Halo ratio from ground based all-sky imaging

Abstract

Abstract. The halo ratio (HR) is a quantitative measure characterizing the occurrence of the 22• halo peak associated with cirrus. We propose to obtain it from the scattering phase function (SPF) derived from all-sky imaging. Ground based fisheye cameras are used to retrieve the SPF by implementing the necessary image transformations and corrections. These consist of geometric correction of lens distortion by utilizing positions of known stars in a camera image, transforming the images from the zenith-centred to the light-source-centred system of coordinates, correcting for the air mass and for vignetting, the 5 latter using independent measurements from a sun photometer. The SPF is then determined by averaging the image brightness over the azimuth angle and the HR by calculating the ratio of the SPF at two scattering angles in the vicinity of the 22• halo peak. In variance from previous suggestions we select these angles to be 20• and 23• , on the basis of our observations. HR time series have been obtained under various cloud conditions, including halo cirrus, non-halo cirrus and scattered cumuli.While the HR measured in this way is found to be sensitive to the halo status of cirrus, showing values typically >1 under halo 10 producing clouds, similar HR values, mostly artefacts associated with bright cloud edges, can also be occasionally observed under scattered cumuli. Given that the HR is an ice cloud characteristic, a separate cirrus detection algorithm is necessary to screen out non-ice clouds before deriving reliable HR statistics. Here we propose utilizing sky brightness temperature from infrared radiometry: both its absolute value and the magnitude of fluctuations obtained through detrended fluctuation analysis.The brightness temperature data permits the detection of cirrus in most but not all instances.

Fish-eye camera and image processing for commanding a solar tracker

“…It is noteworthy to mention that the azimuth and elevation are the two coordinates that define the position of a celestial body (sun) in the sky as viewed from a particular location at a particular time. Notice that the fisheye camera has been leveled and oriented toward the North; this is of great importance for the geo-solar calculation in this proposition [18]. In this paper, the images are taken from the fisheye camera ALLSKY 340C .…”

“…Then, they are shifted and rotated to account for the fact that the camera does not point precisely to the zenith, nor is the image top perfectly aligned with the true north. Finally, an empirical scaling factor is applied to the coordinates in the h and k direction to compensate for the distortion created by the lens [18]. Such transformation represents a difference between the sun path follows by the national oceanographic and atmospheric administration of the United States (NOAA) [27], and the proposed solar tracking.…”

“…The elevation of the sun is the angle between the direction of the geometric center of the sun's apparent disk and the observer's local horizon; A stands for the azimuth angle (angle between the sun and the North, measured clockwise around the observer's horizon); Δ stands for the camera's rotation from the North (it may take into account some imperfections of the mechanism and possible de-calibration of the equipment due to its mobility). For the fish-eye cameras ( h, k ) τ is a scale factor; in this paper it is taken as 3.365 px/deg [18]. The above expressions as given in (11),τz=(x−h)2+true(y−ktrue)2…”

Ice Crystal Characterization in Cirrus Clouds II: Radiometric Characterization of HaloCam for the Quantitative Analysis of Halo Displays

“…However, no further details on the calibration method or its application were provided. Recently, Dandini et al (2018) presented a geometrically calibrated all-sky camera with an equisolid angle projection. The camera was equipped with a sun-tracking shadow disk for the observation of halo displays.…”

“…The camera was equipped with a sun-tracking shadow disk for the observation of halo displays. Dandini et al (2018) determined the center pixel coordinates and the azimuthal deviation from true north by comparing star and planet trajectories with their theoretical angular positions.…”