The seasonal cycle of landscape freeze/thaw (FT) state across mid- to high latitudes influences critical processes such as the land surface energy balance, carbon cycle dynamics related to vegetation growth, and hydrological partitioning between surface runoff and infiltration. In this paper, we produce the first daily FT classification for the 2011-2014 period based on L-band radar measurements from Aquarius. The radar FT algorithm used in this paper is based on a seasonal threshold approach, which is also the baseline algorithm applied to higher-resolution (3 km) radar measurements from NASA's Soil Moisture Active/Passive (SMAP) mission (Launched January 31, 2015). The lower frequency (L-band) radar backscatter measurements from Aquarius provide enhanced sensitivity to FT conditions in vegetation canopy, snow and surface soil layers, although the relative radar penetration depth and sensitivity of the FT signal to these landscape elements will vary according to surface moisture and vegetation biomass conditions, and underlying land cover and terrain heterogeneity [1], [2]. Evaluation of the seasonal threshold FT algorithm using Aquarius was performed using surface air and soil temperatures from selected stations in the Snow Telemetry (SnoTel) network. Analysis identified good agreement during the fall freeze-up period with flag agreement exceeding the 80% SMAP accuracy target when summarized on a monthly basis. Disagreement was greater during the spring thaw transition due in part to uncertainty in characterizing thaw from in situ measurements. Unlike the fall season, stronger agreement in the spring was identified when the reference state was characterized with air temperature compared to soil temperature.

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landscape elements higherresolution 3 km radar measurements hydrological partitioning between surface runoff and infiltration vegetation growth seasonal threshold approach which land cover and terrain heterogeneity lower frequency lband radar backscatter measurements daily ft classification period ft conditions in vegetation canopy snow and surface soil seasonal cycle of landscape freezethaw ft state smap accuracy land surface energy balance carbon cycle dynamics thaw radar penetration depth baseline algorithm aquarius nasas soil moisture activepassive smap mission

S.H.Yueh, A.Colliander, R.S.Dunbar,X.Xu, C.Derksen,.Freeze/Thaw Detection and Validation Using Aquarius’ L-Band Backscattering Data. (),1370-1381.

S.H.Yueh, et al."Freeze/Thaw Detection and Validation Using Aquarius’ L-Band Backscattering Data"

S.H.Yueh, A.Colliander, R.S.Dunbar,X.Xu, C.Derksen,"Freeze/Thaw Detection and Validation Using Aquarius’ L-Band Backscattering Data"