In this study, atmospheric circulation anomalies associated with heavy precipitation events in Iran during the 1990–2022 period were investigated through the analysis of 500-hPa geopotential height and temperature fields, using both observational daily precipitation records and gridded datasets. Heavy precipitation days were first identified based on the 95th percentile threshold, after which associated atmospheric patterns were extracted using Principal Component Analysis (PCA). The results indicated that in autumn and winter, heavy precipitation events predominantly occurred over western Iran and the Caspian Sea coasts, while in spring their maximum occurrence was observed over the western and northern regions of the country. Cluster analysis revealed five patterns of negative geopotential height anomalies, including the expansion of the polar vortex, a mid-tropospheric cut-off low accompanied by strong geopotential height and temperature anomalies, a geopotential height anomaly linked to a Mediterranean trough, and the expansion of a European low-pressure system together with its interaction with the Mediterranean trough over the region. From a temporal perspective, the first and second patterns mainly occur in January and February, concurrent with cold-air outbreaks from higher northern latitudes. The mid-tropospheric cut-off low pattern, characterized by a −180 geopotential meter anomaly, represents the most dominant atmospheric pattern associated with heavy precipitation events in Iran, accounting for 157 occurrences and 34% of the total frequency. Accordingly, the most influential thermal pattern governing heavy precipitation in Iran is the polar cold-air outbreak surrounding the mid-tropospheric cut-off low.
Introduction
Climate hazards, particularly heavy precipitation, have increased in frequency and intensity under ongoing climate change and pose significant risks to human societies. In arid and semi-arid regions such as Iran, heavy rainfall frequently triggers flooding and substantial socio-economic damage. These extreme events are largely controlled by large-scale atmospheric circulation anomalies, especially at mid-tropospheric levels. Negative anomalies in geopotential height and temperature at 500 hPa strengthen cyclonic circulation, enhance dynamic ascent, and favour the development of deep troughs or cut-off lows. Identifying the dominant mid-level circulation patterns associated with heavy precipitation is therefore essential for improved understanding, prediction, and water-resource management in Iran. The present study examines the role of 500 hPa geopotential-height and temperature anomalies in the occurrence of heavy precipitation across the country.
Material and Methods
Daily precipitation data from 100 synoptic stations for the period 1990–2022 were obtained from the Iran Meteorological Organization. Geopotential height and temperature fields at 500 hPa were extracted from the ECMWF archive. Heavy-precipitation days were identified, and the corresponding mid-level anomaly fields were calculated relative to the long-term climatology. Principal component analysis (PCA) followed by cluster analysis was applied to the anomaly fields to extract the dominant circulation patterns. The contribution of each pattern to the total variance and its temporal frequency were quantified. Selected representative cases were subjected to detailed synoptic analysis. Statistical relationships between the magnitude of the geopotential-height anomaly and precipitation intensity were also examined. Data processing was performed in MATLAB and Excel; maps were produced with GrADS and Surfer.
Results and Discussion        
Spatially, the highest frequencies and intensities of heavy precipitation (>100 mm day⁻¹) occur over the western Caspian Sea basin, the central and northern Zagros, and parts of north-eastern Iran. Temporally, events are concentrated in the cold season, with a clear maximum in March and a minimum in October.
Cluster analysis of the 500 hPa anomaly fields identified five distinct synoptic patterns that together account for the majority of heavy-precipitation events:

Pattern 1 is characterised by pronounced meridional flow and the deepening of a mid-level trough accompanied by a cut-off low west of Iran. It develops under a blocking high over the northern Caucasus–Russia region. The resulting negative geopotential-height and temperature anomalies intensify the thermal gradient and promote strong ascent, producing heavy rainfall mainly over western and south-western Iran. This pattern is essentially confined to winter.
Pattern 2 (116 events, ≈25 % of cases) features a strong European blocking ridge that generates a cut-off low (inverted-omega type) over the eastern Mediterranean. The associated intense negative height and temperature anomalies enhance dynamic lifting and moisture transport from southern water bodies. Precipitation is widespread, frequently exceeding 30–50 mm over eastern, southern and north-western Iran. The pattern peaks in December–March and occasionally appears in April.
Pattern 3 (60 events, ≈16 %) consists of a coupled European–Mediterranean trough system. Deepening of the Atlantic–European low and simultaneous amplification of the Mediterranean trough place most of Iran beneath the trough axis. Broad negative height and temperature anomalies favour extensive moderate-to-heavy rainfall, especially over western Iran. Frequency is highest in April, May and November.
Pattern 4 is linked to an omega-type blocking over the North Atlantic–Europe. Meridional amplification on the eastern flank of the block extends a deep trough southward to approximately 25° N. Strong cold-air advection over the eastern Mediterranean produces marked negative temperature anomalies and frontogenesis, resulting in widespread heavy precipitation (often >15 mm, locally up to 90 mm) across more than half of the country. The pattern is most frequent in February and April.
Pattern 5 (157 events, ≈34 % of total variance) is the dominant mode. It is a hybrid blocking configuration that isolates a cut-off low (cold pool) over the eastern Mediterranean. Geopotential-height anomalies reach −180 to −250 gpm and temperature anomalies reach −6 to −12 °C at the centre of the low. Intense cyclonic ascent draws moist air from the Arabian Sea, Oman Sea, Persian Gulf and Red Sea, generating heavy precipitation along the trough axis across large parts of Iran. This pattern occurs throughout the cold season, with a maximum in March, and is observed in a greater number of months than the other patterns.

In all five patterns, heavy precipitation is accompanied by a pronounced negative anomaly of both geopotential height and temperature at 500 hPa over the eastern Mediterranean and western Iran. The common dynamical ingredient is enhanced mid-level ascent associated with the trough or cut-off low.
Conclusion
Heavy precipitation in Iran is primarily driven by large-scale mid-tropospheric circulation anomalies that appear as deep troughs or cut-off lows over the eastern Mediterranean–western Iran sector. The five identified patterns share a common signature of strong negative geopotential-height and temperature anomalies that promote dynamical ascent and moisture convergence. The most frequent and impactful configuration is the mid-level cut-off low (Pattern 5), which alone accounts for approximately one-third of all heavy-precipitation events. Recognition of these recurring anomaly patterns provides a useful synoptic framework for forecasting and risk management of extreme rainfall in Iran.