Time-variations of zeroth-order vegetation absorption and scattering at L-band

Surface soil moisture and vegetation optical depth (VOD), as an indicator of vegetation wet biomass, from passive microwave remote sensing have been increasingly applied in global ecology and climate research. Both soil moisture and VOD are retrieved from satellite brightness temperature measurements assuming a zeroth order radiative transfer model, commonly known as the tau-omega model. In this model the emission of a vegetated surface is dependent on soil moisture, vegetation absorption and vegetation scattering. Vegetation scattering is normally represented by the single scattering albedo, ω, and is commonly assumed to be a time-invariant calibration parameter to achieve high accuracy in soil moisture estimation. Therefore, little is known about ω dynamics in the context of its ecological information content. Furthermore, VOD and ω are functions of more fundamental absorption and scattering coefficients κₐ and κₛ. In this study, we retrieve both VOD and ω as well as κₐ and κₛ in two separate tau-omega model frameworks using known soil moisture information. Our sensitivity analysis confirms that vegetation attenuation has a strong impact on the sensitivity of the retrieval to noise. If vegetation attenuation approaches zero, ω is weakly constrained, which leads to strong ω dynamics. If vegetation attenuation is very high, VOD, κₐ and κₛ are increasingly less constrained and susceptible to noise, while ω becomes quasi time-invariant. Coinciding with our sensitivity study, global retrievals from SMAP brightness temperatures exhibit large ω dynamics in drylands and deserts, decreasing rapidly with increasing vegetation cover. In drylands ω might peak during the dry season at the seasonal VOD minimum. With increasing vegetation attenuation, ω dynamics start to follow leaf phenology before they transition to a nearly time-invariant ω in forested areas. VOD, κₐ and κₛ follow the dynamics of wet biomass and peak after the maximum of leaf area index and precipitation. Our results suggest that zeroth order scattering is generally time-invariant over a majority of the global vegetated areas. In general, time dynamic ω and κₛ might provide additional information on the state of the vegetation canopy, which can be valuable for biosphere studies and relevant for the parameterization of vegetation scattering in soil moisture retrievals.