Abstract

Extreme precipitation is among the most important climatic phenomena affecting the natural environment and human activities, and when concentrated in time and space, it can lead to severe hydrological hazards. Although this phenomenon occurs in most regions of the world, in certain areas—such as southern Iran—it is associated with greater intensity and more destructive impacts due to specific synoptic and thermodynamic conditions as well as complex topography. The penetration of tropical-origin precipitation systems, particularly Sudanese low-pressure systems and their associated troughs, in interaction with steep slopes, unfavorable soil characteristics, and weak vegetation cover, plays a decisive role in intensifying surface runoff and triggering widespread flooding. The objective of this study is to investigate the intensity and frequency characteristics of extreme precipitation across different elevation classes and to analyze their temporal variations during the last three solar cycles (1986–2019). For this purpose, daily precipitation data from 109 synoptic and climatological stations located in southern and southwestern Iran were collected. Following rigorous quality control procedures, data gaps (approximately 4%) were reconstructed. Data normality was confirmed using the Anderson–Darling test. The results indicate that from the coastal belt up to elevations of 1000 m, the intensity of extreme precipitation exhibited an increasing trend across all three solar cycles, whereas a significant decrease was observed within the 1000–1500 m elevation range. At elevations above 1500 m, extreme precipitation intensity increased again, with the maximum recorded in the 2000–2500 m elevation class. Inter-cycle comparison shows that Solar Cycle 23 experienced the highest intensity and frequency of extreme precipitation events, while the monthly distribution reveals peak occurrences in January and March.

Keywords: Extreme precipitation, Altitudinal distribution, Temporal variability, Tropical precipitation systems, Southern Iran, Solar cycles

Extended Abstract

Introduction

Extreme precipitation events are recognized as one of the most significant manifestations of climate variability and instability, exerting profound impacts on natural systems, infrastructure, and human activities across different spatial and temporal scales. When such precipitation occurs with high temporal concentration and spatial continuity, it often results in severe hydrological hazards, including flash floods, landslides, and widespread environmental and socioeconomic damage. Although extreme precipitation is a global phenomenon observed in most climatic regions of the world, its intensity, frequency, and destructive potential vary considerably depending on regional atmospheric dynamics, thermodynamic conditions, and physiographic characteristics.



Southern Iran represents one of the most vulnerable regions of the country to extreme and torrential precipitation events. This vulnerability is largely attributed to its geographical location, proximity to warm tropical moisture sources such as the Persian Gulf, the Oman Sea, and the Red Sea, and its frequent exposure to tropical synoptic systems. Among these systems, Sudanese low‑pressure systems and their associated troughs play a dominant role in generating intense rainfall over southern and southwestern Iran. The dynamic structure of these systems, when combined with steep topographic gradients, complex mountainous terrain, unfavorable soil properties, and limited vegetation cover, substantially enhances surface runoff and increases the likelihood of destructive flooding.



Previous studies conducted at global and regional scales have documented an increasing trend in the intensity of extreme precipitation events in various parts of the world, including Central Europe, North America, and parts of West Asia. In Iran, most research has focused on synoptic analyses of heavy rainfall events, moisture transport pathways, and the role of large‑scale atmospheric circulation patterns, particularly the Sudanese low‑pressure system. In addition, several studies have suggested that variations in solar activity and solar cycles may influence precipitation variability and the occurrence of convective and extreme rainfall events. However, despite these efforts, a clear research gap remains in the integrated analysis of extreme precipitation intensity, its elevation‑dependent distribution, and its temporal variability in relation to consecutive solar cycles, particularly in southern Iran.



Accordingly, the primary objective of this study is to analyze the intensity and frequency characteristics of extreme precipitation across different elevation classes in southern Iran and to investigate their temporal variations during the last three solar cycles (1986–2019). By adopting an elevation‑based and cycle‑oriented approach, this research aims to provide a more comprehensive understanding of the spatial–temporal behavior of extreme precipitation and its controlling mechanisms in one of Iran’s most flood‑prone regions.



Materials and Methods

This study utilizes daily precipitation data obtained from 109 synoptic and climatological stations distributed across the southern half of Iran. The stations are located within the provinces of Ilam, Lorestan, Khuzestan, Chaharmahal and Bakhtiari, Fars, Kohgiluyeh and Boyer‑Ahmad, Bushehr, Hormozgan, Kerman, and Sistan and Baluchestan. Together, these stations cover a wide range of climatic, topographic, and elevation conditions, providing a robust dataset for regional‑scale analysis.



Due to inconsistencies in record length among stations, a common 33‑year study period corresponding to three complete solar cycles (cycles 22, 23, and 24), spanning from 1986 to 2019, was selected. To maximize the number of stations included in each solar cycle, only stations with complete and continuous records within each 11‑year cycle were considered in the analysis. As a result, the number of stations varied among cycles, increasing from solar cycle 22 to cycle 24.



