The 18 June 1997 Companion supercells: Multiparametric Doppler radar analysis

On 18 June 1997 two simultaneous supercells 50 km apart swept the Po valley W-NW to E-SE. An exceptional hailfall lasted for more than 3 hours over a strip 200km wide. There are no records of companion supercells over northern Italy, a fact that attributes to the present observations a considerable meteorological interest. The forcing due to a baroclinic wave disturbance created the synoptic conditions favourable for storm development. A closer mesoscale analysis conducted using the Local Analysis and Prediction System (LAPS) indicates that the interaction of the storm systems with the low-level frontal high-humidity band is a key aspect of their evolution. The operational polarimetric Doppler weather radar of S. Pietro Capo®ume was used for a volumetric analysis of the storm system focusing on the microphysical and dynamical structure, and wind patterns. Re¯ectivity and Doppler wind ®elds document the transition from multicell to supercell phase for one of the storms. Differential re¯ectivity (ZDR) ®elds are examined. Positive ZDR columns are detected in connection with the storms strong updraft. Flare echoes hint to the presence of substantial hailshafts. The spectrum width ®eld is used to investigate internal motions. Limited dual Doppler analysis is carried out and results are discussed in terms of storm evolution properties.