Dual-wavelength radar observation of a supercell hailstorm

On April 21, 1999, a convective cold frontal line swept South-West of France. Between 17:00 and 17:30 UTC, on the south end of this eastward stormy system, a rain cell turned off on the right of the mean wind. In less than 30 minutes, this cell burst into a supercell hailstorm which crossed the oriental Pyrénées area along a North-West/South-East axis causing heavy damage. The frontal line evolution and particularly the supercell hailstorm was observed from birth to disappearance by two meteorological radars of the French operational radar network. The first one is a five centimetre wavelength radar located at Toulouse whereas the second one is a ten centimetre wavelength radar situated at Nimes.

The goal of the paper is to discuss the ability of the dual-wavelength reflectivity ratio to detect and identify the development of a supercell hailstorm among a population of ordinary convective rainstorm. We show that, due to the non-Rayleigh scattering of hailstone, the distribution of the dual-wavelength reflectivity ratio displays an identifying signal for the hail-bearing cells. We found that correlatively with this signal the lightning activity of the supercell as well as the ratio between the mean reflectivity and the fractional area of the hail cell are modified.