Global long-term passive microwave satellite-based retrievals of vegetation optical depth.

Vegetation optical depth (VOD) retrievals from three satellite-based passive microwave instruments were merged to produce the first long-term global microwave-based vegetation product. The resulting VOD product spans more than two decades and shows seasonal cycles and inter-annual variations that generally correspond with those observed in the Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI). Some notable differences exist in the long-term trends: the NDVI, operating in the optical regime, is sensitive to chlorophyll abundance and photosynthetically active biomass of the leaves, whereas the microwave-based VOD is an indicator of the vegetation water content in total above-ground biomass, i.e., including wood and leaf components. Preliminary analyses indicate that the fluctuations in VOD typically correlated to precipitation variations, and that the mutually independent VOD and NDVI do not necessarily respond in identical manners. Considering both products together provides a more robust structural characterization and assessment of long-term vegetation dynamics at the global scale.

Figure 2: (a) Correlation coefficient (r) between monthly VOD and NDVI for 1987 through 2006; and trends in annual averages of (b) VOD and (c) NDVI from 1988 through 2006 (unit: change per year). The ranges of annual averages of VOD and NDVI (see Figure 1) are 0 to 1 and 0.1 to 0.8, respectively.