The dependence of L-band Vegetation Optical Depth (L-VOD, τ) on Vegetation temperature TV is investigated for 1165 boreal forest grid cells selected for latitudes > 55° and high radiometric forest fraction FFO≥90%. SMOS Level-3 Brightness Temperatures (BT) at ascending orbits acquired from 2011 to 2022 are used. This is a spatio-temporal extension of our previous study on τTV made over the “Sodankylä grid cell” (Finland) in 2019. It demonstrated the Electromagnetic (EM) reasons for τTV reaching maximum at 0°C and decreasing when TV is moving away from 0°C. The parameterisation of the "L-VOD model" developed in the previous study is simplified and updated to take into account the conservation of salt in sap-water during freezing. The “forward operator” based on the Two-Stream Microwave Emission Model (2S-MEM) is inverted to retrieve τSMOSTV together with effective Ground permittivities εG during seasonal Warming Periods (WPs) determined from ERA-interim air temperatures. The “L-VOD model” parameters TmeltWCwood are estimated for the boreal forest grid cells by minimizing squared differences between τSMOSTV and simulated τsimTV. The vegetation melt-parameter Tmelt represents the “number of degrees below 0°C” at which sap-water melts, and WCwood is the gravimetric wood-Water Content of branches. Reasonable values of TmeltWCwood are achieved for a majority of the boreal forest grid cells. It is found that Tmelt tends to be too high over Northern Europe, a region with longer WP durations compared to other regions of the boreal forest belt. By optimising the scattering albedo used to retrieve τSMOSTV, the correlations between τsimTV and τSMOSTV can be increased, thereby improving the reliability of TmeltWCwood. The results raise the possibility of an alternative method to parameterise the scattering albedo of boreal forests by means of the reality-based nature of τSMOSTV associated with the consistency of Tmelt with expected values. The study also shows that estimating the Above-Ground Biomass (AGB) of boreal forests using L-VOD must take into account the reduction in L-VOD due to the EM-reasons resulting from freezing sap-water in wooden branches, which dominate the radiative transfer at L-band.

中文翻译：

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2011—2022 年北方森林 L 波段植被光学深度的温度依赖性


针对纬度 > 55° 和高辐射森林分数 FFO≥90% 选择的 1165 个北方森林网格单元，研究了 L 波段植被光学厚度 （L-VOD， τ） 对植被温度 TV 的依赖性。使用 2011 年至 2022 年在升轨道上获得的 SMOS 3 级亮度温度 （BT）。这是我们之前在 τTV 上于 2019 年对“Sodankylä grid cell”（芬兰）进行的研究的时空延伸。它证明了 τTV 在 0°C 时达到最大值并在 TV 远离 0°C 时降低的电磁 （EM） 原因。 在先前研究中开发的“L-VOD 模型”的参数化被简化和更新，以考虑冻结过程中汁液中盐分的守恒。基于双流微波发射模型 （2S-MEM） 的“前向算子”被反转，以在由 ERA 临时气温确定的季节性变暖期 （WP） 期间检索 τSMOSTV 和有效接地介电常数 εG。通过最小化 τSMOSTV 和模拟 τsimTV 之间的平方差，估计北方森林网格单元的“L-VOD 模型”参数 TmeltWCwood。植被融化参数 Tmelt 表示树液-水融化的“低于 0°C 的度数”，WCwood 是树枝的重量木材含水量。大多数北方森林网格单元都达到了 TmeltWCwood 的合理值。研究发现，北欧的 Tmelt 往往太高，与北方森林带的其他地区相比，该地区的 WP 持续时间更长。通过优化用于检索 τSMOSTV 的散射反照率，可以增加 τsimTV 和 τSMOSTV 之间的相关性，从而提高 TmeltWCwood 的可靠性。 结果提出了一种替代方法的可能性，即通过与 Tmelt 与预期值的一致性相关的 τSMOSTV 的基于现实的性质来参数化北方森林的散射反照率。该研究还表明，使用 L-VOD 估计北方森林的地上生物量 （AGB） 必须考虑到 L-VOD 的减少，这是由于木枝中冻结的树液水导致的 EM 原因造成的，这在 L 波段的辐射传输中占主导地位。