Assimilation of Vegetation Optical Depth Retrievals from Passive Microwave Radiometry

Remote sensing estimates of vegetation have historically been obtained from multi- and hyperspectral optical and thermal satellite sensors, exploiting the relationship between the stomatal stress and the spectral reflectance of leaves and canopies. These measurements have been used to develop retrievals of variables such as Leaf Area Index (LAI), Normalized Difference Vegetation Index (NDVI), solar induced fluorescence (SIF) and biomass. A significant limitation of the optical and thermal infrared sensors is that cloud cover can severely limit the acquisition of data, restricting the coverage to cloud-free, clear days. Passive microwave measurements, on the other hand, are all-weather and are not limited by cloud cover. Historically, microwave radiometry has usually been used for retrieving estimates of soil moisture. As these measurements are also sensitive to vegetation, the attenuation of the microwave signal from vegetation, described by the vegetation optical depth (VOD) parameter can be used an analog of above-ground canopy biomass. In this study, we present the results of assimilating microwave based VOD retrievals X-, C- and L-band sensors within the NoahMP land surface model. The impact of VOD assimilation on key terrestrial water and carbon budget terms are quantified through comparisons against reference datasets. The study also contrasts the relative utility of assimilating microwave soil moisture and VOD.