Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated layer clouds

Abstract

The properties of planar ice crystals settling horizontally have been investigated using a vertically pointing Doppler lidar. Strong specular reflections were observed from their oriented basal facets, identified by comparison with a second lidar pointing 4 • from zenith. Analysis of 17 months of continuous high-resolution observations reveals that these pristine crystals are frequently observed in ice falling from mid-level mixed-phase layer clouds (85% of the time for layers at −15 • C). Detailed analysis of a case study indicates that the crystals are nucleated and grow rapidly within the supercooled layer, then fall out, forming well-defined layers of specular reflection. From the lidar alone the fraction of oriented crystals cannot be quantified, but polarimetric radar measurements confirmed that a substantial fraction of the crystal population was well oriented. As the crystals fall into subsaturated air, specular reflection is observed to switch off as the crystal faces become rounded and lose their faceted structure. Specular reflection in ice falling from supercooled layers colder than −22 • C was also observed, but this was much less pronounced than at warmer temperatures: we suggest that in cold clouds it is the small droplets in the distribution that freeze into plates and produce specular reflection, whilst larger droplets freeze into complex polycrystals.The

A global view of horizontally oriented crystals in ice clouds from Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)

“…The latter appear in ∼20% of ice clouds layers between −10°C and −15°C globally [Hogan et al, 2004], i.e., in the temperature range where the present study found the highest concentration of oriented crystals. Recent ground-based observations suggest that supercooled layers play an important part in the formation of planar ice crystals susceptible to orientation [Westbrook et al, 2010], at least at midlatitudes. It is unclear at this point how many supercooled layers appear in the CALIOP data set and how they relate to crystal orientation, as techniques for reliable identification of in-cloud phase variation are still experimental, but it is a question that needs to be addressed.…”

“…Planar crystal growth occurs predominantly in the −12°C to −18°C range [Fukuta and Takahashi, 1999], thus present results reinforce the hypothesis that oriented crystals are mostly planar, with in-cloud vertical motions and sedimentation explaining why oriented crystals are detected in a slightly wider temperature range. The higher detection of oriented crystals just above cloud base requires enhanced planar crystals in that area, which can be explained either by (1) sedimentation from a higher, midcloud supercooled layer where crystals are rapidly nucleated (as described by Westbrook et al [2010]) or (2) ascending air from turbulence near the ice-subsaturated cloud base [Miloshevich and Heymsfield, 1997] leading to the nucleation of new pristine ice crystals at colder temperatures higher within the cloud. Future work should investigate the importance of these mechanisms, by (1) studying levels of supersaturation in and around warm and cold ice clouds, which might influence crystal growth and shape [Spichtinger et al, 2004] and (2) correlate supercooled layers with oriented crystals on a global scale.…”

Orientations and aspect ratios of falling snow

“…Combined lidar and dual polarization radar observations and Ka-band cloud radar depolarization ratio observations in winter layer clouds containing pristine planar ice indicate that crystals can fall with orientations that are very nearly horizontal [Matrosov et al, 2005;Westbrook et al, 2010]. From a more theoretical perspective, Klett [1995] has argued that even for high kinetic eddy dissipation rates of 0.1 m 2 s −3 (similar to what might be observed in a convective cloud), based on an inertial subrange model for turbulence, the calculated average tilt of branched plates larger than about 0.1 mm should be expected to be no more than about 10 ∘ .…”

Lidar observation and model simulation of a volcanic-ash-induced cirrus cloud during the Eyjafjallajökull eruption

“…In the case of planar planes of horizontally oriented ice crystals, specular reflection appears (e.g. Westbrook et al, 2010;Sassen and Benson, 2001), which results in a high backscatter coefficient and a low depolarization ratio. Specular reflection occurs under conditions where large planar ice crystals are formed and align horizontally during sedimentation.…”