Cloud Effects in Hyperspectral Imagery from First-Principles Scene Simulations
SPECTRAL SCIENCES INC BURLINGTON MA
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Clouds and cloud fields introduce important backscattering, obscuration, shadowing and radiative trapping effects in visible-NIRnear-infrared-SWIRshort-wavelength infrared hyperspectral imagery of the ground, especially in offnadir slant viewing geometries where cloud thickness effects reduce the cloud-free line of sight CFLOS. An investigation of these effects was conducted using monochromatic, multispectral and hyperspectral scene simulations performed with the Spectral Sciences, Inc. MCScene Monte Carlo code. Cloud fields were obtained from the Cloud Scene Simulation Model CSSM of Cianciolo and Raffensberger. The simulations took advantage of a data-fusionbased noise-removal method that enabled a dramatic reduction in computation time. Illumination levels at the sunlit ground showed enhancements of up to 50 due to cloud scattering. Illumination in the cloud shadows was 20 of the full solar illumination or greater, with cloud optical depths of up to 10. Most of this illumination arises from solar scattering off the cloud tops and sides however, a significant part can be ascribed to radiative trapping between the ground and the clouds, as represented by a local atmospheric spherical albedo. A simulation of a hyperspectral scene with cloud shadows was found to reproduce shadowing effects found in real data. Deeper shadowing is observed with increasing wavelength and in water-band regions, consistent with a previous analysis of cloud shadows in real imagery. The MCScene calculations also predict shadow enhancements of column water vapor retrievals from atmospheric correctioncompensation codes, also in accord with field observations. CFLOS fractions were calculated as a function of off-nadir viewing angle and were found to be very accurately represented by a semi-empirical analytical function of both angle and cloud cover.
- Atmospheric Physics
- Computer Programming and Software
- Optical Detection and Detectors