واکاوی همدیدی-ترمودینامیک و قلمرو فضایی توفان های حاره ای در کشور ایران

ABSTRACT
Tropical cyclones, as thermal low-pressure systems, cause severe rainfall and extensive damage across the coastal and inland regions of southeastern Iran. This study aims to identify and analyze the spatiotemporal patterns of tropical cyclones affecting the region during the period 1986 to 2019. Daily precipitation data from 73 synoptic stations and associated weather phenomenon codes were analyzed across three 11-year solar cycles to detect cyclone-induced events with precipitation ≥20 mm recorded at ≥3 stations. To gain deeper insights into these cyclones, Skew-T diagrams and TRMM satellite data were also employed. During this period, 26 significant events with varying frequency patterns and spatial distributions were identified. The results reveal that cyclones are primarily active in May, June, and October, with the intensity and spatial extent of their impacts exhibiting seasonal variations. In some months, despite fewer cyclone occurrences, the associated rainfall coverage and impacts are more extensive. Synoptic pattern analysis indicates a broad field of specific humidity in the eastern sector of the cyclone, extending throughout the troposphere. In the mid-troposphere, the northward expansion of the Arabian anticyclonic ridge induces cold advection on the western flank of the cyclone, along with sharp thermal and pressure gradients. Additionally, strong negative omega values at the 700 and 500 hPa levels highlight the intense dynamical processes within the cyclone structure. Tropical cyclones in southeastern Iran are characterized by intense rainfall and variable seasonal distribution, with the interplay of moisture, atmospheric instability, and synoptic-scale structures playing a key role in their intensification.
Extended Abstract
Introduction
Climatic hazards, particularly tropical cyclones, cause substantial human and economic losses annually across the globe and are considered among the most significant environmental threats due to their profound impacts. These systems, often accompanied by extreme rainfall, intense winds, and flash flooding, exert severe influence on both coastal and inland regions. In recent years, southeastern Iran notably the provinces of Sistan and Baluchestan, Hormozgan, Kerman, and parts of South Khorasan has repeatedly been affected by such cyclones, resulting in considerable damage to infrastructure, water resources, and the livelihoods of local communities. In this context, a comprehensive and precise analysis of the genesis, development, and interaction of tropical cyclones with synoptic patterns and thermodynamic characteristics is crucial for enhancing forecasting capabilities, reducing vulnerability, and formulating effective disaster management strategies. The identification of impactful events in southeastern Iran, along with the analysis of their spatio-temporal patterns and delineation of their zones of influence, represents a vital step toward a deeper understanding of the dynamics of this climatic hazard within the framework of forecasting systems and risk management policymaking.
 
Methodology
To assess the extent of Iran’s vulnerability to tropical cyclones, daily precipitation data from 73 synoptic stations located in cyclone-prone provinces (Sistan and Baluchestan, Kerman, Hormozgan, Fars, Bushehr, Khuzestan, and South Khorasan) were analyzed over a 33-year period (1986–2019), corresponding to three 11-year solar cycles. Stations with complete data for each cycle were examined separately. To identify tropical cyclone events, days with precipitation exceeding 20 mm at a minimum of three stations, accompanied by weather codes 91–99, were classified as storm days. In total, 26 events were identified, among which two prominent cases—June 6–8, 2007, and June 4–5, 2010—were selected for synoptic and thermodynamic analysis.
Atmospheric variables including geopotential height, specific humidity, omega (vertical velocity), and wind components were retrieved from the NCEP/NCAR reanalysis dataset and analyzed using GrADS software. To examine the vertical structure of the atmosphere during these events, Skew-T diagrams for the Muscat Airport station as the closest representative profile to the storm core were utilized. Additionally, to visualize the spatial extent and intensity of precipitation, TRMM satellite data with a spatial resolution of 0.25 degrees and high correlation with ground-based observations (r > 0.8) were employed, obtained from the Giovanni data portal.
 
Results and discussion
The findings of this study indicate that over the 33-year period, tropical cyclones originating in the Arabian Sea and the northern Indian Ocean exerted a direct and variable influence on the precipitation patterns of southeastern Iran. The intensity and nature of this impact are contingent upon the timing of the event, associated synoptic conditions, and the dynamic–thermodynamic structure of the cyclone. The seasonal frequency of storms follows a discernible pattern from May to October, with a peak in May, a decline through July, and a subsequent increase in August and October, while no events were recorded in September—a trend closely linked to the gradual transition of synoptic systems during the monsoon period. Notably, a lower frequency of storms does not necessarily imply a reduced spatial impact. For instance, in July, a single cyclone may influence a broad geographical area. A case study analysis of the June 2007 and June 2010 cyclones revealed that the 2007 event, characterized by a strong cyclonic structure, abundant moisture, and extensive upper-level divergence, resulted in intense and concentrated rainfall across southeastern Iran—exceeding 300 mm at some stations. The Skew-T diagram from Muscat Airport confirmed the presence of a highly unstable troposphere. In contrast, the 2010 cyclone exhibited a different interaction with prevailing synoptic systems, particularly between a migrating anticyclone and northern cold advection versus southern warm advection, which led to the intensification of the pressure gradient, high winds, and convective rainfall, albeit with a more limited inland penetration. A comparative assessment of these two events suggests that the extent and intensity of tropical cyclone rainfall are not solely determined by internal cyclone characteristics, but are strongly influenced by the synoptic background and the degree of synergy with incoming moisture. These findings are consistent with previous studies that underscore the importance of cyclone interaction with surrounding atmospheric patterns and moisture advection in shaping the distribution and intensity of extreme precipitation events.
 
Conclusion
The findings of this study demonstrate that tropical cyclones originating in the Arabian Sea and northern Indian Ocean play a significant role in generating intense and localized precipitation in southeastern Iran. The magnitude and spatial extent of their impacts are highly dependent on the timing of occurrence, the dynamic–thermodynamic structure of the cyclone, and the surrounding synoptic atmospheric conditions. The seasonal trends and spatiotemporal variability of these storms reflect the complex interplay between large-scale systems and internal convective processes. The analysis of two major events in June 2007 and June 2010 underscores that extreme rainfall is not merely a function of a cyclone’s dynamic strength, but is also heavily influenced by the amount of moisture advection and the nature of its interaction with synoptic-scale flows. These interactions enhance thermodynamic instability, amplify convective energy, and ultimately result in very intense rainfall and sudden flash floods. Therefore, a deeper understanding of the interactive mechanisms between tropical cyclones and large-scale atmospheric patterns is crucial for improving forecasting capabilities and climate hazard management in the region. The present results, consistent with previous research, highlight the critical role of synoptic conditions and moisture transport, and may serve as a foundation for evidence-based policymaking in climate disaster risk reduction and management.
 
Funding
There is no funding support.
 
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved the content of the manuscript and agreed on all aspects of the work declaration of competing interest none.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.