Polarimetric radar observations of the Jersey tornado and hailstorm of 1 – 2 November 2023

At approximately midnight on 2 November 2023, a supercell thunderstorm struck Jersey in the Channel Islands. This thunderstorm formed just ahead of the cold front of an intense extratropical cyclone (Storm Ciarán) which subsequently produced widespread damaging winds over northern France and adjacent areas during the morning of 2 November.

The supercell thunderstorm produced a tornado of intensity T6/IF3 which tracked over eastern parts of Jersey, and hail of diameter ≥5 cm over a swath several kilometres wide in central and eastern parts of the island. The tornado damage track was continuous from the south coast of the island at St Clement, where the tornado made landfall, to the northeast coast at Fliquet where the tornado exited the island.

The tornadic storm passed within a few kilometres of the Channel Islands Doppler, polarimetric, C-band radar, located on the south-western tip of Jersey. In this presentation, radar observations of the storm will be explored, focussing on the signature of a tornado debris cloud in polarimetric fields and a collocated reflectivity ‘debris ball’. The observations meet the accepted operational definition of a Tornado Debris Signature (TDS), comprising a small region of exceptionally low correlation coefficient (<0.8) collocated with differential reflectivity ≤0 dB, reflectivity >35 dBZ and a velocity couplet in the radial wind field. To the best of our knowledge, this is the first documented polarimetric TDS in northwest Europe. In vertical section (constructed from available plan position indicator scans at different elevation angles, translated horizontally to account for the movement of the storm between scan times), the debris signature comprised a column of low correlation coefficient and high reflectivity that tilted north-northeast with height and extended to at least 2 km above ground level.