Assessing the Sensitivity of the Soil Moisture and Ocean Salinity (SMOS) Observed Brightness Temperature for Soil Freeze- Thaw Detection during Wet Snow Conditions

English Abstract:

The soil freeze-thaw cycle imparts a major control on the seasonal hydrology of cold regions. Understanding the soil freezing state is critical to accurately estimating runoff rates, water storage and predicting spring flooding. One of the most promising techniques for monitoring frozen soils is passive microwave remote sensing. Passive microwave satellites, like the Soil Moisture and Ocean Salinity (SMOS) satellite, offers the ability to capture the landscape soil freeze/thaw state. This L-band satellite is sensitive to the changes in the landscape dielectric which dramatically decrease from thawed to frozen state. Though it is widely accepted that L-band passive microwave satellites are sensitive to frozen soils, literature has indicated that the sensitivity may be hindered during wet snow conditions. This study was conducted during the winters of 2012 and 2013 in southern Saskatchewan. The study site, Environment Canada’s Brightwater Creek Soil Moisture Network, is densely instrumented with 22 soil moisture stations over a 10x10km grid. The ability of the SMOS satellite to detect frozen soils was initially accessed by comparing passes with frozen and thawed soil. The results indicated that brightness temperature during frozen soil conditions was significantly (p<0.05) different from thawed soil. Following this comparison, periods with frozen soil and either wet or dry snow conditions were analyzed, using air temperature and snow albedo as an indicator of wet/dry snow conditions. The study found that the brightness temperature during wet and dry snow conditions was not significantly different (p>0.05). Polarization index was also examined, and it too indicated a significant difference between frozen and thawed soils but no significant difference during wet and dry snow conditions. These results suggest that L-band passive satellites like SMOS may be less sensitive to snow conditions in this region.