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

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 reveal 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. 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 was much less pronounced than at warmer temperatures: here we suggest that crystals nucleated from small droplets grow into single plate crystals which are able to produce specular reflection, whilst larger droplets grow into complex polycrystals which are not. The lidar Doppler measurements show that typical fall speeds for the oriented crystals are ≈ 0.3ms−1, with a weak temperature correlation; the corresponding Reynolds number is Re ∼ 10, in agreement with light-pillar measurements. Coincident Doppler radar observations show no correlation between the specular enhancement and eddy dissipation rate, indicating that turbulence does not control crystal orientation in these clouds. Copyright c © 2009 Royal Meteorological Society