Characteristics of the Supercell Cb Thunderstorm and Electrical Discharges on 19 August 2015, North Caucasus: A Case Study

Abstract

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).

Radar Diagnosis of the Thundercloud Electron Accelerator

“…This radar facility, shown in Figure 3 with Mt Aragats rising in the background to the northwest, is a Soviet-era MRL-5 brand. This dual wavelength (S-band and X-band) radar, designed specifically for hail detection, is currently deployed not only in Armenia, but in ∼45 other countries including Cuba (Koloskov et al, 1996;Pena et al, 2000), Poland (Moskowicz et al, 1994), Russia (Abshaev et al, 2019(Abshaev et al, , 2020, and South Africa (Visser, 2001). The characteristics of the MRL-5 radar are summarized in Table 2.…”

Investigation of Thundercloud Features in Different Regions

“…Detailed information about projects where data were obtained and summaries of the Cu characteristics observed in each case were presented in earlier publications [1,[29][30][31][32][33][34][35][36][37][38][39]. All lightning flashes, which were detected in a cloud during a radar scan, were counted and the average frequency for a scan in min −1 was determined.…”

“…The third group consists of one supercell Cb observed in Russia, which originated near the Black Sea, and crossed Caucasus on its way to the Caspian Sea (19 August 2015). It was a deep and long-lived Cb with hail and tornado [34].…”

“…The third group consists of one supercell Cb observed in Russia, which originated near the Black Sea, and crossed Caucasus on its way to the Caspian Sea (19 August 2015). It was a deep and long-lived Cb with hail and tornado [34]. It was analyzed separately from N-Cauc_Gr as it was a rather unique event, and the characteristics of this cloud differed significantly from "typical Cb" in that region.…”

“…Data Availability Statement: Detailed information about projects where data were obtained and summaries of the Cu characteristics observed in each case were presented in our earlier publications [1,[29][30][31][32][33][34][35][36][37][38][39].…”