All precipitation data underwent rigorous quality control procedures. Missing values, accounting for approximately 4% of the total dataset, were identified and corrected using appropriate statistical methods. Data consistency and completeness were verified using spreadsheet‑based statistical checks. Subsequently, the normality of the precipitation data was assessed using the Anderson–Darling test in the Minitab software environment, and the results confirmed that the data were suitable for statistical analysis.



For analytical purposes, the dataset was first divided according to the three solar cycles, each covering an 11‑year period. The stations were then classified into five elevation zones with 500‑meter intervals: 0–500 m, 500–1000 m, 1000–1500 m, 1500–2000 m, and 2000–2500 m. This classification enabled the examination of elevation‑dependent variations in extreme precipitation characteristics.



Extreme precipitation events were identified based on maximum 24‑hour precipitation amounts exceeding 10 mm. These events were further grouped into 10‑mm intensity classes, and their frequency and intensity were calculated for each elevation zone and solar cycle. In addition, monthly distributions of extreme precipitation were analyzed to identify seasonal patterns and intra‑annual variability across the study region.



Results and Discussion

The results indicate that the elevation‑dependent behavior of extreme precipitation in southern Iran does not follow a simple linear pattern. Instead, it reflects a complex interaction between atmospheric dynamics, moisture availability, and topographic influences. From the coastal areas up to elevations of approximately 1000 m, the intensity and frequency of extreme precipitation events exhibit a clear increasing trend across all three solar cycles. This pattern can be attributed to the effective penetration of tropical moisture, enhanced low‑level convergence, and favorable dynamic lifting mechanisms in these lower‑elevation zones.



In contrast, a noticeable and consistent decrease in extreme precipitation intensity is observed within the 1000–1500 m elevation range. This reduction may be associated with local topographic effects, partial weakening of synoptic systems, or changes in atmospheric stability that limit the efficiency of precipitation‑producing processes at these elevations.



Above 1500 m, extreme precipitation intensity increases once again, reaching its maximum values within the 2000–2500 m elevation class. This finding highlights the critical role of orographic lifting, enhanced condensation, and intensified atmospheric instability over mountainous regions. The pronounced increase in extreme precipitation at higher elevations underscores the heightened flood and landslide risk in upland catchments and mountainous basins of southern Iran.



A comparison among the three solar cycles reveals that solar cycle 23 experienced the highest intensity and frequency of extreme precipitation events across most elevation classes. This result suggests a potential link between heightened solar activity and increased atmospheric instability or moisture transport efficiency during this period, although further investigation is required to fully elucidate the underlying mechanisms.



Analysis of the monthly distribution of extreme precipitation events shows a distinct seasonal pattern. The highest frequency and intensity of extreme precipitation occur in January, followed by a marked decrease in February and a subsequent increase in March. After April, extreme precipitation events become sporadic and rare, reflecting the dominance of more stable atmospheric conditions during the warm season. This seasonal behavior is consistent with the temporal activity of Sudanese low‑pressure systems and their interaction with mid‑latitude circulation patterns during the cold season.



Overall, the results demonstrate that both elevation and temporal factors, including solar cycle variability, play crucial roles in shaping the spatial and temporal distribution of extreme precipitation in southern Iran. The observed patterns align well with previous regional studies while providing new insights through their integrated elevation‑ and cycle‑based framework.



Conclusion

This study demonstrates that extreme precipitation in southern Iran is strongly controlled by elevation‑dependent processes, temporal variability, and solar cycle fluctuations. The non‑linear elevation pattern—characterized by increasing intensity from coastal areas to mid‑elevations, a reduction around 1000–1500 m, and a pronounced increase above 1500 m—highlights the combined influence of synoptic dynamics and orographic effects on extreme rainfall generation.



The identification of solar cycle 23 as the period with the highest intensity and frequency of extreme precipitation emphasizes the importance of considering long‑term temporal variability in climate hazard assessments. Moreover, the concentration of extreme precipitation events during the winter months, particularly in January and March, underscores the critical role of cold‑season synoptic systems in driving hydrological extremes in the region.



The findings of this research provide valuable scientific insight for flood risk management, watershed planning, and climate adaptation strategies in southern Iran. By improving the understanding of where and when extreme precipitation is most likely to occur, the results can contribute to more effective early warning systems, land‑use planning, and infrastructure design aimed at reducing vulnerability to hydrological hazards. Future studies are encouraged to further explore the interactions between large‑scale climate oscillations, land‑use changes, and extreme precipitation dynamics in order to enhance regional climate resilience.