Introduction
Examining the intensity and patterns of extreme precipitation in recent years has become one of the important topics of climatological studies, because these events are closely related to global warming and changes in energy within the atmospheric system. Global studies indicate that in many regions, including Central Europe and the western United States, the intensity and frequency of extreme precipitation have shown an increasing trend. In some mid latitude and subtropical regions such as Southeast Asia and the Mediterranean Basin, an increase in precipitation intensity has occurred simultaneously with a decrease in the number of rainy days, which has led to a temporal concentration of rainfall and an increase in hydrological hazards. The results of studies in the Eastern Mediterranean, the Indian subcontinent, and research related to large scale atmospheric factors such as atmospheric rivers and synoptic patterns also emphasize the role of atmospheric systems in strengthening extreme precipitation. In Iran, numerous studies have examined the intensity, frequency, and spatial patterns of extreme precipitation. The results of these studies show that the Sudan Low is the most important factor producing heavy rainfall in the southern part of the country, and its activity is influenced by the Arabian anticyclone and the subtropical jet stream. Some studies have also pointed to the role of solar cycles in the temporal variability of precipitation and have reported a reduction in the temporal distribution of rainfall and a tendency toward convective events during recent cycles. In addition, elevation is also effective in intensifying extreme precipitation, and at elevations higher than 1500 m the effect of topography on increasing rainfall intensity becomes more significant. The importance of this issue in southern Iran becomes greater because this region lies under the simultaneous influence of several moisture sources and atmospheric systems. Proximity to the Persian Gulf, the Gulf of Oman, and the Arabian Sea, together with the influence of Sudanese and Mediterranean systems and southern airflows, creates conditions in which precipitation in this region often has an unstable, convective, and torrential nature. On the other hand, the presence of the extensive Zagros Mountains in the northern and eastern parts of this region causes elevation to play an important role in intensifying or weakening precipitation.
Therefore, studying extreme precipitation in southern Iran without simultaneous attention to atmospheric mechanisms, the elevation of stations, and large-scale temporal variability cannot provide a complete picture of the behavior of these events.Nevertheless, many previous studies have either focused on synoptic factors or examined temporal variations separately, and the simultaneous relationship between solar cycles and the altitudinal distribution of extreme precipitation has received less attention. Therefore, the present study, with an integrated approach, examines the simultaneous role of solar cycles and elevation classes in the changes of extreme precipitation structure in southern Iran. The main objective of the research is to analyze the trends and altitudinal pattern of extreme precipitation intensity in this region during the last three solar cycles in the period 1986–2019.
Materials and Methods
The study area of this research is southern Iran, including the provinces of Ilam, Lorestan, Khuzestan, Chaharmahal and Bakhtiari, Kohgiluyeh and Boyer Ahmad, Fars, Bushehr, Hormozgan, Kerman, and Sistan and Baluchestan. The data used include daily precipitation time series from the main and auxiliary synoptic stations of the Iran Meteorological Organization. Considering the objective of the research in examining the relationship between extreme precipitation and solar activity, the statistical period of the study was adjusted to three complete solar cycles (cycles 22, 23, and 24) corresponding to the years 1986–2019. To increase the accuracy of spatial analyses, in each 11-year cycle only stations with complete data were included in the calculations; therefore, the number of stations increased from 31 in cycle 22 to 59 in cycle 23 and 99 in cycle 24. In the first stage, the maximum 24-hour precipitation data were examined in terms of quality. The results showed that about 96% of the data had no defects, and for the 4% of data with gaps statistical reconstruction methods were used. In order to ensure data reliability, homogeneity and normality tests were also performed using the Anderson–Darling test at the 95% confidence level. In the next step, considering the importance of topography in precipitation distribution, the stations were classified based on 500-meter elevation classes in order to examine the effect of elevation on extreme precipitation intensity in each solar cycle. Then, to identify heavy precipitation events, a threshold of 10 mm was considered based on the R10mm index introduced by ETCCDI. Finally, the frequency of precipitation events was calculated in 10 mm classes, and the changes in their intensity and frequency in different solar cycles and elevation classes were analyzed and compared.
