CWVC Flight II (9 June 2000) radar synopsis

Robin Hogan 18 July 2002

Introduction

The cloud observed in this case was associated with a slow-moving cold front, and the Frontiers animation showed the precipitation to be rather showery in nature. The precipitation passed through Chilbolton by 10:30 UTC, and was followed by altocumulus. The radar and aircraft sampled along the 246 degree radial from Chilbolton, but because of the strong southerly wind, it was not really possible to track the evolution of individual features.

Cloud top was at 5.5-6 km (-14 to -19 degrees C) throughout most of the experiment, but there were occasions early on when cloud extended up to 10 km (e.g. at 08:22:51). The melting layer was at 2.5-3 km. The altitude of the base of the altocumulus later in the day was around 4 km (-5 degrees C). The lidar image indicates that after the precipitation had passed, intermittent low cloud was also present between 0.5 and 2 km altitude.

Microphysics

Throughout the experiment when cloud top was at around 6 km, high values of ZDR (up to around 6 dB) tended to occur at cloud top. Ice falling into this layer from above tended to greatly reduce the ZDR (e.g. 09:08:09 Zh|Zdr). The appearance of the ZDR was sometimes intermittent, with the high values only occurring in low reflectivity regions, while at other times it was more continuous (note that in this image the high ZDR below 2 km is due to insects). It seems likely from these observations that a liquid water layer was present at cloud top for much of the day. At this temperature, the pristine crystals formed in it would have been plates, explaining the ZDR values larger than 4 dB. The intermittent appearance earlier could have been due to convective motions carrying larger particles higher into the cloud. Before the aircraft began sampling at at 09:20 the cloud had a more convective appearance, although due to the radar orientation relative to the wind it was not possible to identify convective cells unambiguously.

Dynamics

It was not easy to get an idea of the shear structure due to the fact that the radar was scanning perpendicular to both the wind and shear vectors. In a horizontal scan at 13:08:11 there is some evidence of east-west oriented waves (Kelvin-Helmholtz) to the east of Chilbolton, although none is present in the vertical scans. There is nonetheless considerable structure at all scales in the velocity images.