Simulation of the vegetation index

In order to investigate atmospheric influences on the NDVI, model calculations of the vegetation index were performed, with the assumption that surfaces reflect according to the cosine law. The two wavebands selected represent the first two NOAA satellite channels. Molecular and aerosol extinction, water vapour absorption and ozone absorption provide the strongest influences on the vegetation index at these wavebands. The NDVI was simulated at the top of the atmosphere for eight sets of atmospheric conditions and eight different sun elevations, from 20 to 90 degrees, with a constant surface albedo of 0.1 for channel 1 and 0.37 for channel 2. The resulting NDVI at the surface is 0.574. The aerosol optical depth is comparatively low and for the model calculations it is assumed to be independent of the wavelength. For presentation purposes, the calculations are given for the mean ozone concentration of 0.30 cm (at standard temperature and pressure). The aerosol optical depth is denoted by TAU and the precipitable water by PPW in the figure.

The results are summarised on the following page.