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The development of extremely powerful thunderstorm which took place on August 19, 2015 is discussed in this paper. High depth hail cloud originated on the Black Sea Coast and classified as a supercell as well as several weaker hailstorms passed more than 1000 km over Northern Caucasus of Russia, the Caspian Sea, and then invaded the territory of Kazakhstan. During more than 20 hours of existence this supercell produced heavy hail, rain, intense lightning discharges, gust and tornado which rarely occurs in the region. The study of the structure and characteristics of the thunderstorm during the formation of electrical discharges and their frequency were of particular interest. According to the forecast, development of convective clouds and separate thunderstorms were expected, though the powerful hail process was not expected due to small vertical temperature gradients and the absence of cold fronts. Supercell was tracked by 5 radars located in this area, which showed its right-hand development with clock-wise deviation from the leading stream on 40-50 degrees to the right and the resulting speed of propagation was about 60-85km/h. The maximum reflectivity factor exceeded value 75dBZ, top of the clouds reached 15-16km and the height of the hail core raised on 11.2km. The size of hailstones size on most of the hail path was 2–3cm, and at the peak of cloud development - 4–5cm. Maximum frequencies of cloud-to-ground flashes of negative and positive polarities reached 30-35min-1 and 60-70min-1 correspondingly, while frequency of cloud-to-cloud flashes was significantly higher and amounted up to 300-500min-1 at the peak of the supercell development. An important fact is that the maximum frequency of flashes of different types coincided in time, showing that the reason of all discharges is similar. Total current of the cloud-to-ground flashes of positive and negative polarities was almost identical in magnitude and differed by sign. It was 200-300 kiloampere at the peak of thunderstorm development. The minimum value of radiation temperature, measured by SEVIRI radiometer installed onboard of Meteosat-10 satellite in 10.8 μm channel, was near to -60ºC. The minimum temperature value on the top of the supercell was comparable to coupled radar and sounding data. The most intensive precipitation flux derived from radiometric measurements was about 22000m3/sec; at the same period radars assessments showed precipitation up to 550mm/h (mixed phase precipitation) and size of hail 4.5cm. The combined satellite-radar-lightning data analysis showed that radar derived characteristics of the supercell reached their maximums earlier than maximum in lightning activity. The highest correlation coefficient between radar and lightning characteristics of the supercell storm was found for pair maximum reflectivity and intensity of LF (0.55) and VHF (0.66) discharges. Estimations of relationship between hail size and lightning activity showed that with increasing hail size, thunderstorm activity increases for both cloud-to-ground and intracloud flashes (on the level 0.46 - 0.59). Analysis of doppler-polarimetric data showed strong inflow zone associated with tornado. Tornadic debris signature was manifested by radar reflectivity factor ZH > 60 dBZ, differential reflectivity ZDR > -1 dB, copolar cross-correlation coefficient ρHV < 0.6, and it was collocated with the tornado vortex signature. Doppler velocities in mesocyclone zone reached values -43 and +63 m/s. Prominent radar echo hook was identified in 1.5 km layer above the ground, while ZDR columns was relatively narrow (4–8 km wide) and not very deep (4.5 km).

Methods, algorithms, and programs of automated radar identification of convective cells in cumulonimbus clouds are considered, that provides the ability to measure the parameters of convective cells, to plot the graphs of the time course of parameters, to compute the direction and speed of the movement, to assess their thunderstorm and hail danger, and to recognize the categories of target objects for the prevention of hail damage and meteorological provision of aviation.

The time course and the relationship of radar and lightning characteristics of the powerful hail clouds of the North Caucasus are considered. The studies were conducted in 2016-2018 at the automated meteorological radar complex ASU-MRL, which provided simultaneous acquisition and processing of radar data from the MRL-5 and 4-point network of LS-8000 lightning detection sensors, visualization of thunderstorm discharges on the background of the radar reflectivity map, as well as measurement complex of radar and thunderstorm cloud characteristics, including their maximum radar reflectivity Zmax, heights, volumes and water content of clouds at different reflectivity values, frequency of intracloud lightning discharges (VHF), “cloud-to-ground” discharges with positive (LF+) and negative polarity (LF-), current and duration of discharges. An analysis of the results showed that lightning discharges begin in the developing hail clouds of the North Caucasus when their maximum radar reflectivity reaches Zmax ≥ 40 dBZ, the frequency of discharges increases with increasing reflectivity, integral water content and enhancing the process of precipitation formation. The highest thunderstorm activity is noted at Zmax≈ 70-75 dBZ. Most low-frequency discharges of LF- and LF+ types are observed on the windward flank in the zone of intensive precipitation formation, and the majority of high-frequency VHF discharges are on their leeward side. Seeding of hail clouds with glaciogenic reagents with anti-hail rockets also leads to an increase in the frequency of discharges of all types.

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