  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 4deg 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 -15C). 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 -22C was also observed, but 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 lidar Doppler measurements show that typical fall speeds for
the oriented crystals are 0.3m/s, 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.
