODIS (MOD11A1/MYD11A1) Land Surface Temperature (LST), to derive ice dates (start and end of break-up and freeze-up dates) and their associated trends (2001-2017 or shorter) [77; 78; 79; 80; 81; 82]. Approaches that use top-of-atmosphere or surface reflectance (e.g., MODIS near-infrared and red bands) are based on threshold values where ice is determined to be present/absent above/below a certain value. High solar zenith angle, which is important during freeze-up for high-latitude lakes, and cloud cover are two factors that affect the quality of lake ice products. Hence, research has also focused on developing ice phenology retrieval algorithms from passive and active microwave observations. At passive microwave frequencies (e.g., 18-37 GHz) used for satellite remote sensing of lake ice cover, nadir emissivity from open water is low (\(\varepsilon=0.443\)- 0.504 at 24 GHz) compared to that of ice (\(\varepsilon=0.858\)-0.908 at 24 GHz) [83]. This makes the determination of the timing of ice formation and decay on lakes feasible from brightness temperature (Tb) measurements.