Abstract
Freeze/thaw (FT) processes at the earth’s surface can have a considerable effect on global carbon, energy, and hydrologic cycles. Therefore, an accurate representation of FT is valuable to adequately monitor and model these processes. In this study, we assess the relationship between satellite-based FT products and modeled surface and soil temperatures over North America. In addition, hourly land surface temperature (LST) from the Geostationary Operational Environmental Satellite (GOES) system is also compared to FT classifications. Utilizing the higher spatial resolution temperatures (~5 km), we assess subgrid-scale variability and its relationship to coarser microwave FT classifications (>25 km). We also examine product agreement and subpixel characteristics across the land cover, climate, and topography. FT classifications are shown to vary widely depending on these variables, leading to an ambiguous definition of frozen and thawed states. Our results suggest that current products can characterize FT transitions with consistent subfreezing surface characteristics in far northern regions (>50 °N). However, uncertainty associated with FT classifications is shown to increase considerably as latitude decreases. Our results also suggest that fractional FT products, utilizing data inputs, such as LST, would provide a considerable improvement in mountainous regions with high intergrid cell heterogeneity, in regions characterized by ephemeral FT events (i.e., regions < 40 °N), as well as during freeze and thaw onset periods. This study also provides insight to improving the representation of surface FT state by providing a clearer definition of the subpixel scale temperature characteristics that govern existing frozen classifications.
Highlights
S EASONAL freezing and thawing cycles at the earth’s surface act as a critical control of global climate and waterManuscript received December 10, 2020; revised May 26, 2021; accepted July 14, 2021
The results reiterate this finding, as FT products observe frozen conditions over much continental United States (CONUS) when significantly less than 50% of ensemble subpixel SoilT values are modeled to be frozen
We identify thresholds at which FT classifications begin, improve the understanding of what frozen classifications represent, and observe seasonal changes in the governing characteristics of the FT observations over a diverse range of climate regions
Summary
S EASONAL freezing and thawing cycles at the earth’s surface act as a critical control of global climate and waterManuscript received December 10, 2020; revised May 26, 2021; accepted July 14, 2021. S EASONAL freezing and thawing cycles at the earth’s surface act as a critical control of global climate and water. Extensive research has been devoted to characterizing the effects of freeze/thaw (FT) transitions on carbon, water, and energy cycling, further revealing the link between FT and these global systems. Surface FT state can directly influence temperature regimes both at microscales and macroscales, which is relevant to hydrologic, weather, and climate modeling [8]–[11]. Global FT processes contribute to changes in groundwater storage, soil moisture, and infiltration rates, which can directly impact flood intensity through increases in runoff from snowmelt and rain on frozen ground events [15]–[19]. Accurate FT classification employing environmentally relevant and consistent definitions of “frozen” or “thawed” states can be crucial in developing a better understanding of global cycles and for modeling the related processes
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