*Correspondence author: Nan Li, School of Atmospheric Physics, Nanjing University of Information Science and Technology, China
How to cite this article:Li Gao, Luan Li, Nan Li, Yongjiang Yu, Ben Niu. Analysis of Hail Processes Caused by a Supercell Storm. Oceanogr Fish Open
Access J. 2020; 11(5): 555823. DOI: 10.19080/OFOAJ.2020.11.555823
Hail, as a product of convective storms, is mostly produced by powerful supercells. The hail processes of a supercell, occurred in Zhejiang Province of China on March 21, 2019, are analyzed by using the conventional data and the S-band dual polarization radar data of Ningbo. The results show that: the front of the high trough, the shear line of 850hPa and the surface cold front provide suitable dynamic conditions for the supercell; the maintenance of the surface front and the strong vertical wind shear extending eastward are the main reasons for the eastward propagation and long life history of the storm. The appearance of hook echo, front inflow notch (FIN), rear inflow notch (RIN) and bow echo provide the basis for the forecast of hail. The large reflectivity factor, the jump of the vertical integrated liquid water content (VIL) and the VIL density (VILD) larger than 6g/m3 have better indication significance for hail occurrence.
Keywords:Supercell; Long life history; Reflectivity; VIL; VILD
Because of the characteristics of high organization and severe disaster, supercell storms have been concerned since 1950s. Most hail is produced by strong supercell storms in mature stages. Triggered by a cold front, a series of convective storms occurred in the middle of Zhejiang Province of China on March 21, 2019, and lasted for 6 hours, which caused hail in Jinhua, Shaoxing, Taizhou, Ningbo and other nearby cities. They were of small scale and high difficulty in weather forecast (Figure 1). Conventional observation data and dual polarization radar data are used to analyze these hail processes, in order to improve the early warning of hail.
At 08:00 (Beijing Time, the same below) on the 21st, the hail area was in front of the 500hPa cold temperature trough. 200, 500, 700 and 850hPa all had strong southwest airflow transportation. The west of Zhejiang Province was in the left front of the right frontal zone of high-level jet and the exit of low-level jet, and the coupling of high-level and low-level jet was conducive to the development of convective upward movement . At the same time, there was a northeast-southwest shear line in the middle of Zhejiang Province at 850hPa, and the surface cold front pressed southward to the northwest of Zhejiang
(Figure 2a). The dry cold air with high speed was embedded in the wet area, and superposed on the warm energy tongue formed by the strong southwest jet in the lower layer, resulting in the increase of vertical decline rate of the temperature and the significant stratification of upper dry and lower wet, as well as the vertical shear of the environmental wind in the middle and lower layers. At 08:00 on the 21st, the vertical wind shear of 0-6km reached 48m/s. With the sounding of Hongjia (Figure 2b) in the south of Zhejiang, it can be found that the cloud bottom was low under high humidity in the lower layer, and the curve of temperature and dew point was in a shape of bell mouth from bottom to top. The vertical structure of upper dry and lower wet was conducive to the occurrence of hail. At the same time, the height of 0℃ and -20℃ was 4.2km and 7.1km respectively, which reached the index threshold of hail in Zhejiang.
At 8: 42, a convective cell of mesocyclone style with symmetrical structure in the storm-relative mean radial velocity map (SRM) was located at 235°/ 182km (Figure 3a), southwest of Ningbo radar. The maximum rotation speed was 21m/s, which
met the standard of strong mesocyclone . There were found
a hook echo on the south side of the storm, a wide inverted “V”
with front inflow notch (FIN) on the east of the hook echo, and a
rear inflow notch (RIN) on the west (Figure 3b), all of which were
classic features of supercell storms. The presence of FIN and RIN
indicated a strong front inflow and rear downdraft, which showed
that the supercell had developed to a mature stage. As a result, the
hail first occurred at 8:53.
At 9: 32, there was a convective cell, connecting with the
supercell through a cloud bridge at the back (Figure 3c), and it
had a mesoscale convective vortex (MCV) with a rotation speed of
7m/s. Afterwards, it gradually integrated into the main supercell
storm. The combination made the storm maintain and strengthen,
and the maximum rotation speed was also significantly increased
(Figure 3d). At 9:54, in the southwest of the supercell storm,
convective cells were continuously generated, and the maximum
Zh rapidly increased to 50dBZ, and then integrated into the
supercell storm, and afterwards, the characteristics of bow echo
was shown at 10:12 (Figure 3e). When a supercell is embedded in
a mesoscale convective system (such as squall line, etc.), it usually
has a longer life and causes more serious disasters . In this
process, the rotation speed of the supercell maintained at the level
of mesocyclone, and the hail occurred for the third time at 10:40.
Up to 11:14, the main echo moved to the sea, and the supercell had
three hail processes in three hours.
Figure 4 shows the evolution of the supercell storm observed
by Ningbo radar. The white column is the height of the storm, and
the red solid line of the abscissa is the duration of the three hail
processes . In the process of storm development, the maximum
reflectivity (DBZM) of the three hailstorms was above 59dBZ, and
the supercell increased like ladder with the maximum value.
The DBZM reached 76dBZ in the third hailstorm. Through the
evolution of the maximum reflectivity height (MHT), it shown
that the initial convective storm was in the middle troposphere,
the height of the mass center was high, and it gradually extended
downward. The convective storm was very strong in the whole life, the maximum reflectivity was above 60dBZ and the top
of the storm was over 8km. The height of the storm’s mass
center fluctuated three times, corresponding to the three hail
processes. At 9:32, MHT, VIL and VILD increased rapidly, and
the maximum value of VIL reached 60g/kg.
Wu  found that there was a positive correlation between
VIL and hail size through the study on characteristics of a series
of supercells, and the hail occurrence corresponded to an obvious
VIL jump (16-20 g / kg) in the mature stage. At the same time,
the probability of hail occurrence was greater when VILD was ≥
4g/m3. Therefore, we often pay attention to the jump of VIL and
VILD ≥ 4g/m3. The sustained high VIL (60-70g/kg) is usually
used as the forecast index of hail larger than 5cm. By contrast,
the maximum VIL less than 40-50g/kg is often ignored for hail
smaller than 2cm in the nowcasting. Through the analysis of
the hail processes, the VIL had a jump of 11-14g/kg before hail
falling, and the jump of VIL was less than the traditional index,
but the VILD was more than 6g/m3 and the maximum reflectivity
Zh was up to 70 dBZ, therefore we should pay more attention in
operational forecast (Table 1).