Characterization of the effects of cloud heterogeneity on cloud fraction and cloud radiative effects

Properties and extent of clouds have been examined by high resolution digital photography from the surface. High resolution is important for accurate cloud radiative effect calculations and improvements of climate models. The novel approach presented here makes use of a commercially available digital camera to detect cloud and sky radiation in red, green, and blue with a spatial resolution of 30 mm at 1km, close to the Rayleigh diffraction limit. Cloud effect on zenith radiant intensity found to be highly variable even on spatial scales as low as 30 mrad field of view; cloud fraction even in such small field of view is highly sensitive (several tens of percent) to threshold and resolution. The spectral response of the camera was characterized using an integrating sphere with a Mercury Xenon lamp source and monochromator and photodiode with NIST traceable calibration. The convolution of the camera's spectral response and direct normal solar irradiance spectrum allowed us to determine the expected response to solar and blue-sky radiation.

The spectral and spatial responses of the camera were used in the calculation of cloud fraction based on different techniques such as hybrid methods and fractal analysis. Cloud images are also used in conjunction with measurements at Upton, Long Island, New York during the Summer of 2014 from collocated Cimel sun- and sky photometer and Microwave Radiometer are used to determine effects of cloud heterogeneity on retrieval of cloud optical thickness and liquid water path.