In addition to its polarisation capability, the Chilbolton radar is able to measure the line-of-sight component of the wind. In the following plots the radial velocity, radar reflectivity (Z) and differential reflectivity (ZDR) are plotted for Runs 17, 19, 21 and 22 of Ascent 2. Overlayed on them are the aircraft values of vertical velocity, liquid water content, and FSSP number concentration respectively. Also, the radial velocity has been used to calculate the radial component of horizontal divergence; this is shown by the white contours overlayed on ZDR (where the solid lines indicate divergence and the dashed lines show convergence). One would expect that a rising plume would have convergence beneath and divergence aloft.
Previously it had been assumed that the aircraft-measured updrafts were directly associated with high ZDR feature, probably rising up the trailing edge of the high ZDR. However, there is evidence from these plots that the updrafts are associated with the tops of small ascending turrets of high Z and low ZDR, which are rising into the high ZDR region. These turrets measure around 1km by 1km and rise vertically, in contrast to the high ZDR region that has more the appearance of a fallstreak. The aircraft-measured updrafts also tend to coincide with perturbations of the radial velocity contours.
In the light of these observations, do we need to rethink our mechanisms for generating high ZDR (by Hallet-Mossop)? It is possible that the high Z turrets and the high ZDR region are completely separate features which just happened to occur in around the same place. Perhaps the needles observed in run 19 were the only aspect of the high ZDR region that was actually sampled by the aircraft; everything else was forced by the high Z turrets. If this is the case then we cannot really say that the high ZDR was caused by the ascending air. What are the particles responsible for the high Z and low ZDR regions? Graupel?
In the following images, magenta arrows have been added to show the location of the main updrafts. Note also that the four scans are at different azimuths in order to track the centre of the aircrafts flight pattern as it drifted with the wind. Hence the radar-measured velocities change somewhat as a slightly different component of the wind is measured. Also, the high ZDR region is not sampled in its centre in the last two runs, so the fact that the ZDR looks lower is not necessarily an indication of the weakening of the feature.
