A simple parameterization for a boreal forest radiative transfer model at microwave frequencies

Abstract

The impact of forest cover on passive microwave measurements is important but not well documented, particularly for dense boreal forests. The objective of this study was to evaluate a parameterization of the forest emissivity, for dense needleleaf forests at four Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) frequencies (37, 19, 10.7, and 6.9GHz). The τ–ω radiative transfer model was used, but considering transmissivity (γ) at an incidence angle of 55° for the optical thickness (τ). The approach was as follows: 1) high resolution airborne data at 19 and 37GHz were used to derive the equivalent or effective forest parameters (γ–ω) knowing the contribution of the snow–soil emission from non-forested pixels, 2) these γ–ω values were verified using winter radiometric in situ measurements from different forest sites where the brightness temperature (Tb) values above and under the canopy were considered, 3) these γ–ω values were applied and the frequency was extended on a large AMSR-E summer dataset for dense boreal forest sites. The airborne-derived values were consistent with in situ measurements as well as with satellite-based observations. We found that at AMSR-E frequencies, transmissivity values spectrally decreased from 0.67 to 0.41 for frequencies from 6.9 to 37GHz. The single scattering albedo (ω) can be considered frequency independent at ω≈0.064±0.005. A soil parameterization (QH model) at different frequencies was also proposed for boreal forests. We discuss these new γ–ω values and the soil parameterization within the context of the relevant literature. The results are of particular interest for various remote sensing applications based on passive microwave sensor observations over boreal forests, such as the retrieval of soil moisture and the monitoring of snow characteristics under the canopy.