Comparison of Single Channel and Seasonal Threshold Approach for SMAP Freeze-Thaw Retrievals with In Situ Soil Temperature within the Contiguous United States

Seasonal freeze-thaw (FT) impacts much of the northern hemisphere and is an important control on the water, energy and carbon cycle. Owing to greater emission depths, satellite-based L-band FT retrievals should correspond more accurately to the in situ soil FT state than when longer wavelength bands are used. One limitation of L-band FT studies so far is that studies have usually been limited to boreal and higher latitudes (>45^oN). Thus, the applicability, performance and differences between different L-band FT retrieval approaches (such as a single channel or seasonal threshold approach) for sites that are known to freeze south of 45^oN is not clear. The SMAP FT team released an updated SMAP FT product (L3_FT_P, Version 2) which now includes global-scale FT retrievals. This study uses in situ soil temperature data from different national networks at sub-boreal latitudes within the contiguous United States to assess SMAP FT retrieval performance and provides insight to the strengths and weaknesses of the two different FT retrieval methods.