7 Years HaloCam – Long-term Observations of Halo Displays for Remote Sensing of Ice Crystal Properties in Cirrus Clouds

Since September 2013, HaloCam has been recording images of halo displays formed by cirrus clouds. Stationed on the rooftop platform of LMU‘s Meteorological Institute Munich (MIM), the calibrated sun-tracking camera system is capturing the sky around the sun every 10 seconds. This unique long-term dataset allows the retrieval of valuable information about microphysical properties of the observed cirrus clouds. These properties govern the impact of cirrus clouds on the radiation budget of the Earth’s atmosphere and surface and, consequently, on climate.

Halo displays are optical phenomena which arise from refraction and reflection of sunlight by ice crystals. Depending on shape and orientation of the crystals more than 50 different displays can occur (https://www.meteoros.de/themen/halos). One of the most frequent halo displays is the 22° halo, a circle around the sun at an angle of 22°, which is formed by refraction of light by randomly oriented hexagonal crystals. Other examples are the 22° parhelia (sundogs), caused by refraction from horizontal oriented hexagonal ice crystal plates, as well as the upper and lower tangent arcs, formed by oriented ice crystal columns.

Types and intensities of such halo displays can be attributed to certain ice crystal properties, especially distributions of crystal size, shape, orientation, and surface roughness. These determine how much sunlight is transmitted or reflected. The retrieval algorithm is assisted by machine learning methods (Random Forest Classifier) and based on comparing the observed with synthetic images from radiative transfer simulations using libRadtran (http://www.libradtran.org). Since cirrus clouds cover about 30% of the globe, an improved characterization of their optical and microphysical properties is key to better understand their role in our changing climate.

• Forster, L., Seefeldner, M., Baumgartner, A., Kölling, T., and Mayer, B.: Ice crystal characterization in cirrus clouds II: radiometric characterization of HaloCam for the quantitative analysis of halo displays, Atmos. Meas. Tech., 13, 3977–3991, https://doi.org/10.5194/amt-13-3977-2020, 2020.

• 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., 10, 2499–2516, https://doi.org/10.5194/amt-10-2499-2017, 2017.