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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-2019-475
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-475
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Feb 2020

Submitted as: research article | 07 Feb 2020

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This preprint is currently under review for the journal AMT.

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

Linda Forster1, Meinhard Seefeldner1, Andreas Baumgartner2, Tobias Kölling1, and Bernhard Mayer1,2 Linda Forster et al.
  • 1Ludwig-Maximilians-Universiät, München, Germany
  • 2Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany

Abstract. This study presents the weather-proof RGB camera HaloCam_RAW, which is part of the automated halo observation system HaloCam and designed for the quantitative analysis of halo displays. We present a procedure for both the geometric and absolute radiometric characterization of HaloCam_RAW and demonstrate its application in a case study. The geometric calibration was performed using a chessboard pattern to estimate camera matrix and distortion coefficients. For the radiometric characterization of HaloCam_RAW, dark signal and vignetting effect were determined to correct the measured signal. Furthermore, the spectral response of the RGB sensor and the linearity of its radiometric response were characterized. The absolute radiometric response was determined by cross-calibrating HaloCam_RAW against the completely characterized specMACS imager. For a typical measurement signal the relative (absolute) radiometric uncertainty amounts to 2.8 % (5.0 %), 2.4 % (5.8 %), and 3.3 % (11.8 %) for the Red, Green, and Blue channel, respectively. The absolute radiometric uncertainty estimate is larger mainly due to the inhomogeneity of the scene used for cross-calibration and the absolute radiometric uncertainty of specMACS. Geometric and radiometric characterization of HaloCam_RAW were applied to a scene with a 22° halo observed on 21 April 2016. The observed radiance distribution and 22° halo ratio compared well with radiative transfer simulations assuming a range of ice crystal habits and surface roughness. This application demonstrates the potential of developing a retrieval method for ice crystal properties, such as ice crystal size, shape and surface roughness using calibrated HaloCam_RAW observations together with radiative transfer simulations.

Linda Forster et al.

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Latest update: 17 Feb 2020
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Short summary
We present a procedure for both the geometric and absolute radiometric characterization of the weather-proof RGB camera HaloCamRAW, which is part of our automated halo observation system HaloCam, designed for the quantitative analysis of halo displays. By comparing the calibrated HaloCamRAW radiances of a 22° halo scene with radiative transfer simulations, we demonstrate the potential of developing a retrieval method for ice crystal properties, such as size, shape, and surface roughness.
We present a procedure for both the geometric and absolute radiometric characterization of the...
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