Estimation of relative canopy absorption and scattering at L-, C- and X-bands

Abstract

Quantification of microwave vegetation properties (i.e. vegetation optical depth (VOD) and single scattering albedo (ω)) is of interest not only to reliably estimate soil moisture from the Earth's microwave emission, but also for broad applications across plant physiology and hydrology. Estimated VOD is often used as a biomass proxy or to investigate plant water dynamics on multiple scales, as it is dependent on the vegetation water content, dry woody biomass, and canopy structure. For most but not all microwave satellite remote sensing applications, VOD and ω are assigned a priori from auxiliary vegetation information and used as inputs to retrieve soil moisture. Alternatively, polarized brightness temperature has been used to estimate VOD and ω. Retrieval algorithms typically use the zeroth-order solution to the radiative transfer equation (tau-omega model) to simultaneously characterize surface and vegetation emission. In this study we investigate one year of estimated VOD and ω at L- (1.4 GHz), C– (6.9 GHz) and X-band (10.7 GHz), from SMAP and AMSR2 satellites respectively. Since VOD and ω describe absorption and scattering integrated over a path length, we derive estimates of relative absorption and scattering through normalization by a vegetation height derived from GLAS light detection and ranging (lidar) measurements.We assess whether relative attenuation is independent of vegetation height and land cover class. We also test how the relative absorption and scattering estimates follow a spectrum across microwave frequencies. We apply this analysis globally and find relative canopy absorption and scattering to follow distinct patterns, relatively independent of the spatial distributions of vegetation height. We find that relative canopy absorption generally peaks for shorter vegetation (5–10 m) and high density (shrub lands) while high forest canopies have relatively lower values. The VOD values that include the path length (taller vegetation) however are higher for forests relative to shorter vegetation. Seasonal amplitudes of VOD are higher for C- and X-band than for L-band and have maxima in regions with strong rainfall seasonality. This study presents first results for global relative canopy attenuation, based on one consistent retrieval for three frequencies, and discusses temporal dynamics as well as the role of multiple frequencies in assessing attenuation in canopies.