Analysis of the spatial and temporal distribution and changes of thunderstorms in southwestern Iran over the last three solar cycles

Introduction
Natural phenomena, due to their inherent nature, while having beneficial effects, can also have destructive consequences. Natural hazards claim the lives of thousands of people every year in different parts of the world, and a high percentage of these casualties are the product of weather hazards. Among the types of weather hazards, we can mention thunderstorms, regional floods, etc. Thunderstorms are local and mesoscale weather systems that form in a limited area of 20 to 50 km and depend on the height of convective clouds, and in addition to lightning and strong winds, they are often accompanied by heavy rainfall. Iran, due to its special geographical location, has always faced fundamental challenges in the field of water resources. The southwest region of Iran, due to its mountainous location, large-scale climatic systems affecting the region, access to moisture sources in the warm southern seas, is prone to the formation of thunderstorms. This region includes the provinces of Khuzestan, Chaharmahal Bakhtiari and Kohgiluyeh and Boyer-Ahmad, which are among the most important commercial, port, industrial and agricultural regions of Iran. Thunderstorms, in addition to the damage that can be caused to the region's infrastructure, agriculture, soil and vegetation in the event of torrential rains, air and sea transportation are also severely affected by phenomena related to thunderstorms, such as the occurrence of severe gusty winds and lightning. The results of previous research show that thunderstorms are influential and variable weather phenomena in different regions of the world and Iran. Due to the dispersion and significant effects of thunderstorms in different regions of Iran, statistical and spatial analysis of these phenomena on a monthly scale, especially in the south of the country, seems necessary. This analysis can help to better understand the temporal and spatial patterns of the occurrence of thunderstorms and be effective in planning natural hazard management and more accurate forecasting of weather conditions.
 
Materials and Methods
This research analyzed thunderstorm data from synoptic reports of the Iran Meteorological Organization over a long-term statistical period from 1986 to 2018. The study period was selected to coincide with three 11-year solar cycles (cycles 22 to 24). This timeframe, in addition to covering long-term and comprehensive climate changes, allows for the investigation of the potential effects of solar cycle variations on the occurrence and intensity of thunderstorms. Solar cycle 22 corresponds to the period from 1986 to 1996, cycle 23 from 1997 to 2007, and cycle 24 from 2008 to 2018. Solar flare data were obtained from the Royal Observatory of Belgium. The number of stations studied varied in each cycle (8 stations in cycle 22, 17 stations in cycle 23, and 26 stations in cycle 24). Thunderstorm data were organized monthly and annually in Excel software, and the frequency trend of this phenomenon was analyzed.Subsequently, in order to analyze and display the temporal-spatial changes of thunderstorms, the inverse distance weighting method was used. This analysis was performed on a monthly average basis and for each solar cycle. The inverse distance weighting method is based on Taylor's law or the first law of geography, which states that closer geographical units have a greater impact on each other. Finally, in order to display and analyze temporal-spatial changes, the inverse distance weighting (IDW) method was used by ArcMap software, which has the ability to estimate the distribution of data at points lacking information with minimal error. Spatial distribution maps were generated based on the monthly average of data in each solar cycle.
