Correcting for Circumsolar and Near-Horizon Errors in Sky Cover Retrievals from Sky Images

C.N Long

doi:10.2174/1874282301004010045

Abstract

Fractional sky cover amounts retrieved from sky imagery are overestimated significantly at times due to occurrences of “whitening” near the sun, and near the horizon for low sun, in the images. This phenomenon occurs due to forward scattering of visible light by aerosols and haze, and the intensity range limitations of the detectors of the cameras used to record the sky images. Our results suggest that when the problem occurs, the magnitude of the overestimate is typically on the order of about 10% to 20% fractional sky cover. To help alleviate this problem, a statistical analysis of the time series of the areas in the image near the sun position and along the horizon centered on the solar azimuth angle has been developed. This statistical analysis requires that images be captured frequently, at least once per minute. For times when the overestimation is detected as occurring, a correction is applied to the retrieved sky cover amounts. When the sky cover amount correction is applied, analysis indicates that the result better matches the actual sky conditions present, as noted by visual inspection of the sky images in question. In addition, frequency-of-occurrence histogram comparisons show that the adjusted results improve the agreement with other methodologies and expectations. Thus, the methodology presented here helps produce more accurate fractional sky cover retrievals.

Highlights

In the cloudless sky, the area near the sun is most often whiter and brighter than the rest of the hemisphere due to the forward scattering by aerosols and haze
The analysis included data from more than seven months of data at Pacific Northwest National Laboratory, and for the Atmospheric Radiation Measurement (ARM) Cloudiness Inter-comparison Campaign (CIC) field experiment held at the ARM Southern Great Plains site in Oklahoma
There exists some uncertainty in fractional sky cover retrievals from sky images, this uncertainty does not appear to be greater than that attached to human observations for the retrievals used in this study

Summary

Introduction

The area near the sun is most often whiter and brighter than the rest of the hemisphere due to the forward scattering by aerosols and haze. Even a slight haze or moderate aerosol loading will make a large angular area of the horizon whiter and brighter when the sun is low on the horizon. One of the problems in using digital cameras such as those used in the sky imager included here is the intensity range limitations of the camera detector. With no a priori knowledge of the aerosol or haze loading that can be used in some way to predict an increased brightness, these pixels are often interpreted as “cloudy” in the sky-imager retrievals when a human observer would label them as “cloudless.” This problem has been recognized previously, for instance by Pfister

Methods

Results

Conclusion