Thunderstorms are among the most important short-term atmospheric hazards in urban and piedmont areas, and identifying the synoptic patterns governing their occurrence is of great importance for climatic risk management. The present study was conducted to investigate the synoptic patterns influencing thunderstorm occurrences in Tehran Province during solar cycles 22, 23, and 24 over the statistical period 1986–2019. For this purpose, daily data from synoptic stations in Tehran Province, based on the present weather code ww, were obtained from the Iran Meteorological Organization and, after quality control, thunderstorm events were identified according to the criteria of spatial simultaneity and repeated observations. The events were then classified according to duration, from one-day to five-day and longer events, and the 90th percentile threshold was used to extract extreme cases. Atmospheric data related to the selected samples were acquired from the NCEP/NCAR reanalysis database at the 1000, 925, 850, 700, and 500 hPa levels, and composite maps of geopotential height, specific humidity, wind components, and omega were prepared and analyzed. The results showed that more than 70% of thunderstorm systems in Tehran Province had a one-day duration, and that the highest frequency of occurrences was mainly observed in spring, especially in April and May. Among the 49 extreme one-day systems, 14 synoptic patterns were identified, of which the “Saudi Low–Migratory High,” “Pakistan Low–Migratory High,” and “Sudan Low–European Cyclone” patterns had the highest frequencies. The findings indicate that the coincidence of low-level moisture advection, dynamic instability in the middle troposphere, and the role of the Alborz topography constitutes the main mechanism for the development of extreme thunderstorms in Tehran Province.
Extended Abstract
Introduction
Thunderstorms are among the most significant short term and high impact atmospheric hazards, primarily due to their abrupt onset and their capacity to produce destructive phenomena such as severe downbursts, intense rainfall, hail, lightning strikes, and sudden reductions in visibility. These events can disrupt transportation networks, damage energy infrastructures, threaten public safety, and impose substantial economic losses. In this context, Tehran Province—characterized by its dense population, concentrated urban development, and critical infrastructures, including airports, power facilities, and communication networks—is considered one of the most vulnerable regions to convective hazards in Iran. Its location along the southern foothills of the Alborz Mountains creates a unique atmospheric environment in which large scale synoptic systems, regional scale circulations, and local topographic processes interact. These interactions often enhance atmospheric instability and intensify convective development. Despite the importance of this hazard, comprehensive synoptic studies focusing on thunderstorm events in Tehran remain limited. Accordingly, the present study aims to identify, classify, and analyze the dominant synoptic patterns associated with severe thunderstorm occurrences in Tehran Province, in order to provide a scientific foundation for improved forecasting, hazard assessment, and early warning development. The novelty of this study lies in integrating an objective 90th-percentile statistical threshold with a multi-decadal chronological classification based on solar cycles (22–24) to evaluate the recurrence and dynamic configurations of multi-level synoptic patterns (MSLP, 850, and 500 hPa) governing high-impact convective storms over the complex topography of Tehran Province.
Material and Methods
This research follows an applied and descriptive–analytical approach. The database consists of all thunderstorm events recorded at synoptic stations across Tehran Province during the 1986–2019 period, which covers solar cycles 22, 23, and 24. Thunderstorm days were identified using present weather codes (ww) obtained from the Iran Meteorological Organization. The data underwent a rigorous quality‑control process to ensure temporal continuity, observational consistency, and spatial coherence among stations. Thunderstorm occurrences were then filtered based on spatial simultaneity and repeated observations across the stations. To assess temporal persistence, events were classified into one‑day, two‑day, three‑day, four‑day, and five‑day‑plus systems. Extreme cases were extracted using the 90th percentile (P90) threshold to differentiate high‑impact events from ordinary ones. After identifying extreme events, atmospheric variables associated with each case were extracted from the NCEP/NCAR reanalysis database at standard pressure levels of 1000, 925, 850, 700, and 500 hPa. Variables included geopotential height, specific humidity, zonal and meridional wind components, and vertical velocity (omega). Composite maps were generated for each extreme event, and synoptic patterns were diagnosed through comparative and overlay analyses. This process allowed the recognition of distinct synoptic configurations and the extraction of their governing atmospheric mechanisms. Finally, a frequency analysis was conducted to determine the most recurrent synoptic structures responsible for severe convective activity in Tehran Province.
Results and Discussion
Statistical analysis indicates that thunderstorms in Tehran are predominantly short-lived, with over 70% classified as one-day events. Seasonally, activity peaks during April and May, driven by the synergy of intensified surface heating, moisture availability, and thermodynamic instability during this transition period. Synoptic evaluation of the 49 selected events identified 14 distinct patterns, with three configurations proving most recurrent and influential: (1) Saudi Low–Migratory High, (2) Pakistan Low–Migratory High, and (3) Sudan Low–European Cyclone.
In the first two configurations, the juxtaposition of thermal lows over the Arabian Peninsula or Pakistan with migrating anticyclones generates robust pressure gradients, promoting strong lower-tropospheric convergence and enhancing upward motion along the Alborz foothills. The third pattern, involving the Sudan Low coupled with an upper-level European cyclone, fosters a dynamically active environment characterized by enhanced cyclonic vorticity, widespread vertical motion, and intense thermodynamic instability. Moisture advection analysis confirms that the Persian Gulf and Gulf of Oman serve as the primary moisture sources, while the Caspian Sea occasionally contributes supplementary water vapor. When this moisture influx coincides with upper-level divergence and mid-level ascent, deep convective clouds develop rapidly. Furthermore, the topography of the Alborz Mountains plays a critical role by inducing orographic uplift, which localizes and amplifies convective potential over the southern slopes. Consequently, high-impact thunderstorms in Tehran stem from a complex multi-scale interaction involving synoptic forcing, thermodynamic instability, moisture transport, and local topographic enhancement.
Conclusion
The monthly distribution of thunderstorms shows a peak frequency during April and May, highlighting the dominance of convective instability during the seasonal transition. Synoptic analysis identifies the ‘Arabian Low – Migratory High’ pattern as the primary lower-tropospheric driver (accounting for 17 of the 49 selected events). This configuration intensifies surface thermal and pressure gradients across Southwest Asia, while its cyclonic circulation, coupled with middle-tropospheric instability, channels essential moisture from the Caspian and Black Seas to fuel convective development over Tehran. Spatially, thunderstorm occurrences are concentrated in elevated northeastern stations (e.g., Firuzkuh) and southern plain areas (e.g., Imam Khomeini Airport), driven by the combined effects of orographic uplift and surface thermal inflows. Given the strategic importance and high event frequency at Imam Khomeini International Airport, establishing rapid, impact-based convective early warning systems is vital for aviation safety.