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

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doi:10.5194/amt-2017-17
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
17 Mar 2017
Review status
This discussion paper is under review for the journal Atmospheric Measurement Techniques (AMT).
Ice Crystal Characterization in Cirrus Clouds: A Sun-tracking Camera System and Automated Detection Algorithm for Halo Displays
Linda Forster1, Meinhard Seefeldner1, Matthias Wiegner1, and Bernhard Mayer1,2 1Chair of Experimental Meteorology, Ludwig-Maximilians-Universität, München, Germany
2Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany
Abstract. Halo displays in the sky contain valuable information about ice crystal shape and orientation: e.g. the 22° halo is produced by randomly oriented hexagonal prisms while sundogs indicate oriented plates. HaloCam, a novel sun-tracking camera system for the automated observation of halo displays is presented. An initial visual evaluation of the frequency of halo displays for the ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) field campaign showed that during the six weeks sundogs were observed more often than 22° halos. Thus, the majority of halo displays was produced by oriented ice crystals. During the campaign about 33 % of the cirrus clouds produced 22° halos, sundogs or upper tangent arcs. To evaluate the HaloCam observations collected in Munich from January 2014 to June 2016, an automated detection algorithm for 22° halos was developed, which can be extended to other halo types as well. This algorithm detected 22° halos in about 2 % of the time for this dataset. The frequency of cirrus clouds during this time period was estimated by co-located ceilometer measurements using temperature thresholds of the cloud base. About 25 % of the detected cirrus clouds occurred together with a 22° halo which implies that these clouds contained a certain fraction of smooth, hexagonal ice crystals. HaloCam observations complemented by radiative transfer simulations and measurements of aerosol and cirrus optical thickness provide a possibility to retrieve more detailed information about ice crystal roughness. This paper demonstrates the feasibility of a completely automated method to collect and evaluate a long-term database of halo observations and shows the potential to characterize ice crystal properties.

Citation: Forster, L., Seefeldner, M., Wiegner, M., and Mayer, B.: Ice Crystal Characterization in Cirrus Clouds: A Sun-tracking Camera System and Automated Detection Algorithm for Halo Displays, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2017-17, in review, 2017.
Linda Forster et al.
Linda Forster et al.
Linda Forster et al.

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Short summary
Halo displays are produced by scattering of sunlight by smooth, hexagonal ice crystals. Consequently, the presence of a halo should contain information on particle shape. This study presents HaloCam, a novel sun-tracking camera system, and an automated detection algorithm to collect and evaluate long-term halo observations. Two-year HaloCam observations revealed that about 25 % of the detected cirrus clouds occurred together with a 22° halo indicating the presence of smooth, hexagonal crystals.
Halo displays are produced by scattering of sunlight by smooth, hexagonal ice crystals....
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