Bin DENG, Jihai XUAN, Wenlin ZHOU, Zhenfang ZHAI

Hefei Meteorological Bureau, Hefei 230061, China

In China, hail, thunderstorm, gale,tornado and short-time rainstorm are main disastrous severe thunderstorm weather in summer[1]. Thunder and lightning occur randomly and cause a disaster rapidly, so power,aerospace, chemical, insurance, construction, tourist scenic spot and other sectors propose increasingly high demand for monitoring and early warning of thunder and lightning[2]. Studying thunderstorm activity laws based on a variety of observation data can provide scientific references for prediction and early warning of thunderstorm. Moreover, precise forecast of thunderstorm activity can reduce the caused loss as little as possible[3]. Therefore, studying temporal and spatial variation of thunder and lightning in Hefei City has an important significance to monitoring,early warning and prediction of thunder and lightning as well as lightning protection and disaster reduction in Hefei and surrounding areas.

Data and Methods

Based on ground observational data during 1981-2010 and lightning location data in 2010-2013 in Hefei City, annual, seasonal and monthly average of thunderstorm days were analyzed to discuss the changing laws of thunderstorm days,and the distribution laws of thunderstorm days over the past 30 years were compared with the lightning location data.

Results and Analysis

Laws of thunderstorm activity in Hefei City

Multi-year average of thunderstorm days As shown in Fig.1, multi-year average of thunderstorm days in Lu-jiang County was the maximum(31 d),and it was 26 d in Chaohu and Hefei,while it ranged from 23 to 25 d in Feidong,Feixi and Changfeng.It is clearly seen that multi-year average of thunderstorm days was more in the south but less in the north, because Chaohu and Lujiang are situated along rivers.

Annual variation of thunderstorm days According to Fig.2, the multiyear average of thunderstorm days from 1981 to 2010 was 26 d,and there were 11 years with more than 26 days of thunderstorm,7 years with less than 26 days of thunderstorm and 12 years with about 26 days of thunderstorm each.On the whole,annual average of thunderstorm days was relatively more. In addition, the maximum was up to 40 d,while the minimum was only 14 d, so annual distribution of thunderstorm days in Hefei during 1981-2010 was extremely uneven.

Seasonal variation of thunderstorm days There were obvious seasonal variation in thunderstorm days in Hefei City during 1981-2010. That is,thunderstorm days were the most in summer (from June to August), followed by spring and autumn, while there was nearly no thunderstorm in winter(Fig.3).

Monthly variation of thunderstorm days As shown in Fig.4,the monthly variation curve of thunderstorm days in Hefei City from 1981 to 2010 had a peak. That is, the thunderstorm days increased slowly from February to May but rapidly from June to August (the maximum appeared in July) and then decreased fast from September to December.There were more thunderstorm days in Lujiang and Chaohu than other regions.

Distribution laws of cloud-toground lightning in Hefei City

Temporal variation in frequency of cloud-to-ground lightning According to Fig.5, the monthly variation curves of total frequency of cloud-toground lightning and frequency of negative cloud-to-ground lightning in Hefei City from 2010 to 2013 had a peak each. In other words, cloud-to-ground lightning was frequent in July and August, and total frequency in the two months accounted for more than 76% of total frequency in a year;cloud-to-ground lightning appeared 1 157 times in each month on average, and the maximum occurred in August. Positive cloud-to-ground lightning rarely happened,and its changing trend was consistent with that of total frequency of cloud-to-ground lightning;a small peak of positive cloud-toground lightning also appeared in August.However,the intensity of positive cloud-to-ground lightning exceeds that of negative cloud-to-ground lightning and then brings great damage to people, buildings, trees and so forth, so positive cloud-to-ground lightning should be paid more attention to.

As shown in Fig.6, the frequency of cloud-to-ground lightning increased obviously in the afternoon, and the maximum (28 times) appeared at 20:00. Hourly average of lightning frequency was 10.8 times, and lightning frequency exceeded the average from 12:00 to 21:00.

