The Frontiers animation shows two distinct bands of precipitation to the west of Chilbolton which were moving to the NNE, associated with a cold front. The ice feeding the precipitation extended towards Chilbolton, up to an altitude of 10 km, and through the day the ice cloud base descended (as shown in the daily lidar image) until 16:30 UTC when rain was detected at the surface at Chilbolton. The radar was pointing at the 259° azimuth, so was not quite parallel to the wind vector.
The animation of a sequence of Chilbolton radar scans shows very clearly the sedimentation of the ice and the formation of rain at the melting layer, which was just below 2 km. Between 11 and 11:45 UTC there is evidence of rapid growth of ice particles between 4 and 5 km; here the radar reflectivity factor increases quite rapidly from around 0 dBZ to 25 dBZ. The clue as to the cause of this is in the Zdr images; at 10:55:06 and 10:56:38 (for example), high values of Zdr are observed protruding out of each side of the ice cloud at an altitude of around 4.5 km (around -11 degrees). This suggests the presence of a layer of liquid water at this altitude, and as large ice particles fell into it from above they grew rapidly. The decrease in Zdr from 1 to around 0.5 dB as they fell through the layer could be an indication of riming, which tends to produce more spherical particles. The high values of Zdr protruding at each side are due to much smaller crystals growing by deposition in the supersaturated environment, as was observed in Flight 1. This hypothesis is supported by the lidar image, which shows reflective layers indicative of liquid water at various altitudes between 3 and 6 km as the cloud base descended.
Some of the later images show ZDR up to 7 dB in the melting layer, caused by the melting of pristine crystals.
The radar measured a wind towards Chilbolton of up to around 35 m/s at cloud top (this unfolded velocity data will soon be available from BADC), decreasing to close to 0 m/s at the height of the melting layer. This shear was certainly important in determining the overall slanted shape of the cloud, as indicated in the animation, although there was no evidence of Kelvin-Helmholtz wave activity at any time in the day.
