CWVC Flight IV (21 November 2000) radar synopsis

Robin Hogan 18 July 2002

Introduction

A broad band of precipitation swept across southern England from a south-westerly direction, and rain was first measured at Chilbolton at 13:45 UTC. The radar azimuth was 259°, so was farly close to being parallel to the incoming wind. Initially the cloud was remarkably uniform, but later on an embedded convective cell appeared, as shown in this animation. The appearance of the Frontiers animation also indicated the cloud to be a little more convective towards the end of its traverse of southern England. The lidar could not penetrate the liquid cloud base, which was at around 500 m.

Microphysics

The most interesting feature of this flight is the embedded convective plume of elevated radar reflectivity factor compared with its surroundings. ZDR in and around the plume was close to 0 dB, although ice was falling into the region from above, so if Hallett-Mossop was active and any pristine crystals were formed, one would expect them to be aggregated rapidly. The animation also shows a region of high ZDR (up to 6 dB) to appear between 3 and 4 km after around 15:30 when ice is no longer falling into this region from above. This suggests that a layer of liquid water present at 4 km, and as in other cases, the pristine crystals were only able to grow (or were only visible to the radar) when larger aggregates were not present.

Dynamics

According to the radar, the shear was generally quite strong at an altitude of around 2.5 km, around 20 m/s per km. A suggestion of small amplitude Kelvin-Helmheltz waves was apparent in the radar velocity animation from the same time as a convective plume was observed. It is possible that the waves triggered the embedded convection, as in flight 1. Later on larger amplitude waves were evident (note that these animations are from unfolded velocity data which will be available on BADC soon).