the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A high-resolution (0.05°) global seamless continuity record (2002–2023) of near-surface soil freeze-thaw states via passive microwave and optical satellite data
Abstract. The global near-surface soil freeze-thaw (FT) states are crucial for understanding complex interactions with hydrological, ecological, and climatic processes. However, current remote sensing of FT states primarily relies on passive microwave remote sensing, which, despite its all-weather monitoring capabilities, suffers from low spatial resolution. This limitation restricts its application to hydroclimatological scales, precluding its use in finer-scale studies such as soil erosion and hydrometeorological applications. To address this, this study introduces a novel downscaling approach that integrates passive microwave and optical satellite data to generate a long-term (2002–2023), high-resolution (0.05°) dataset of global near-surface FT states, ensuring daily seamless continuity. The dataset was validated against in situ measurements, demonstrating that the high-resolution product maintains an overall accuracy of 83.78 %, consistent with the coarse-resolution microwave-based dataset, while offering enhanced spatial detail. Comprehensive global trend analyses provided new insights into the dynamics of freeze-thaw cycles, revealing that the average annual number of frost days in regions north of 45° N is 187.8 ± 12.7 days, with 14.35 % of the area showing a decreasing trend in frozen persistence. Additionally, the average annual number of freeze onset dates is 240.3 ± 7.2, and 9.10 % of the area exhibits a trend of delayed freeze onset. The high-resolution record enables accurately monitoring FT states and providing detailed information, for a refined understanding of hydrological and ecological effects globally. The global 0.05° near-surface soil FT state dataset is freely available at https://doi.org/10.11888/Cryos.tpdc.301551 (Zhao et al., 2024b).
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Status: open (until 17 Jun 2025)
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RC1: 'Comment on essd-2025-62', Andrey Kalugin, 27 May 2025
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The article is well-structured, and the technique and results are thoroughly explained. However, there are a few important comments:
1. At what depths was in-situ data used to confirm soil freezing beyond 5 cm? Freezing to a depth of 5 cm is a significant criterion for some agricultural activities, but it is insufficient for the majority of hydrological and ecological tasks, particularly in the context of climate change.
2. How do the authors incorporate data on permafrost distribution into the FT product, both in extended areas and mountainous regions? How do authors account for geographical and temporal variations in permafrost indicators between 2002 and 2023?
3 The authors do not provide a convincing number of point verifications of the FT product in various physiographic conditions using statistical techniques. In addition, a comparison of soil freezing depth inaccuracies in various natural zones is required.
4. Can you explain the lack of freeze-thaw data at 0.25º resolution in Fig. 4?Citation: https://doi.org/10.5194/essd-2025-62-RC1
Data sets
Global 0.05° near-surface freeze/thaw state dataset v2.0 (2002-2023) Tianjie Zhao, Defeng Feng, Pei Yu, and Ziqian Zhang https://doi.org/10.11888/Cryos.tpdc.301551
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