A practical method for simulating AVHRR-consistent NDVI data series using narrow MODIS channels in the 0.5-1.0 mu m spectral range

Over the past two decades, a key indicator of climate change has been the l ong time series of global maps of the normalized difference vegetation inde x (NDVI), derived from remotely sensed data acquired with a series of NOAA advanced very high resolution radiometer (AVHRR) instruments from space. Th ese NDVI values are calculated from relatively broad AVHRR channels in the red (similar to 0.58-0.68 mu m) and near-infrared (similar to 0.73-1.0 mu m ) regions. Continuation of this long term data set is extremely valuable fo r climate-related research, However, sometime in the coming decade, the AVH RR time series measurements will no longer be continued, Instead, the measu rements will be made using newer generation satellite instruments having na rrower channels and improved spatial resolution, For example, the moderate resolution imaging spectroradiometer (MODIS) onboard the Terra spacecraft h as several narrow channels in the 0.4-1.0 spectral range. The NDVI values d erived from the MODIS red channel (similar to 0.62-0.67 mu m) and near-IR c hannel (similar to 0.841-0.876 mu m) will be biased compared to those deriv ed from the broader AVHRR channels because of differences in channel positi ons and widths for the two instruments. The narrow MODIS near-IR channel is -only slightly affected by atmospheric water vapor absorption, while the br oad AVHRR near-IR channel is strongly affected by water vapor absorption, A s a result, the largest bias comes from the near IR channels on the two ins truments. To a lesser extent, the bias also comes from the differences betw een the red channel positions and the widths of MODIS and AVHRR instruments , In this paper, we describe a practical method for simulating AVHRR NDVI v alues using several narrower MODIS channels in the 0.4-1.0 mu m spectral ra nge, including the MODIS green channel (similar to 0.545-0.565 mu m) and th e ater vapor absorption; channel (similar to 0.915-0.965 mu m), For this st udy, hyperspectral imaging data acquired with the airborne visible/infrared imaging spectrometer (AVIRIS) are used in the simulation of MODIS and AVHR R channels. This same method can, in principle, be used to derive AVHRR-con sistent NDVI data series from other future satellite sensors that have simi lar capabilities as MODIS.