The Role of Summer and Winter Shamal Winds in the Occurrence of Dust Storms in Western Iran

ABSTRACT
We analyzed meteorological data from 33 synoptic stations spanning 1987-2022 to investigate dust storms in western Iran. Dusty days were defined as instances where suspended dust coincided with horizontal visibility below 1 km at a minimum of three nearby synoptic stations. We employed ERA-Interim data to identify synoptic patterns associated with dust storms. By analyzing surface pressure and geopotential height maps at lower atmospheric levels, we distinguished the summer and winter Shamal Wind patterns, both of which play a crucial role in dust transport to western Iran. Of the 229 recorded dust events, 70 were linked to summer Shamal Winds and 25 to winter Shamal Winds. Summer Shamal Wind storms predominantly originated from dust sources in northern Iraq, particularly around Lake Tharthar, Nineveh Province, and Kirkuk Province. In contrast, winter Shamal Wind storms showed a reduced contribution from central Iraqi lakes (15%), with dust primarily sourced from eastern and southeastern Iraq, northern Saudi Arabia, Kuwait, and southwestern Iran.
Dust storms are among the most significant climatic phenomena affecting arid and semi-arid regions worldwide. The interannual variability of atmospheric general circulation patterns (Kaskaoutis et al., 2012; Jin et al., 2018) plays a crucial role in determining the frequency and intensity of dust events, making this relationship a subject of interest for researchers (Yu et al., 2015; Alizadeh-Choobari et al., 2016; Mashat et al., 2017; Beyranvand et al., 2019, 2023). One of the primary meteorological drivers of dust activity in the Middle East is the Shamal Wind. Walters (1990) categorizes Shamal Wind events into two types: short-term events lasting 24 to 36 hours with wind speeds of approximately 15.5 m s-1, and long-term events persisting for 3 to 5 days with wind speeds reaching 25.7 m s-1. Winds between 9 and 11.2 m s-1 can lift dust into the atmosphere. The impact of summer Shamal Winds on dust events has been emphasized in prior research (e.g., Yu et al., 2015; Francis et al., 2017; Ranjbar Saadat Abadi et al., 2022). Meanwhile, the winter Shamal Wind, a northwesterly wind lasting between 3 and 7 days from December to early March (Pakhirehzan et al., 2018), is categorized based on duration as one type lasting 1 to 3 days and another lasting 3 to 5 days. During winter Shamal events, strong frontal winds develop due to interactions between the polar frontal jet stream and the subtropical jet stream, with surface wind speeds in the central Persian Gulf reaching 15 to 20 m s-1.
 
Methodology
We analyzed horizontal visibility and present weather from 33 synoptic stations across Iran to identify dust storms in western Iran from 1987-2022. Dust events are where suspended dust coincides with horizontal visibility below 1 km in at least three synoptic stations. To assess the contribution of Shamal Winds, we utilized the hourly European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim) data. The variables analyzed included sea-level pressure, geopotential height, temperature, horizontal (u) and meridional (v) wind components, vertical wind speed, divergence, convergence, and relative humidity across multiple atmospheric levels.
 
Results and discussion
Our analysis indicates that the highest frequency of dust events in Iran occurred between 2008 and 2012. Seasonal distribution analysis revealed that 37.2% of dust events occurred in spring, 36.3% in summer, 18.8% in winter, and 7.7% in autumn. The Shamal Wind is a dominant meteorological pattern responsible for dust storms in Southwest Asia. Over half of the summer Shamal Wind events occurred in 2000, 2005, 2008, 2009, 2010, and 2012. In fewer than 15% of cases, low-level jet streams (winds exceeding 15 m s-1) were observed at 850 hPa. However, in more than 80% of cases, near-surface wind speeds at latitudes 35°N or 31°N exceeded 10 m s-1. Approximately 65% of Shamal Wind cases were associated with frontal systems, low-pressure tongues, and low-pressure centers with pressures below 1000 hPa extending from central to northwestern Iraq. Analysis of geopotential heights at upper levels suggests that the development of subtropical high pressure plays a key role in the occurrence of summer Shamal Winds. Convergence between this ridge and the monsoon trough, particularly at 700 hPa, contributes to summer Shamal wind formation. Of the 20 dust storms linked to winter Shamal Winds, 75% lasted one day, while 25% persisted for two to three days. These storms occurred between October and May. In over 70% of cases, the development of the Siberian High extending into Turkey generated a strong pressure gradient, facilitating Shamal Wind formation. Dust events associated with the winter Shamal Wind were most frequent in 2008, 2011, and 2012.
 
Conclusion
Our analysis of the Pakistan-Afghanistan low-pressure system during dust events caused by summer Shamal Winds revealed that in 35.7% of cases, pressures in both locations were nearly similar, while in 37.5% of cases, pressures were lower in southwestern Afghanistan, and in 26.8% of cases, pressures were lower in Pakistan. Khosravi et al. (2015) found that the low pressure in Pakistan has increased over time, though this trend has weakened between 1980 and 2015. Our findings suggest that during dust events triggered by summer Shamal Winds, the low-pressure system over Afghanistan was slightly stronger and more influential than that over Pakistan. Comparing dust events driven by summer and winter Shamal Winds, we found that tilting vorticity was stronger in winter, and wind speeds during winter Shamal events exceeded those of summer Shamal events. In dust storms caused by the summer Shamal Wind, the primary dust sources were in the northern half of Iraq, particularly around Sarsar Lake, Nineveh Province, and Kirkuk Province. In contrast, dust storms driven by winter Shamal Winds exhibited a reduced role of central Iraqi lakes as dust sources (contributing only 15% of emitted dust), with eastern and southeastern Iraq, northern Saudi Arabia, Kuwait, and southwestern Iran playing a more prominent role.