Unravelling the microphysical structure of complex layer clouds using Doppler radar, Doppler lidar and polarimetric measurements

The microphysics of frontal layer clouds is often complex, with a variety of different processes (nucleation, vapour growth, aggregation) occurring at different levels. In this work we present observations of such clouds using a variety of remote-sensors based at the Chilbolton Observatory in the UK, in an attempt to unravel this structure. We show that vertical profiles of radar Doppler spectra may be used to identify regions embedded within the main cloud layer where ice nucleation is occurring, based on the formation of a second population of ice particles with very slow fall speeds at that height, which are then observed to grow (by vapour deposition and aggregation), ultimately merging into the main Doppler peak as they fall over a depth of 1-2km. This signature is commonly observed in the temperature range -10 to -20C, corresponding to the planar crystal growth regime. Coincident zenith-pointing Doppler lidar measurements confirm this interpretation, with strong specular reflections observed from the pristine oriented planar crystals, while the lidar Doppler velocity closely tracks the secondary peak in the radar Doppler spectrum profile. A weak increase in differential reflectivity can sometimes be observed at the same level, however this often masked by the presence of larger polycrystal/aggregated particles formed at cloud top.

We argue that the nucleation of ice in these layers is evidence of the presence of liquid water droplets, some of which freeze to produce the planar crystals. Long term statistics from Chilbolton have shown that ice forms almost exclusively in supercooled clouds in the temperature range -10 to -20C.

Finally, a case study from the recent APPRAISE-CLOUDS field campaign provides further evidence for this behaviour. A precipitating frontal cloud system was observed using the UK's FAAM (www.faam.ac.uk) instrumented aircraft whilst coincident measurements using scanning S-band polarimetric radar and vertically pointing Ka-band Doppler radar were made. The in-situ observed crystal habits and distribution of liquid water confirm the picture developed above, with complex polycrystalline particles formed at cloud top (-25C) with plate/dendrite crystal types nucleated in an embedded layer of liquid water droplets near the -15C level, coincident with bimodality in the Doppler spectrum and intermittent regions of elevated differential reflectivity from the oriented crystals.