Pegahfar et al. (2026) Vorticity budget analysis of a mesoscale convective vortex during the July 2022 flash flood in Northern Tehran

Identification

• Journal: Scientific Reports
• Year: 2026
• Date: 2026-01-13
• Authors: Nafiseh Pegahfar, Maryam Gharaylou, Omid Alizadeh
• DOI: 10.1038/s41598-026-35778-x

Research Groups

• Atmospheric Sciences Research Center, Iranian National Institute for Oceanography and Atmospheric Science, Tehran, Iran
• Institute of Geophysics, University of Tehran, Tehran, Iran
• Geography Department, Humboldt-Universität zu Berlin, Berlin, Germany

Short Summary

This study investigates the dynamical processes of a Mesoscale Convective Vortex (MCV) associated with the 27 July 2022 flash flood in northern Tehran, revealing that horizontal advection, divergence, and tilting primarily enhanced cyclonic vorticity, while vertical advection acted as a compensating force, collectively leading to the extreme rainfall event.

Objective

• To examine the dynamical processes leading to the formation and intensification of a Mesoscale Convective Vortex (MCV) associated with the 27 July 2022 heavy rainfall and flash flood in northern Tehran, and to clarify its role in producing the observed extreme precipitation.

Study Configuration

• Spatial Scale: Northern Tehran, Iran (35°14′–36°17′ N, 50°14′–53°6′ E), specifically the Imamzadeh Davood flood event location. Vertical extent from 1000 hPa (100 kPa) to 100 hPa (10 kPa) (troposphere).
• Temporal Scale: 27 July 2022 flash flood event. Detailed analysis focuses on 20:00 to 23:00 UTC on 27 July 2022.

Methodology and Data

• Models used: Vorticity budget equation (ignoring friction and subgrid-scale effects) applied to reanalysis data.
• Data sources:
  • ERA5 reanalysis (European Centre for Medium-Range Weather Forecasts 5th Generation Reanalysis): Hourly data at 0.25° × 0.25° horizontal resolution, including zonal wind (u), meridional wind (v), vertical pressure velocity (ω), and pressure fields from 1000 hPa (100 kPa) to 100 hPa (10 kPa).
  • GPM (Global Precipitation Measurement) satellite: 24-hour accumulated rainfall data at 0.1° (~10 km) horizontal resolution.
  • Infrared satellite imagery (EUMETSAT).
  • Doppler radar observations (Tehran).

Main Results

• A coherent positive vorticity center emerged south of the flood site in the late afternoon and migrated northward as convection intensified.
• All four vorticity budget terms—horizontal advection, vertical advection, divergence (stretching), and tilting—made non-negligible contributions to the local vorticity tendency.
• Horizontal advection dominated the early development of the MCV, producing positive vorticity tendencies of approximately 10⁻⁸ s⁻² in the 700–600 hPa (70–60 kPa) layer roughly three hours before rainfall onset.
• Divergence and tilting further amplified cyclonic vorticity below approximately 700 hPa (70 kPa), with tilting peaking near 850 hPa (85 kPa) at approximately 1.25 × 10⁻⁸ s⁻² at 20:00 UTC.
• Vertical advection consistently acted as a compensating term, partially offsetting the low-level vorticity growth, with magnitudes of approximately –0.5 × 10⁻⁸ s⁻² at 850 hPa (85 kPa).
• The combined effect of these processes was a vertically coherent vortex column, aligned temporally with radar-observed convective organization.
• Extreme rainfall resulted from the interaction between a warm, humid monsoonal inflow from lower latitudes (specific humidity exceeding 0.01 kg/kg) and a cooler air mass advected from higher latitudes, which enhanced low-level convergence, mesoscale ascent, and vortex stretching.
• Cloud-top temperatures ranged from 228.15 K to 208.15 K (–45°C to –65°C), indicating strong convective activity, with radar reflectivity reaching 40 dBZ.
• The MCV was most pronounced around 850 hPa (85 kPa).

Contributions

• Provides the first detailed vorticity budget analysis of a Mesoscale Convective Vortex (MCV) associated with an extreme precipitation event in Iran, a region characterized by complex topography.
• Highlights the critical importance of MCV-related vorticity processes in triggering high-impact precipitation events in mountainous mid-latitude regions.
• Underscores the necessity for improved representation of mesoscale convective mechanisms in operational forecasting systems.
• Demonstrates that all four vorticity budget terms (horizontal advection, vertical advection, divergence, and tilting) contribute significantly to MCV development, with vertical advection consistently acting as a compensatory force, offering a nuanced perspective compared to some prior studies.
• Emphasizes the crucial role of low-level processes, modulated by mesoscale circulation and vertical wind shear, in generating extreme rainfall in complex terrain.

Funding

Not applicable.

Citation

@article{Pegahfar2026Vorticity,
  author = {Pegahfar, Nafiseh and Gharaylou, Maryam and Alizadeh, Omid},
  title = {Vorticity budget analysis of a mesoscale convective vortex during the July 2022 flash flood in Northern Tehran},
  journal = {Scientific Reports},
  year = {2026},
  doi = {10.1038/s41598-026-35778-x},
  url = {https://doi.org/10.1038/s41598-026-35778-x}
}