Temperature dependence of L-band vegetation optical depth over the boreal forest from 2011 to 2022

The dependence of L-band Vegetation Optical Depth (L-VOD, $\tau$) on Vegetation temperature $T_V$ is investigated for 1165 boreal forest grid cells selected for latitudes > 55° and high radiometric forest fraction $\mathrm{FFO}\ge 90\%$. 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 $\tau \left(T_V\right)$ made over the “Sodankylä grid cell” (Finland) in 2019. It demonstrated the Electromagnetic (EM) reasons for $\tau \left(T_V\right)$ reaching maximum at 0°C and decreasing when $T_V$ 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 ${\tau }_{\mathrm{SMOS}}\left(T_V\right)$ together with effective Ground permittivities ${\epsilon }_G$ during seasonal Warming Periods (WPs) determined from ERA-interim air temperatures. The “L-VOD model” parameters $\left(T_{\mathrm{melt}},{\mathrm{WC}}_{\mathrm{wood}}\right)$ are estimated for the boreal forest grid cells by minimizing squared differences between ${\tau }_{\mathrm{SMOS}}\left(T_V\right)$ and simulated ${\tau }_{\mathrm{sim}}\left(T_V\right)$. The vegetation melt-parameter $T_{\mathrm{melt}}$ represents the “number of degrees below 0°C” at which sap-water melts, and ${\mathrm{WC}}_{\mathrm{wood}}$ is the gravimetric wood-Water Content of branches. Reasonable values of $\left(T_{\mathrm{melt}},{\mathrm{WC}}_{\mathrm{wood}}\right)$ are achieved for a majority of the boreal forest grid cells. It is found that $T_{\mathrm{melt}}$ 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 ${\tau }_{\mathrm{SMOS}}\left(T_V\right)$, the correlations between ${\tau }_{\mathrm{sim}}\left(T_V\right)$ and ${\tau }_{\mathrm{SMOS}}\left(T_V\right)$ can be increased, thereby improving the reliability of $\left(T_{\mathrm{melt}},{\mathrm{WC}}_{\mathrm{wood}}\right)$. 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 ${\tau }_{\mathrm{SMOS}}\left(T_V\right)$ associated with the consistency of $T_{\mathrm{melt}}$ 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.