Thawing of soils in Siberia observed by the ERS-1 scatterometer between 1992 and 1995

Based on multi-temporal radar backscatter measurements over Siberia, obtained from the ERS-1 scatterometer at various incidence angles, a method was developed for monitoring the state (frozen/thawed) of the upper layer of the soils. In spring and to some lesser extent in fall, the normalized radar cross section (NRCS) data show distinct variations which can be attributed to thawing and freezing soils. In spring, during snow melt, the NRCS first decreases by up to 5 dB and shortly later, when the soils start thawing, increases dramatically. In autumn, the freezing of the soils is a more gradual process. Due to the huge extent of the Siberian test site, it takes more than three months for the entire region to be thawed. For the years 1992 to 1995, maps of the time period of thawing were constructed for the Siberian test site which reveal the geographical distribution and the interannual variability of the "onset of thawing". A first comparison with monthly SSM/I snow cover data and in situ air temperature data confirms the good behavior of the proposed technique.