Results and Discussion
The results of this study show that the maximum 24-hour precipitation on the southern coasts of the country does not follow a linear trend. In other words, with increasing elevation precipitation does not show a uniform increasing pattern. Instead, precipitation variability is a combination of convective and orographic processes in which thermodynamic processes dominate over topographic effects. As a result, below the 500 m level, with the onset of convective flows, convective rainfall begins, but extreme precipitation with values exceeding 100 mm, which occurs at higher elevation classes, does not occur. Access to abundant moisture sources from the warm southern seas and the mechanisms of atmospheric systems affecting the region with tropical characteristics cause convective eddies to form along the coastal belt as soon as the first unstable conditions dominate the region. Subsequently, coastal topography increases the intensity of convective instability. These conditions continue up to an elevation of about 1000 m, after which extreme 24-hour precipitation shows a noticeable decrease in the 1000–1500 m elevation class. Above 1500 m, the increasing trend continues up to about 2500 m, where meteorological stations are located. In fact, the 1000–1500 m elevation class represents a turning point where the convective structure of systems changes into a combined convective–orographic structure. It seems that the moisture advected into the incoming systems in the lower layer is discharged by intense convective showers, and in the 1000–1500 m elevation class convective systems face a lack of moisture due to the discharge of moisture in the lower layer. In addition, according to the synoptic and thermodynamic pattern governing precipitation systems in this region, the moisture sources feeding the systems differ at different levels. In these patterns, in the lower levels, due to strong moisture advection—especially during the dominance of the Siberian and Tibetan high pressure tongues—moisture is advected into the Sudan system in the lower layer, and with the addition of moisture sources from the Intertropical Convergence Zone above the 850 hPa level through southern flows ahead of troughs, heavy and flood producing rainfall in higher elevations is greatly intensified (Mashayekh et al., 2023). However, in some particular synoptic patterns, convection can also occur at the lower levels if sufficient moisture is available, producing torrential rainfall. For example, during the precipitation system of 28 Azar 1404, the 24-hour precipitation amount at Khark Island station reached 137.9 mm and at Qeshm station 135.2 mm. In some events, precipitation intensity has even exceeded 200 mm and has been accompanied by widespread and destructive floods, including the floods of 27–28 Azar 1404 and 6–7 Esfand 1402. In addition, in the elevation classes of 1500–2000 and 2000–2500 m the intensity of 24-hour precipitation has increased. The number of months with an average precipitation of more than 100 mm has increased significantly.During the 33-year statistical period, in the elevation class of 1500–2000 m a total of 21 months and in the class of 2000–2500 m 42 months with a monthly average
24-hour precipitation of more than 100 mm were reported. In terms of the month of occurrence of the highest 24 hour means, December occurred four times, January three times, March four times, and November four times. Therefore, the main concentration of extreme precipitation in southern Iran in terms of temporal distribution is between November and March. Among these months, February has the lowest frequency in producing extreme precipitation. This issue highlights the combined role of orographic uplift and the strengthening of convective instability. Comparison of solar cycles also showed that cycle 23 experienced the highest intensity and frequency of extreme precipitation. The examination of monthly distribution also indicates that the peak occurrence of extreme precipitation is in January, followed by a noticeable decrease in February and a renewed increase in March, while after April such events occur rarely and in a scattered manner.
Conclusion
Therefore, the occurrence of torrential rainfall—especially in the elevated areas of Fars, Bushehr, and Hormozgan provinces, where precipitation mostly occurs in liquid form—represents one of the most important climatic hazards in these provinces. Considering that these events can be predicted within the framework of synoptic patterns and that their main temporal concentration occurs in December and January, issuing effective early warnings before the occurrence of such torrential rainfall and proper runoff management can play an important role in reducing the damages caused by these phenomena.The results of this study also showed that the systems responsible for the main precipitation of both the warm and cold seasons in southern Iran, either independently or in combination with other atmospheric systems, have particular dynamical and thermodynamical structures that show considerable similarity with tropical precipitation systems. Among them, the Sudan Low, in a suitable configuration with the Arabian and Siberian anticyclonic systems, plays the most important role in supplying moisture for precipitation during the cold season in this region (Jafari et al, 2021; Lashkari et al, 2020; Mohammadi et al, 2022; Lashkari et al, 2015; Mohammadi et al, 2019; Lashkari and Mohammadi, 2023; Zarrin et al, 2005; Mohammadi et al, 2012).