Detecting soil freeze/thaw onsets in Alaska using SMAP and ASCAT data

摘要

Microwave remote sensing, both active or passive, can provide useful information about the freeze/thaw (F/T) state of soil near the surface. Here we apply an edge detection algorithm on time series of indicators derived from measurements of SMAP L-band radiometer and ASCAT C-band scatterometer to detect the freeze/thaw onsets of surface soil. Comparing these results against the onsets derived from in situ measurements in Alaska, we demonstrate that this algorithm is an effective approach to detect onsets of the soil FIT transition. More specifically, our results show that the thawing onsets estimated from the SMAP data occurred 5 to 13 days earlier than the onsets estimated from the in situ measurements, which is likely due to the influence of snowmelt on the radiometer signal. The thawing onsets estimated from the ASCAT data were about 6 days later than the in situ onsets. Our estimated freezing onsets from each microwave remote sensing dataset were close to the in situ onsets (1-5 days). We also compare our estimated onsets with those from the SMAP Level 3 F/T product and the mean biases for thawing and freezing onsets are 1 +/- 2 and 1 +/- 3 days, respectively. Furthermore, we illustrate the complementary nature of the SMAP and ASCAT measurements and the potential for combining these two to differentiate snowmelt from soil thawing events.