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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/amt-2018-3
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
28 Feb 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).
Halo ratio from ground based all-sky imaging
Paolo Dandini1, Zbigniew Ulanowski1, David Campbell1, and Richard Kaye2 1School of Physics Astronomy and Mathematics, University of Hertfordshire, Hatfield, AL10 9AB, UK
2School of Engineering and Technology, University of Hertfordshire, Hatfield, AL10 9AB, UK
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 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 cumulus. While the HR measured in this way is found to be sensitive to the halo status of cirrus, showing values typically > 1 under halo producing clouds, similar HR values, mostly artefacts associated with bright cloud edges, can also be occasionally observed under scattered cumulus. 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.
Citation: Dandini, P., Ulanowski, Z., Campbell, D., and Kaye, R.: Halo ratio from ground based all-sky imaging, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2018-3, in review, 2018.
Paolo Dandini et al.
Paolo Dandini et al.
Paolo Dandini et al.

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Short summary
The halo ratio indicates the strength of the 22° cirrus halo, and gives valuable information on cloud properties. We propose to obtain it from all-sky images by applying a range of transformations and corrections and averaging brightness azimuthally over sun-centred images. The ratio is then taken at two angles from the sun, 20° and 23°, in variance from previous suggestions. While we find ratios > 1 to be linked to halos, they can occur under scattered cumuli as artefacts due to cloud edges.
The halo ratio indicates the strength of the 22° cirrus halo, and gives valuable information on...
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