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Importance of ice crystal shape in Arctic cirrus clouds as retrieved from halo photography

Cirrus clouds influence Earth’s radiative energy budget as they scatter and absorb solar radiation (0.3 to 4.0 µm wavelength), and absorb and emit terrestrial radiation (4 to 100 µm). They either cool or warm the Earth’s climate depending on which of the two effects dominates. Beside the optical thickness and effective ice crystal radius, the sign of the radiative forcing of cirrus clouds depends crucially on the ice crystal shape and orientation.

Image may contain: Sky, Light, Lighting, Photography.

Image reference:

Left: Forster et al. (2017).: Ice crystal characterization in cirrus clouds. Right: 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

To provide measurements of the ice crystal morphology on a regular basis, we are developing and calibrating a camera system mounted on a sun-tracking device to automatically observe halo displays at the Alomar Observatory on Andøya in the Norwegian Arctic. Halo displays are the result of refraction and reflection of solar radiation by hexagonal ice crystals. Thus, brightness analysis of the halo observations will be used to retrieve scattering phase functions of the ice crystals and, thus, deduce ice crystal properties such as shape, orientation, and roughness. The installation at the Alomar Observatory will allow for a direct comparison with ice crystal shape retrievals from depolarization ratio measurements by a tropospheric lidar.

This comprehensive dataset of microphysical properties of Arctic cirrus clouds will be used as input in radiative transfer simulations to study effects of the ice crystal morphology on the radiative forcing of Arctic cirrus clouds. This is especially important in the rapidly warming Arctic as clouds play a major role in its amplified response to climate change, and persistent observations as provided by the halo camera observations are scarce in high latitudes.

The prospective candidate should have basic programming skills (Python is preferred) and an interest in the remote sensing of clouds. The project will begin in Fall 2020.

Tags: Meteorology, Halo photography
Published Oct. 26, 2020 1:37 PM - Last modified Aug. 12, 2022 3:43 PM

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