Ghasemifar et al. (2026) Widespread extreme precipitation events over Iran: Large-scale patterns and their associated global indices

Identification

• Journal: Atmospheric Research
• Year: 2026
• Date: 2026-03-01
• Authors: Elham Ghasemifar, Irfan Ur Rashid, Céline Planche, Jean-Luc Baray, Mansour Almazroui, Manoranjan Mishra
• DOI: 10.1016/j.atmosres.2026.108937

Research Groups

• Université Clermont Auvergne, CNRS INSU, Laboratoire de Météorologie Physique UMR 6016, F-63000 Clermont-Ferrand, France
• Research and Development Division, Pakistan Meteorological Department (PMD), Islamabad, Pakistan
• Institut Universitaire de France, Paris, France
• Center of Excellence for Climate Change Research/Department of Meteorology, King Abdulaziz University, Jeddah, Saudi Arabia
• Climate Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, UK
• Department of Geography, Fakir Mohan University, Vyasa Vihar, Nuapadhi, Balasore, Odisha, India

Short Summary

This study characterizes widespread extreme precipitation events (WEPEs) over Iran, identifying their frequency, intensity, duration, and regional patterns using 25 years of satellite data. It reveals that WEPEs are primarily driven by deep troughs over the Red Sea/Arabian Peninsula and are strongly linked to the Circumglobal Wave Train (CGT), with the North Atlantic Oscillation (NAO) indirectly modulating CGT variability.

Objective

• To investigate how large-scale circulation patterns control the frequency, intensity, and spatial extent of widespread extreme precipitation events (WEPEs) in Iran.

Study Configuration

• Spatial Scale: Iran, divided into three distinct sub-regions: R1 (western and northwestern Iran), R2 (eastern, central, and southeastern Iran), and R3 (southern Iran).
• Temporal Scale: 2000–2024 (25 years).

Methodology and Data

• Models used:
  • ECMWF ERA5 global reanalysis (daily 0.25°×0.25° data for 500 hPa and 200 hPa geopotential height, 500 hPa vertical velocity (omega), 850 hPa and 200 hPa zonal and meridional wind components, specific humidity, mean sea level pressure, and vertical integral of eastward and northward moisture fluxes).
• Data sources:
  • Integrated Multi-satellitE Retrievals for GPM (IMERG) final product (IMERG-F) for precipitation data (30-minute temporal, ~0.1° spatial resolution).
  • Precipitation data from 39 synoptic weather stations across Iran (2000-2024) from the Iran Meteorological Organization (IRIMO).
  • Global climate indices from NOAA Physical Sciences Laboratory: El Niño Southern Oscillation (ENSO) via Niño 3.4 index, North Atlantic Oscillation (NAO), and Indian Ocean Dipole (IOD).
  • Circumglobal Wave Train (CGT) index derived from the first leading mode of 200 hPa meridional wind over [100°W-100°E, 20°N-80°N].
  • Methodology: WEPEs defined as events where daily precipitation exceeded the 95th percentile over a spatially connected area greater than 10% of Iran's land area. Regionalization performed using Ward's hierarchical clustering method. Composite analysis, lead-lag analysis, and Pearson's correlation coefficients were used to investigate atmospheric drivers and teleconnections.

Main Results

• A total of 153 WEPEs were identified across Iran from 2000-2024, with 135 events occurring predominantly in winter (80 events) and spring (55 events).
• WEPEs are relatively rare, with a return period of approximately 60 days (or ~6 events per year) for events covering >10% of Iran's land area, compared to ~25 localized extreme precipitation events per year.
• Non-significant increasing trends were observed for both the intensity (+0.1 mm/year) and frequency (+0.01 events/year) of WEPEs, while duration showed a slight non-significant decreasing trend (–0.01 hours/year).
• Three distinct regions were identified: R1 (west and northwest), R2 (east and southeast), and R3 (south). R1 exhibited the highest frequency (~37 events) and intensity (~90 mm/hour), while R3 showed the longest event durations (up to 520 hours over 25 years).
• Large-scale circulation patterns associated with WEPEs are characterized by a pronounced low-pressure system (deep trough) over or in close proximity to the Red Sea and Arabian Peninsula, with the strongest signal during winter.
• Upper-level geopotential height fields reveal well-defined low-height anomalies, shifting from the Red Sea towards the northern and eastern Arabian Peninsula. In winter, an east-west dipole pattern (low HGT over Red Sea/Arabian Peninsula, high HGT over Afghanistan/Pakistan) enhances atmospheric instability and upward motion.
• Strong integrated vapor transport (IVT) from the Red Sea and Persian Gulf, resembling atmospheric river-like moisture plumes, consistently supports WEPE formation.
• Lead-lag analysis of 200 hPa geopotential height anomalies shows distinct seasonal evolution: in winter, high HGT systems over Europe precede WEPEs, pushing low HGT systems eastward; in spring, a broad, coherent low-pressure system extends from the Mediterranean towards Iran, strengthening over the Red Sea and Arabian Peninsula during events.
• WEPEs are primarily controlled by the Circumglobal Wave Train (CGT), with the negative phase of CGT being dynamically favorable for enhanced precipitation. The North Atlantic Oscillation (NAO) indirectly influences WEPEs by modulating CGT variability, particularly in spring, while ENSO and IOD play secondary or seasonally limited roles.

Contributions

• Provides the first comprehensive national-scale assessment of widespread extreme precipitation events (WEPEs) across Iran, including their synoptic drivers and seasonal dependencies.
• Utilizes 25 years of high-resolution, satellite-based IMERG data, overcoming limitations of sparse station networks in capturing spatial variability of extreme precipitation.
• Establishes a robust climatology of WEPEs and their associated large-scale circulation patterns, allowing for confident interpretation and generalization of synoptic-scale results.
• Offers a region-based assessment of WEPE characteristics, providing practical value for hydrological and agricultural risk management in Iran.

Funding

• Elham Ghasemifar was fully supported by a grant (MOPGA) from the French Ministry of Europe and Foreign Affairs.

Citation

@article{Ghasemifar2026Widespread,
  author = {Ghasemifar, Elham and Rashid, Irfan Ur and Planche, Céline and Baray, Jean-Luc and Almazroui, Mansour and Mishra, Manoranjan},
  title = {Widespread extreme precipitation events over Iran: Large-scale patterns and their associated global indices},
  journal = {Atmospheric Research},
  year = {2026},
  doi = {10.1016/j.atmosres.2026.108937},
  url = {https://doi.org/10.1016/j.atmosres.2026.108937}
}