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 an approximation to 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 camera characterization 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 and 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 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-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 permit the detection of cirrus in most but not all instances.
Highlights
Cirrus clouds are composed of ice crystals
Halo ratio time series were obtained on 6 July 2016 between 08:00 and 11:00, when halo and nonhalo cirrus alternated with scattered cumuli and on 7 July 2016 between 12:00 and 15:00, when mostly cirrus occurred
In variance from the previous definitions of the halo ratio (HR), it was determined as the ratio of the scattering phase function (SPF) at slightly larger angles: 23 and 20◦, which correspond to the locations of the maximum and minimum we typically found in the measured SPF, respectively
Summary
It is well established that because of their high global coverage their impact on the Earth’s climate is significant and to quantify it the microphysical and radiative properties of cirrus have to be better represented in atmospheric models (Baran, 2012) This is not trivial as the ice crystals which compose cirrus can take on a wide variety of non-spherical shapes and have sizes ranging from a few microns up to over a millimetre, making detailed characterization of cirrus difficult and light scattering by cirrus highly challenging to model. Vignetting increases with the aperture and decreases with the focal length It is quantified by comparing daytime image data with sun photometer data under clear sky. To form a correction function symmetric about the zenith, the original curve was “mirrored” about the zenith (see Fig. 9, red curve), a Gaussian of the form shown in Eq (5) was fitted to the mean of the original and the mirror curve (see Fig. 9, dashed curve)
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