03 Mar 2022
03 Mar 2022
Status: a revised version of this preprint was accepted for the journal ACP.

Ice Crystal Characterization in Cirrus Clouds III: Retrieval of Ice Crystal Shape and Roughness from Observations of Halo Displays

Linda Forster1 and Bernhard Mayer1,2 Linda Forster and Bernhard Mayer
  • 1Meteorologisches Institut, Ludwig-Maximilians-Universität, München, Germany
  • 2Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany

Abstract. In this study, which is the third part of the HaloCam series after Forster et al. (2017) and Forster et al. (2020), we present a novel technique to retrieve quantitative information about ice crystal optical and microphysical properties making use of ground-based imaging observations of halo displays. Comparing HaloCam’s calibrated RGB images of 22° and 46° halo observations against a look-up table of simulated radiances, this technique allows the retrieval of size and shape of randomly oriented crystals as well as the fraction of smooth and rough ice crystals for cirrus clouds. We analyzed 4400 HaloCam images between September 2015 and November 2016 showing a visible 22° halo. The optical properties of hexagonal 8-element columns with a mean ice crystal effective radius of about 20 μm and a mixture of 37 % smooth and 63 % rough crystals on average best match the HaloCam observations. Implemented on different sites, HaloCam in combination with the machine-learning based halo detection algorithm HaloForest can provide a consistent dataset for climatological studies of ice crystal properties representing typical cirrus clouds. Representative ice crystal optical properties are required for remote sensing of cirrus clouds as well as climate modeling. Since ground-based passive imaging observations provide information about the forward scattering part of the ice crystal optical properties, the results of this work ideally complement the results of satellite-based and airborne studies.

Linda Forster and Bernhard Mayer

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-128', Ping Yang, 15 Mar 2022
  • RC2: 'Comment on acp-2022-128', Anonymous Referee #2, 24 Mar 2022

Linda Forster and Bernhard Mayer

Linda Forster and Bernhard Mayer


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Short summary
We present a novel retrieval using ground-based imaging observations of halo displays together with radiative transfer simulations to help improve our understanding of ice crystal properties representative for cirrus clouds. Analysis of 4400 calibrated HaloCam images featuring a 22° halo revealed aggregates of hexagonal columns of 20 µm effective radius with a mixture of about 37 % smooth and 63 % severely roughened surface as best match in general.