Determination of cloud contributions to Earth's radiation budget and anthropogenic perturbations remains a major measurement problem in part because of sub-pixel structure that is unresolved by satellite instruments such as MODIS (250 m resolution in shortwave). Here initial results are presented characterizing clouds by surface-based digital photography looking vertically upwards with high resolution in horizontal distance (~ 1 cm for cloud at 1 km), time (seconds), and radiance (parts per thousand). Digital photography from the surface looking upward affords the advantage over satellite imagery in quantitative interpretation of measured signal in that the background radiance is the black of outer space, with contributions to downwelling radiance only from light scattering by air (Rayleigh scattering), aerosol particles, and cloud droplets, with no confounding contribution to the signal from surface-leaving radiance. A key limitation to the method is that it is restricted to daytime (sunlit) conditions, as the measurement relies on scattering of solar radiation. Also, the measurements are hyperlocal; at the maximum focal length of the camera employed, the field of view of approximately 20 mrad corresponds, for a cloud at 1 km, to 20 m. Images will be presented showing variation in cloud radiance on scales of less than 1 m. Zenith radiance is inverted to yield corresponding images of cloud optical depth COD for optically thin clouds (COD less than about 3) with estimated accuracy of about 10%.
This page was last updated 2016-06-24.
Return to Stephen E. Schwartz Publications Page