Abstract
Abstract. In the Northern Hemisphere, seasonal changes in surface freeze–thaw (FT) cycles are an important component of surface energy, hydrological and eco-biogeochemical processes that must be accurately monitored. This paper presents the weekly polar-gridded Aquarius passive L-band surface freeze–thaw product (FT-AP) distributed on the Equal-Area Scalable Earth Grid version 2.0, above the parallel 50° N, with a spatial resolution of 36 km × 36 km. The FT-AP classification algorithm is based on a seasonal threshold approach using the normalized polarization ratio, references for frozen and thawed conditions and optimized thresholds. To evaluate the uncertainties of the product, we compared it with another satellite FT product also derived from passive microwave observations but at higher frequency: the resampled 37 GHz FT Earth Science Data Record (FT-ESDR). The assessment was carried out during the overlapping period between 2011 and 2014. Results show that 77.1 % of their common grid cells have an agreement better than 80 %. Their differences vary with land cover type (tundra, forest and open land) and freezing and thawing periods. The best agreement is obtained during the thawing transition and over forest areas, with differences between product mean freeze or thaw onsets of under 0.4 weeks. Over tundra, FT-AP tends to detect freeze onset 2–5 weeks earlier than FT-ESDR, likely due to FT sensitivity to the different frequencies used. Analysis with mean surface air temperature time series from six in situ meteorological stations shows that the main discrepancies between FT-AP and FT-ESDR are related to false frozen retrievals in summer for some regions with FT-AP. The Aquarius product is distributed by the U.S. National Snow and Ice Data Center (NSIDC) at https://nsidc.org/data/aq3_ft/versions/5 with the DOI https://doi.org/10.5067/OV4R18NL3BQR.
Highlights
IntroductionLandscape freeze–thaw (FT) state transitions show highly variable spatial and temporal patterns, with measurable influences to climate (IPCC, 2014; Peng et al, 2016; Poutou et al, 2004), hydrological (Gouttevin et al, 2012; Gray et al, 1984), ecological (Kumar et al, 2013; Black et al, 2000) and biogeochemical processes (Panneer Selvam et al, 2016; Xu et al, 2013; Schaefer et al, 2011)
Seasonal freezing and thawing affect over half of the Northern Hemisphere
Despite some regional discrepancies in forested landscape, very good agreements between freeze–thaw product (FT-AP) and FT Earth Science Data Record (FT-ESDR) were found in this land cover, suggesting that the sensitivity of L and Ka bands to FT is more similar in forested landscape
Summary
Landscape freeze–thaw (FT) state transitions show highly variable spatial and temporal patterns, with measurable influences to climate (IPCC, 2014; Peng et al, 2016; Poutou et al, 2004), hydrological (Gouttevin et al, 2012; Gray et al, 1984), ecological (Kumar et al, 2013; Black et al, 2000) and biogeochemical processes (Panneer Selvam et al, 2016; Xu et al, 2013; Schaefer et al, 2011). The surface FT state affects the latent heat exchange and the energy balance at the interface between the soil surface and the overlying medium. Comprehensive in situ observational long-term datasets for soil state characteristics across terrestrial environments are still limited or inadequate, mostly for northern remote regions. And temporally continuous information on soil freeze–thaw changes is lacking for the regions of both seasonal frozen ground and permafrost
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