Results and Discussion
Based on the results, the Ahvaz station, with the highest number of occurrences (927 cases), had the most reports of thunderstorms. In general, in the northern areas of the province or towards the highlands and foothills, such as the Dezful and Masjed Soleiman stations, the number of thunderstorm occurrences has been increasing. In this period, 738 and 723 cases of thunderstorm potential were reported for each station, respectively. In contrast, the Abadan station, with 374 cases, had the lowest number of occurrences. In cycle 22, an inverse relationship was observed between the number of sunspots and the occurrence of thunderstorms, such that the most thunderstorms occurred in the final years of the cycle. This trend was repeated in cycle 23, and in 2007, coinciding with the lowest number of sunspots, the most thunderstorms (more than 320 cases) were reported. This pattern continued in cycle 24, with the most thunderstorms occurring at the beginning and end of the cycle. In summary, it can be said that the number of thunderstorms has an inverse relationship with the number of sunspots, and the final years of each cycle experienced the highest number of thunderstorms. The results show that in January, the storms in the second decade (1997-2007) had the highest frequency, and the spatial extent of this phenomenon is evident in the Dezful and Imamzadeh Jafar stations. In February, the most events were recorded in cycle 23, which indicates the possible influence of solar cycles on the temporal distribution of this phenomenon. In March, the peak of the storms was observed in cycle 23, with the Dezful and Yasuj stations recording the highest numbers. In cycle 24, although the distribution pattern is similar to cycle 23, the frequency of events has decreased. This analysis emphasizes that climatic fluctuations and solar cycles affect the spatial and temporal distribution of thunderstorms. In April, despite the decrease in rainfall in the southwest of the country, the dispersion of thunderstorms does not change significantly compared to winter, but their concentration increases in the northwest of the region. In May, the highest activity is seen in northern Khuzestan and Chaharmahal and Bakhtiari, especially in Shahrekord during cycle 22 and in Ahvaz and Dezful during cycles 23 and 24. In June, with the strengthening of the Arabian anticyclone and the reduction of the penetration of rainfall systems, thunderstorms decreased in the west, but increased in the east of the country under the influence of monsoon systems. In July, the most occurrences were recorded in the east of the region, especially Shahrekord (cycle 22), Yasuj (cycle 23), and eastern Chaharmahal and Bakhtiari and Kohgiluyeh and Boyer-Ahmad (cycle 24). This increase is due to the penetration of monsoon systems. In August, the distribution pattern is similar to July, and the main centers are located in Shahrekord (cycle 22), Yasuj and Dogonbadan (cycle 23), and Sisakht (cycle 24). In September, the focus of occurrences remains in the east of the region, especially in Dogonbadan, Sisakht, Kuhrang, and Izeh. In October, thunderstorms, moving to the northwest and west, recorded the highest frequency in the Dezful, Ahvaz, and Kuhrang stations. In November, under the influence of winter systems, the highest occurrences were reported in Ahvaz (more than 48) and Dezful (43), and in cycle 24 the number of occurrences reached 685 cases. In December, thunderstorms were concentrated in Izeh, Ahvaz, and Yasuj, and the highest activity was observed in cycles 23 and 24.
Conclusion
From a temporal perspective, the analysis of thunderstorm frequency in solar cycles 22 to 24 indicates significant differences in the occurrence of this phenomenon between these cycles. The results show that the frequency of thunderstorms in solar cycle 22 was lower compared to the next two cycles (23 and 24). There is also an inverse relationship between the number of sunspots and the frequency of thunderstorms. So that in all three solar cycles, the lowest number of thunderstorms occurred in the middle of the cycle and the highest number of thunderstorm occurrences occurred in the early or late years of the cycle. In examining the temporal distribution, the highest number of occurrences was reported in the years of solar cycle 23, which indicates the significant impact of solar activity on the occurrence of this phenomenon. In total, in the 33-year statistical period, the highest occurrence was related to the year 2006 and the lowest occurrence of thunderstorms was reported in 1990. In terms of monthly distribution, the highest number of thunderstorms is related to April and November, and the lowest occurrence of thunderstorms is reported from September. The results show that the most thunderstorm activities in southwestern Iran occur in the spring and winter 
seasons, while significant activities have also been recorded in the fall and summer seasons. However, thunderstorms can occur throughout the year. In fact, the seasonal distribution shows that in the spring, about 41% of the total reports of thunderstorms have been recorded, which is due to the increased atmospheric instability along with the temperature and humidity potential of the seas. In contrast, the summer season with only 8% of the occurrences shows the least thunderstorm activity, which is due to the edge of the summer pattern and the dynamic stabilities resulting from it. In the fall and winter seasons, 24 and 29 percent of the occurrences have been reported, respectivel  y. Thunderstorms in the southwestern part of Iran due to the thermodynamic characteristics of the incoming systems from the south are an integral phenomenon of these systems. These systems are generally accompanied by destructive phenomena such as thunder, lightning and strong winds. Considering the increasing trend of this phenomenon in the last three solar cycles, the awareness of users and institutions affected by this phenomenon, such as pilots, drivers of land and sea transport fleets, and farmers, of the characteristics, signs, spatial ranges, and timing of this phenomenon is of great importance. This awareness includes knowing the wind speed, the height of the systems, and the ability of the storms to intensify the wind speed, which can be effective in planning flights and preventing possible dangers on road and sea transport and agricultural products and urban structures.