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, z) on Vegetation temperature T V is investigated for 1165 boreal forest grid cells selected for latitudes > 55 degrees 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 spatiotemporal extension of our previous study on z(TV) made over the "Sodankyla<spacing diaeresis> grid cell" (Finland) in 2019. It demonstrated the Electromagnetic (EM) reasons for z(TV) reaching maximum at 0 degrees C and decreasing when T V is moving away from 0 degrees 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 z SMOS ( T V ) together with effective Ground permittivities eG during seasonal Warming Periods (WPs) determined from ERA-interim air temperatures. The "L-VOD model" parameters ( T melt WC wood ) are estimated for the boreal forest grid cells by minimizing squared differences between z SMOS ( T V ) and simulated zsim(TV). The vegetation melt-parameter T melt represents the "number of degrees below 0 degrees C" at which sap-water melts, and WC wood is the gravimetric wood- Water Content of branches. Reasonable values of ( T melt WC wood ) are achieved for a majority of the boreal forest grid cells. It is found that T 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 zSMOS(TV), the correlations between zsim(TV) and z SMOS ( T V ) can be increased, thereby improving the reliability of ( T melt WC wood ). 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 z SMOS ( T V ) associated with the consistency of T melt with expected values. The study also shows that estimating the Above-Ground Biomass (AGB) of boreal forests using LVOD 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.