Cloud – vegetation interaction: use of Normalized Difference Cloud Index for estimation of cloud optical thickness

A new technique to retrieve cloud optical depth for broken clouds above green vegetation using ground-based zenith radiance measurements is developed. By analogy with the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Cloud Index (NDCI) is defined as a ratio between the difference and the sum of two zenith radiances measured for two narrow spectral bands in the visible and near-IR regions. The very different spectral behavior of cloud liquid water drops and green vegetation is the key physics behind the NDCI. It provides extra tools to remove the radiative effects of the 3D cloud structure. Numerical calculations based on fractal clouds and real measurements of NDCI and cloud liquid water path confirm the improvements. 1 . Introduction We study the removal of ambiguity in the interpretation of measured downwelling radiances in the simultaneous presence of broken clouds and green vegetation. A three-dimensional (3D) cloud structure triggers complex radiative effects (e.g., Marshak et al., 1998) that make remote sensing of cloud properties difficult or impossible. The key to solving this problem is the very different spectral behavior of cloud liquid water drops and green vegetation (Figs. 1a and 1b). For example, cloud optical properties, and hence cloud reflectivities, change little between 0.65 and 0.86 μm, while the vegetated surface albedo, ρsur, changes from 0.1 to 0.5 (e.g., Tucker, 1979) between the same two wavelengths. This spectral contrast in surface albedo suggests using ground measurements at both wavelengths not independently, but as an algebraic combination (a spectral index). Based on this idea, we have developed a new technique to retrieve cloud optical depth for broken clouds above green vegetation using ground zenith radiance measurements in two narrow spectral bands in the visible (VIS) and near-IR (NIR) regions. For a spectral band in the NIR region, the green vegetation acts as a powerful reflector that “illuminates” horizontally inhomogeneous clouds from below. This provides the extra information needed to largely remove the radiative effects of the 3D cloud structure, especially in the case of broken clouds; this in turn allows the retrieval of cloud optical depth using traditional one-dimensional (1D) radiative transfer theory. 0 0.1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .4 0 .6 0 .8 1 1.2 1 .4 1 .6 1 .8 su rf ac e re fle ct an ce a t 30 0 m wavelength (μm ) VIS n e a r I R m i d I R ( a ) 0.75 0.8 0.85 0.9 0.95 1 0 1 0 2 0 3 0 4 0 5 0 6 0 0.4 0 .6 0 .8 1 1.2 1 .4 1 .6 1 .8 g a n d ω 0 (d im e n s io n le s s ) x in c io n , σ