On 28 July 2000, a number of thunderstorms were observed with the Chilbolton radar. On several occasions during the most intense phases of the storms, negative KDP was obsered in the developing tower. This indicates that the crystals were predominantly vertically aligned, something that cannot occur by natural inertial forces alone. Hence the only possible explanation is that strong electric fields were aligning the crystals along electric field lines. This is a potentially useful way to detect the electrification of anvil cirrus, which can be a hazard for aircraft.
The figures below show differential phase shift, Z, ZDR, LDR, and velocity towards the radar, for two successive scans around 30 seconds apart. It can be seen that in this short interval the differential phase shift flips from around -5 degrees over 5 km to +10 degrees. The most likely explanation for this sudden change in orientation is that there was a lightning strike that instantly dissipated the growing electric field, after which the crystals fell back to their preferred horizontal orientation. There is a clear increase in LDR when the crystals are vertically aligned, but no corresponding signature in ZDR. Clearly there is a lot of scope for further analysis of this case, including matching up these sudden changes in orientation with high resolution spherics data.
| Scan 1 | Scan 2 (30 seconds later) |
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