Vegetation phenology monitoring with SeaWinds scatterometer in eastern Asia

Vegetation phenology tracks plants´ lifecycle events, revealing the response of vegetation to global climate changes. Microwave backscatter is insensitive to signal degradation from solar illumination and atmospheric effects and could provide an alternative data source to optical remote sensing in phenology studies. In this study, we analyzed a time series of Ku-band radar backscatter measurements from the SeaWinds scatterometer on board the Quick Scatterometer (QuickSCAT) to examine its effectiveness for vegetation phenology monitoring across eastern Asia. Phenological metrics including the start of season (SOS) and end of season (EOS) were derived from time series of backscatter data with a weighted curve fitting method. Comparing with MODIS phenology products, backscatter detects earlier greenup dates for the grasslands in Kazakhstan and eastern Tibetan Plateau and forests in southern and northern China areas. For agricultural lands in the middle of China and northern India, the backscatter data shows later greenup dates than MODIS data. The backscatter EOS is later the MODIS senescence dates in most areas, whereas showing spatial patterns agreeing with regional climate gradients. In the grasslands in Kazakhstan, Mongolia and China, the EOS detected by MODIS is later than backscatter data. The bias of backscatter phenological metrics and MODIS products might be caused by the temporal shifts between backscatter increase and canopy greenups. Overall, the results indicate that SeaWinds backscatter is sensitive to seasonal canopy dynamics across a range of vegetation types and provides a quantitative view that is independent of optical/NIR remote sensing instruments.