{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,5]],"date-time":"2026-03-05T23:44:15Z","timestamp":1772754255316,"version":"3.50.1"},"reference-count":65,"publisher":"MDPI AG","issue":"19","license":[{"start":{"date-parts":[[2022,9,22]],"date-time":"2022-09-22T00:00:00Z","timestamp":1663804800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["42075150"],"award-info":[{"award-number":["42075150"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["21ZR1405500"],"award-info":[{"award-number":["21ZR1405500"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["FOR2131"],"award-info":[{"award-number":["FOR2131"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/100007219","name":"Natural Science Foundation of Shanghai","doi-asserted-by":"publisher","award":["42075150"],"award-info":[{"award-number":["42075150"]}],"id":[{"id":"10.13039\/100007219","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/100007219","name":"Natural Science Foundation of Shanghai","doi-asserted-by":"publisher","award":["21ZR1405500"],"award-info":[{"award-number":["21ZR1405500"]}],"id":[{"id":"10.13039\/100007219","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/100007219","name":"Natural Science Foundation of Shanghai","doi-asserted-by":"publisher","award":["FOR2131"],"award-info":[{"award-number":["FOR2131"]}],"id":[{"id":"10.13039\/100007219","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Deutsche Forschungsgemeinschaft (DFG)","award":["42075150"],"award-info":[{"award-number":["42075150"]}]},{"name":"Deutsche Forschungsgemeinschaft (DFG)","award":["21ZR1405500"],"award-info":[{"award-number":["21ZR1405500"]}]},{"name":"Deutsche Forschungsgemeinschaft (DFG)","award":["FOR2131"],"award-info":[{"award-number":["FOR2131"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>Knowing the freeze-thaw (FT) state of the land surface is essential for many aspects of weather forecasting, climate, hydrology, and agriculture. Microwave L-band emission contains rather direct information about the FT-state because of its impact on the soil dielectric constant, which determines microwave emissivity and the optical depth profile. However, current L-band-based FT algorithms need reference values to distinguish between frozen and thawed soil, which are often not well known. We present a new FT-state-detection algorithm based on the daily variation of the H-polarized brightness temperature of the SMAP L3c FT global product for the northern hemisphere, which is available from 2015 to 2021. Exploiting the daily variation signal allows for a more reliable state detection, particularly during the transition periods, when the near-surface soil layer may freeze and thaw on sub-daily time scales. The new algorithm requires no reference values; its results agree with the SMAP FT state product by up to 98% in summer and up to 75% in winter. Compared to the FT state inferred indirectly from the 2-m air temperature and collocated soil temperature at 0\u20137 cm of the ERA5-land reanalysis, the new FT algorithm has a similar performance to the SMAP FT product. The most significant differences occur over the midlatitudes, including the Tibetan plateau and its downstream area. Here, daytime surface heating may lead to daily FT transitions, which are not considered by the SMAP FT state product but are correctly identified by the new algorithm. The new FT algorithm suggests a 15 days earlier start of the frozen-soil period than the ERA5-land\u2019s estimate. This study is expected to extend the L-band microwave remote sensing data for improved FT detection.<\/jats:p>","DOI":"10.3390\/rs14194747","type":"journal-article","created":{"date-parts":[[2022,9,22]],"date-time":"2022-09-22T23:07:55Z","timestamp":1663888075000},"page":"4747","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":14,"title":["A Novel Freeze-Thaw State Detection Algorithm Based on L-Band Passive Microwave Remote Sensing"],"prefix":"10.3390","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8957-6636","authenticated-orcid":false,"given":"Shaoning","family":"Lv","sequence":"first","affiliation":[{"name":"Department of Atmospheric and Oceanic Sciences, Institute of Atmospheric Sciences, Fudan University, Shanghai 200438, China"},{"name":"Zhuhai Fudan Innovation Research Institute, Zhuhai 519000, China"},{"name":"Institute for Geosciences-Meteorology, University of Bonn, Auf dem Huegel 20, 53121 Bonn, Germany"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1146-3628","authenticated-orcid":false,"given":"Jun","family":"Wen","sequence":"additional","affiliation":[{"name":"The Plateau Atmosphere and Environment Key Laboratory of Sichuan Province, Chengdu University of Information Technology, Chengdu 610225, China"}]},{"given":"Clemens","family":"Simmer","sequence":"additional","affiliation":[{"name":"Institute for Geosciences-Meteorology, University of Bonn, Auf dem Huegel 20, 53121 Bonn, Germany"},{"name":"Cloud and Precipitation Exploration Laboratory (CPEX-Lab) of Geoverbund ABC\/J, Auf dem Huegel 20, 53121 Bonn, Germany"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2166-5314","authenticated-orcid":false,"given":"Yijian","family":"Zeng","sequence":"additional","affiliation":[{"name":"Department of Water Resources, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, 7500AE Enschede, The Netherlands"}]},{"given":"Yuanyuan","family":"Guo","sequence":"additional","affiliation":[{"name":"Department of Atmospheric and Oceanic Sciences, Institute of Atmospheric Sciences, Fudan University, Shanghai 200438, China"}]},{"given":"Zhongbo","family":"Su","sequence":"additional","affiliation":[{"name":"Department of Water Resources, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, 7500AE Enschede, The Netherlands"}]}],"member":"1968","published-online":{"date-parts":[[2022,9,22]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1","DOI":"10.2136\/vzj2016.01.0010","article-title":"Hydrologic Impacts of Thawing Permafrost-A Review","volume":"15","author":"Walvoord","year":"2016","journal-title":"Vadose Zone J."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"171","DOI":"10.1038\/nature14338","article-title":"Climate change and the permafrost carbon feedback","volume":"520","author":"Schuur","year":"2015","journal-title":"Nature"},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Zeng, Y., Su, Z., Barmpadimos, I., Perrels, A., Poli, P., Boersma, K.F., Frey, A., Ma, X., de Bruin, K., and Goosen, H. 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