Fine temporal resolution freeze and thaw states using combination of microwave land surface emissivity estimated

Abstract

Monitoring freeze-thaw (FT) transitions in high latitude regions are critical to enhancing our knowledge about the prediction of biogeochemical transitions, carbon dynamics, climate change, and impacts on boreal-arctic ecosystems. Since land surface emissivity depends primarily on the surface characteristics, it would contains valuable information about the surface, especially regarding freeze and thaw states. The surface characteristics in terms of microwave emission changes whenever water undergoes phase changes at constant temperature. This study aims to investigate the potential of using emissivity estimates from various microwave sensors such as the Advanced Microwave Scanning Radiometer — Earth Observing System (AMSR-E), Special Sensor Microwave Imager (SSM/I), AMSR2, and the Global Precipitation Measurement (GPM) Microwave Imager (GMI). It employs data fusion techinques to construct diurnal estimates in order to accurately predicting the exact time of the freeze-thaw transition for each land cover type and region. The results reveal that emissivity difference values from low and high frequencies (such as 6.9GHz and 89GHz) at horizontal polarization have a strong correlation with ground-based soil temperature values at 5cm depth. A novel threshold-based approach specific to different land cover types is proposed for daily FT detection from the use of three years (August 2012–July 2015) of emissivity estimates at different frequencies. Ground-based soil temperature observations are used as reference to develop threshold values for FT states. Preliminary evaluation of the proposed approach with independent ground observations for the year 2015 shows that the use of land emissivity estimates for high-latitude FT detection is promising with fine temporal resolution (at least 4 times a day).