Investigation of Supercells in China : Environmental and Storm Characteristics

Xiaoding Yu∗ Xiuming Wang Juan Zhao Haiyan Fei

( China Meteorological Administration Training Center)

Abstract Based on the Doppler weather radar data, More than 200 supercell events in China were identified

during eight-year period from 2002 to 2009. Their spatial distribution, seasonal and diurnal variations are

given. Using routine sounding and surface observational data, several important parameters

representing the major features of the environment in which these supercells occurred were investigated,

these parameters include convective available potential energy (CAPE), 0-6km deep layer vertical wind

shear, 700-500hPa temperature difference, and surface dewpoints. The results show that the value

ranges corresponding to the peak frequencies for the CAPE and the 0-6km wind vector difference

distributions are from 1000 to 2500 J/kg and from 15 to 25m/s, respectively. With the base data from

more than 50 Doppler weather radars, the characteristics describing the intensity, structure, and

evolution of China’s supercells were analyzed. The average diameter of mesocyclones associated with

supercells is 6.2 km, with the range of the peak frequency for diameter distribution being between 4km

and 7km. The value ranges corresponding to peak frequencies for mesocyclone rotational speed and

duration are from 15 to 20m/s and 40 to 50 minutes, respectively. About 63% of supercells display the

BWER structure, and the value range of peak frequency for supercell maximum reflectivity distribution is

between 65 dBz and 70dBz，with the extreme reflectivity value being 76 dBz.

1 Introduction

Supercell storms are the severe convective storms possessing deep and persistent mesocyclones (Doswell and Burgess, 1993; Doswell,2001) and they are responsible for most of extreme hail events, significant tornado events and part of the significant damaging straight wind events (Johns and Doswell, 1992). Up to now, most studies on supercells are done in United States, so in this paper we will give an overview on the supercells in China, with emphasis on their environmental conditions and storm characteristics detected by the Doppler weather radar.

2. Spatial and Temporal Distribution

During the eight-year period from 2002 to 2009, we have identified 224 supercell events，a supercell event means a convective storms process in which a single or multiple supercells have been identified during a 24h period and over an area of a single radar umbrella. China Meteorological Administration (CMA) began to deploy its operational Doppler weather radar network from 1999, up to now, over 160 Doppler weather radars have been put into operation. Figure 1 gives spatial distribution of these supercell events. The supercell events have been identified nearly in all parts of China except the large part

                                                              *corresponding author email address ：xdyu1962@126.com

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One example of the supercell embedded in the cluster of convective stroms is given in Figure 15. The supercell event occurred around mid-night on 14 June 2005 in Anhui province, it produced 120mm size hail and damaging wind. The figure shows the supercell constitutes the front part of a cluster of convective storms, with a mesocyclone of moderate strength, inflow notch, arc shape outflow boundary and BWERs, as well as three body scattering spike and side lobe echoes due to the large hail in the supercell.

Figure 16 shows an example of supercell embedded in a squall line in Hainan Island, the south end of Chinese continent, occurring in the afternoon on 29 May 2008. It looks like a HP supercell with a front inflow notch and a mesocyclone of moderate strength, having produced a F1 tornado.

Similar to the situation in the United States (Doswell 2001), nearly 100% of the convective storms that produced 50 mm or larger hail and significant tornadoes (F2 or above) are supercell storms. The peak frequency for significant tornadoes occurs in July, most of them were produced by the supercells in cluster or supercells in QLCSs, usually accompanied by flash heavy rain ( 30mm or greater for one hour). The discrete supercells produce large hail, straight line damaging wind, and occasionally tornadoes.

6. Summary and Conclusion

Based on the Doppler weather radar data, More than 200 supercell events in China were identified during eight-year period from 2002 to 2009. Their spatial distribution, seasonal and diurnal variations are given. Using routine sounding and surface observational data, several important parameters representing the major features of the environment in which these supercells occurred were investigated. And based on the Doppler weather radar data, the characteristics of supercells were studied. The following conclusion are obtained.

1) Most of the supercell events occurred in eastern, central, southern and north-eastern China; The great majority of supercells occurring from April to August and no supercell has been identified in January and February; The supecell events occur the most frequently from 14:00 to 21:00 local time, with peak value during the period 15:00 to 16:00 local time. The lowest frequency time period for supercell occurrence is between 00:00 to 12:00 local time; For eastern, central and southern China, the frequency of the supercell events is on the order of 2.0 to 3.0 events per 105 square kilometers per year.

2) Most of the supercell events occurred in an environment with the CAPE between 500 to 3000 J/kg and 0-6 km wind vector difference between 10 to 30m/s. The value ranges corresponding to the peak frequencies for both the CAPE and the 0-6km wind vector difference distributions are from 1000 to 2500J/kg and from 15m/s to 25m/s, respectively.

3) For over 85% supercells, their mesocyclones rotation speed is between 15 to 25 m/s, with extreme value as high as 35m/s; The majority of mesocyclones diameter is between 4 to 7km, with minimum, average, and maximum value being 2.0, 6.2 and 14.5 km, respectively; Correspondingly, the vertical vorticity for the great majority of

mesocyclones is between 0.5 to 2.5×10-2 s-1 , with maximum value over 4.0×10-2 s-1 .

4) The great majority of supercell storms have a life span of 30 to 130 minutes, with several peak frequencies being 40 to 50 minutes, 70 to 80 minutes and 90 to 100 minutes, the longest supercell duration among the 224 supercell events is nearly 3.5 hours; The most of supercells have a maximum reflectivity of 55 to 75 dBz, with peak frequency range from 65 to 70dBz; About 63% of supercell storms display the BWER structure and 37% of supercells have not shown clear BWER; The moving speed is between 5.0 to 20.0 m/s for the great majority of supercells, with peak frequency range being from 10.0 to 15.0 m/s, that is, 36 to 48 km/h; The highest value of moving speed is 25m/s, corresponding to 90km/h.

5) As in the United States, the organization modes of the supercells in China can be divided into three categories: 1) discrete cells, 2) cell in clusters, and 3) cell in quasi-linear convective systems (QLCSs); Similar to the situation in the United States, nearly 100% of the convective storms that produced 50 mm or larger hail and significant tornadoes (F2 or above) are supercell storms; The peak frequency for significant tornadoes occurs in July, most of them were produced by the supercells in cluster or supercells in QLCSs, usually accompanied by flash heavy rain ( 30mm or greater for one hour). The discrete supercells produce large hail, straight line damaging wind, and occasionally tornadoes.

Acknowledgments This study was supported by China NSF Grant 41175043 and Chinese Government’s 973 Project “Study on the Evolution Mechanisms and Detection Technology of Severe Convection in China”.

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