ElectroMagnetic (EM) reasons resulting in temperature dependence of L-band Vegetation Optical Depth (L-VOD) are currently overlooked in remote sensing products. Discrepancies in retrievals of geophysical surface properties over vegetated areas can result from this incompleteness. This perception motivated to explore EM considerations in how temperature drives L-VOD of a boreal forest. Thereto, a novel physics-based model is developed and evaluated to assess L-VOD sensitivities to canopy temperature and some other model parameters. The L-VOD model is compared to L-VOD derived from close-range L-band brightness temperatures measured through the tree canopy at the Finnish Meteorological Institute's Arctic Research Center (FMI-ARC) in Sodankylä (Finland) during a 4-week and a 1-day period in 2019. Furthermore, the model's ability to explain L-VOD retrieved from brightness temperatures of the “Soil Moisture and Ocean Salinity” (SMOS) satellite over the “Sodankylä grid cell” is investigated. Experimental L-VOD are maximal at around 0 °C and decrease when canopy temperature is moving away from zero degree Celsius. This temperature response, observed at different temporal- and spatial scales, is captured by the proposed L-VOD model and explained by freezing tree sap-water and the dependence of water permittivity on temperature. The demonstrated EM-induced temperature dependence suggest caution with interpreting satellite-based L-VOD, because increased L-VOD around the freezing point is not solely due to increased biomass or rehydration of the vegetation. Further, our study can find future application to compensate L-VOD for EM-induced temperature sensitivity. This potentially leads to improved explanatory power of temperature normalized L-VOD for characterization of forest phenology. Furthermore, we suggest examining the presence and strength of the demonstrated L-VOD temperature response as a practical L-VOD retrieval quality assessment method under steady forest phenology.

导致 L 波段植被光学深度 (L-VOD) 温度依赖性的电磁 (EM) 原因目前在遥感产品中被忽视。这种不完整性可能导致植被区域地球物理表面特性反演的差异。这种看法激发了探索温度如何驱动北方森林 L-VOD 的 EM 考虑因素。此外，开发并评估了一种新的基于物理的模型，以评估 L-VOD 对冠层温度和一些其他模型参数的敏感性。L-VOD 模型与 L-VOD 的 L-VOD 进行了比较，该 L-VOD 是在 4 周内通过位于 Sodankylä（芬兰）的芬兰气象研究所北极研究中心 (FMI-ARC) 的树冠测得的近距离 L 波段亮度温度，并且2019 年为期 1 天。此外，模型' 研究了解释从“Sodankylä 网格单元”上的“土壤湿度和海洋盐度”(SMOS) 卫星的亮度温度获取的 L-VOD 的能力。实验 L-VOD 在 0 °C 左右达到最大值，并在冠层温度远离零摄氏度时降低。这种在不同时间和空间尺度上观察到的温度响应被提议的 L-VOD 模型捕获，并通过冻结树液和水介电常数对温度的依赖性来解释。已证明的 EM 诱导的温度依赖性表明在解释基于卫星的 L-VOD 时要谨慎，因为在冰点附近增加的 L-VOD 不仅仅是由于生物量增加或植被再水化。此外，我们的研究可以找到未来的应用，以补偿 L-VOD 的 EM 引起的温度敏感性。这可能会提高温度归一化 L-VOD 的解释能力，用于表征森林物候。此外，我们建议检查已证明的 L-VOD 温度响应的存在和强度，作为稳定森林物候条件下实用的 L-VOD 检索质量评估方法。