Abstract
AbstractResults 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° to −11°, 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 1 km across, that contained supercooled liquid‐water droplets with an effective diameter of 24 µm 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 ZDR (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 ZDR assuming horizontal alignment of the crystals. We argue that the high concentrations of small ice crystals (up to 103 l−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, low‐ZDR aggregates which appeared to rapidly accrete the columns. A number of plumes of high ZDR 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°, the regions of high ZDR 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 ZDR observations are in good agreement with our theoretical finding that columns alone cannot produce values of ZDR 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. Copyright © 2002 Royal Meteorological Society
Highlights
Fronts are responsible for the bulk of the precipitation that falls in the mid-latitudes, and most current forecast models assume that this ‘large-scale’ precipitation is formed by a uniform distribution of ice particles within the model grid box which grow by deposition and aggregation as they fall, melting to produce rain
In this paper we have presented the first detailed simultaneous remote and in situ measurements of such a region. We summarise these results and discuss the merits of the ZDR parameter for mapping out regions containing pristine crystals
These turrets were found by the aircraft to be associated with narrow updraughts of 1–2 m s 1 containing 0.1–0.2 g m 3 ¥ of liquid water and concentrations of small ice crystals up § ̈ to 2 orders of magnitude larger than the ‘ambient’ values only 5 km away
Summary
Fronts are responsible for the bulk of the precipitation that falls in the mid-latitudes, and most current forecast models (which typically have horizontal resolutions between 15 km and 60 km) assume that this ‘large-scale’ precipitation is formed by a uniform distribution of ice particles within the model grid box which grow by deposition and aggregation as they fall, melting to produce rain. Hogan et al (1999) observed high-ZDR radar signals at the 15 C top of a frontal cloud, and using data from an airborne lidar showed them to be associated with a thin but horizontally extensive liquid water layer.
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