Monthly variation in the maximum intensity of cloud-to-ground lightning According to Fig.7, the maximum intensity of cloud-to-ground lightning in Hefei City from 2010 to 2013 varied greatly in different months. It was the highest in July, reaching 617 kA; it exceeded the average 230 kA in July and August, which coincides with the high-incidence period of thunderstorm(from June to August)in Hefei.

Comparison between thunderstorm days and the frequency of cloud-toground lightning in Hefei

There were obvious seasonal and monthly variations in thunderstorm days in Hefei City from 1981 to 2010.That is, thunderstorm was frequent in summer;the monthly variation curve of thunderstorm days had a peak in July,and thunderstorm days increased fast from June to August. The monthly variation curves of total frequency of cloud-to-ground lightning and frequency of negative cloud-to-ground lightning in Hefei City from 2010 to 2013 also had a peak each. In other words,cloud-to-ground lightning was frequent in July and August, and the maximum frequency appeared in August. It is clearly seen that both thunder and lightning were very frequent in summer, especially in July and August.However, there are certain differences between the two kinds of data in the distribution laws, and the reasons are shown as follows. Firstly, thunderstorm and lightning is monitored based on thunder at meteorological stations at present, and thunder size, background noise and obstacles on the propagation path can affect observers’judgment.Moreover,no matter how many times observers hear thunder within a day,the day is named a thunderstorm day. These human factors can influence conventional observation of thunderstorm. Secondly,lightning position indicator is sensitive and can record lightning that can not be recorded by observers, so the data obtained by it can supplement observation data. However, there are few data obtained by lightning position indicator in Hefei City now,and surrounding noise, terrain and so forth may affect the monitoring of lightning position indicator and then the quality of its observation data. Hence, it is needed to combine data of lightning position indicator and long-term artificial observation data to study the detection efficiency of lightning position indicator.

Conclusions

In Hefei City, multi-year average of thunderstorm days from 1981 to 2010 was the maximum (31 d)in Lujiang County, and it was more in the south but less in the north, which is closely related to terrain. On the whole, annual average of thunderstorm days in Hefei City was relatively more during 1981-2010. Additionally,the maximum was above two times more than the minimum, so annual distribution of thunderstorm days in Hefei was extremely uneven. Moreover,there were obvious seasonal and monthly variation in thunderstorm days in Hefei City.Thunderstorm days were the most in summer, followed by spring and autumn, while there was nearly no thunderstorm in winter.Monthly average of thunderstorm days in Hefei City had a peak in July.

From 2010 to 2013, the monthly variation curves of total frequency of cloud-to-ground lightning and frequency of negative cloud-to-ground lightning in Hefei City had a peak each. In other words, cloud-to-ground lightning was frequent in July and August, especially August. Positive cloud-toground lightning rarely happened, and its changing trend was consistent with that of total frequency of cloud-toground lightning;a small peak of positive cloud-to-ground lightning also appeared in August. The frequency of cloud-to-ground lightning increased obviously in the afternoon, and the maximum appeared at 20:00;lightning frequency exceeded the average(10.8 times) from 12:00 to 21:00. The maximum intensity of cloud-to-ground lightning in Hefei City varied greatly in different months.It was the highest in July;it exceeded the average in July and August, which coincides with the highincidence period of thunderstorm(from June to August)in Hefei.

There are certain differences between the two kinds of data in the distribution laws, and the reasons are shown as follows. Firstly, conventional observation of thunderstorm can be influenced by many human factors.Secondly, there are few data obtained by lightning position indicator in Hefei City now, and surrounding noise, terrain and so forth may affect the monitoring of lightning position indicator and then the quality of its observation data. Therefore, it is needed to combine data of lightning position indicator and long-term artificial observation data to study the detection efficiency of lightning position indicator.

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