Cloud-vegetation interaction: Use of Normalized Difference Cloud Index forestimation of cloud optical thickness

A new technique to retrieve cloud optical depth for broken clouds above gre en vegetation using ground-based zenith radiance measurements is developed. By analogy with the Normalized Difference Vegetation Index (NDVI), the Nor malized Difference Cloud Index(NDCI) is defined as a ratio between the diff erence and the sum of two zenith radiances measured for two narrow spectral bands in the visible and near-IR regions. The very different spectral beha vior of cloud liquid water drops and green vegetation is the key physics be hind the NDCI. It provides extra tools to remove the radiative effects of t he 3D cloud structure. Numerical calculations based on fractal clouds and r eal measurements of NDCI and cloud liquid water path confirm the improvemen ts.