Field Notes: The Polar Field Services Newsletter

Picturing Optical Displays at Summit Station, Greenland

Summiteer Lance Roth kite-skis under optical skies. Photo: Ed Stockard

Summit Station manager Ed Stockard and ICECAPS tech Patrick Wright have been collaborating to produce photographs of optical displays as well as photomicrographs of the snow crystals that produce these displays. Atmospheric optics, including halos, arcs, parhelia, and parhelic circles, occur due to the refraction and reflection of light by ice crystals in the atmosphere, often called “diamond dust.” See www.atoptics.co.uk for more information and terminology on ice halos.

During phase I of Summit’s winter season, Ed has captured many great photos of optics including solar and lunar halos and fogbows. On August 23, Patrick and Lance Roth went kite skiing under very nice optical displays, and Ed was able to get the full fish-eye photo of Lance and his kite shown above. The optics in this photo include a 22-degree halo, bright parhelia and a parhelic circle, and a partial circum-zenithal arc. Displays such as this are often a result of very small simple prisms: columns, hexagonal plates, and triangular plates, pictured below.

For the ICECAPS cloud study, researchers photogrraph ice crystals like these at Summit Station. Photo: Patrick Wright

 

Although the corresponding ice crystal photo shown above was not taken on the same day, similar conditions were recently photographed by Patrick using the Ice Particle Imaging Camera at the ICECAPS Mobile Science Facility (MSF) [Von Walden, U of Idaho, PI]. Sub-mmcrystals  such as these are common during optical displays. See http://www.flyozone.com/snowkites/en/news/headlines/20626 for more photos of this day as well as Ed’s description of the event.

While a halo appears on the ice sheet surface, none appears in the sky. Why? Photo: Ed Stockard

On September 8, a unique optical display was observed, showing a distinct halo on the snow surface, but without atmospheric optics. Precipitation over the previous few hours was dominantly an ice crystal form called bullet rosettes, shown in the photo below. These crystals covered the ice sheet surface in a thin but uniform layer, creating a beautiful surface halo that Ed captured in this corresponding image. Bullet rosette crystals are a result of nucleation of multiple crystals growing away from a central point at random orientations, and they often break apart into isolated bullet-shaped columns. Well-formed surface halos are somewhat rare, so this was a great opportunity to photograph the phenomenon and to investigate its cause.

These flower-shaped crystals, photographed on the snow surface at Summit Station, are called rosettes. Photo: Patrick Wright

In addition, measurements are being made at the ICECAPS Mobile Science Facility that can help to determine the possible causes of atmospheric optics. A LiDAR instrument (Light Detection and Ranging) emits a laser beam through a window in the MSF roof into the atmosphere above Summit, which is then reflected off ice crystals and aerosols and returns back through the window to a detector on the instrument.

The ICECAPS LiDAR shoots a green beam from the roof of the Mobile Science Facility--it matches the aurora borealis also seen in the night sky. Photo: Ed Stockard

The LiDAR detects any “hydrometeor” in the atmosphere, primarily giving data on cloud height and depth, and particle phase (supercooled liquid vs. ice). Also, the LiDAR detects reflected light in three polarization orientations, and new techniques show potential to use this polarization data to detect ice crystal orientation, which could play some role in optical displays. If periods of strong atmospheric optics can be correlated with the LiDAR data, it may be possible to investigate the extent that ice crystals are similarly oriented during optical display conditions.— Patrick Wright

The ICECAPS project is funded by the U.S. National Science Foundation, which funds and manages the Summit Station in cooperation with the government of Greenland.

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