Properties of embedded convection in warm-frontal mixed-phase cloud from aircraft and polarimetric radar

Results are presented from a case-study in which the macrophysical and microphysical characteristics of a warm-frontal mixed-phase cloud were investigated using simultaneous aircraft and polarimetric-radar measurements. A region of embedded convection was located and sampled at various stages of its ascent through the cloud from -5 degreesC to - 11 degreesC, and evidence is presented that it was triggered by Kelvin-Helmholtz instability near the melting layer. High concentrations of small crystals were observed in and above narrow convective turrets, around I kin across, that contained supercooled liquid-water droplets with an effective diameter of 24 pm and riming ice particles. The area surrounding the turrets was found to contain pristine columns, and was strikingly visible to the radar as a broad plume of high differential reflectivity Z(DR) (around 3 dB), contrasting with the 0-0.5 dB range usually found in ice clouds. The columns observed in situ had axial ratios of around 5:1, in close agreement with the values predicted theoretically from Z(DR) assuming horizontal alignment of the crystals. We argue that the high concentrations of small ice crystals (up to 10(3) 1(-1)) observed in this case were splinters produced via the Hallett-Mossop mechanism in the riming process, and these then grew rapidly by deposition in the water-saturated environment into pristine columns. Later in the ascent the picture was complicated by the presence of a fallstreak containing large, loq-Z(DR) aggregates which appeared to rapidly accrete the columns. A number of plumes of high Z(DR) were observed by the radar on the same day, each of which persisted for at least half an hour. In the vicinity of cloud top (-15 degreesC), the regions of high Z(DR) tended to spread out horizontally, and attain values as high as 7 dB. At these temperatures plates and dendrites are known to grow, so these Z(DR) observations are in good agreement with our theoretical finding that columns alone cannot produce values of Z(DR) greater than 4 dB, no matter how extreme their axial ratios, whereas planar crystals can attain values up to 10 dB. Embedded convection could clearly be important in determining the distribution of ice and liquid water in frontal clouds, which affects both cloud glaciation and the radiation budget, and is important for aircraft icing.