Monitoring global vegetation using AVHRR

The availability of AVHRR time series for the past two decades has attracted many scientists to investigate interannual variability and trends in land surface conditions. To study interannual variability of land surface processes using these data, the change in radiances due to two varying factors-sensor sensitivity and illumination conditions-must be accounted for. This study analyzes the behaviour of global AVHRR shortwave data over land, processed using updated post-launch calibration, for a 12-year period from April 1985-October 1997, and investigates their usefulness for the monitoring of global land surface processes. The author focuses on verifying the stability of post-launch calibrations for NOAA-11 and -14 by assuming correct calibration of NOAA-9 and by excluding the illumination effects, modeled with NOAA-9 data. The residual trends in data averaged over global, stable aggregates, such as deserts and rainforests, are then attributed to errors in calibration. These trends should be removed from the rest of the data to alleviate their misidentification as the real trends in the Earth´s climate system and to make statistical studies of anomalies more reliable. This study illustrates the potential of improving the available global AVHRR time series to infer more reliable conclusions from inter-annual variability studies. Spectral dependence of residual trends was examined. Preliminary assessments suggest that a desert calibration that has been widely used may not be sufficiently accurate for applications to vegetated areas