واکاوی نقش چرخندهای حرارتی و دینامیکی در توفان‎های گردوغبار غرب ایران

1. Introduction



The undeniable connection between interannual changes in atmospheric circulation patterns and the dust cycle is clear. One of the most important atmospheric patterns in the dust cycle, especially in the rise and early stages of dust storms, is the activity of cyclones. In the Middle East region and especially in the Mediterranean, studies indicate the effectiveness of climatic variables (such as wind, pressure, temperature, cloudiness, precipitation, thunderstorms, floods, waves, dust storms, avalanches) from Formation and passage of cyclone disturbances. The age of studies related to the discussion of cyclogenesis in the Mediterranean region dates back to the middle of the 20th century the affectability of Syria, Iraq, Iran and other countries in the region from the cyclones of Mediterranean is clear. In cases where there is not enough moisture to cause precipitation, the cyclone mechanism acts as a determining factor in collecting and transporting dust from susceptible areas. Sometimes they create big and historic storms that, depending on their path, can severely affect areas even far away from the dust sources.

2. Research Methodology

In order to identify dust storms, in this study, the data related to horizontal visibility and the code of phenomena in the current weather for the time period of 1987-2022 have been used in 33 synoptic stations in Iran. The above data was taken from Iran Meteorological Organization. An event of dust storm is considered a dusty day if at least once a day the horizontal visibility in a station becomes less than or equal to 1 km due to dust in suspension in the present weather phenomenon (the synoptic code 06 is considered which represents dust in suspension). Then Considering the above conditions in the last three decades (1987-2022), a total of 229 dust storm events (391 dust storm days) were identified and in order to achieve the circulation patterns of the identified days, the sea level pressure map at 00, 06, 12, and 18 hours of global coordination, as well as the geopotential height map at the level of 500 hpa, were drawn and analyzed.

3. Results and discussion



Among all the investigated events, 47 events were caused by dynamic cyclone that occurred in all seasons. 18 occurrences have been the result of the formation of thermal cyclone, in this case, occurrences have been observed in all seasons, although with a very low frequency in the autumn season. Analysis of dynamic cyclone indicated in terms of the time of occurrence in a year, only 7 cases occurred in the hot season, 6 cases in the cold season, and the rest in the spring and autumn seasons. So that about 70% of the events related to dynamic cyclone occurred in the months of March to June, of which half of this percentage is related to the month of April.

Out of the 47 cases of storms caused by dynamic cyclone, about 45% of the storms were one-day events, which covered less than 10 stations in the west and southwest of the Iran. Almost 40% of the incidents have lasted for 2 days, and 4 storms in 2010-2012 have reached a significant extent. In 10% of the cases, the duration of the dust storm was 3 days, and two three-day storm in 2009 and 2012 covered a larger area of the Iran. Storms with a longer time period including 4 and 5 days have been observed only once in the studied period. The most occurrences of dynamic whirlwinds occurred in 1994, 2007, 2008 and 2010.

In the thermal cyclone, the maximum frequency was related to 2008. In nearly 60% of cases, the thermal cyclone has formed in the southeast of Iraq, southwest of Iran, Kuwait, northeast of Arabia and the north of the Persian Gulf. In other cases, a thermal cyclone has appeared on the border between Iraq and Saudi Arabia. Storms caused by thermal cyclones in the region have lower intensity than storms caused by dynamic cyclones. The centers located in the southern half of Iraq, northern Saudi Arabia, Kuwait and southwestern Iran have been more effective in the occurrence of dust in these sources than other parts in the region.



4. Conclusion

The most occurrences of dynamic cyclone were in 1994, 2007, 2008 and 2010. In half of the cases, the cyclone created in the central areas of Iraq, especially the central lakes of Tharthar, Rezazah and Habaniyya, has caused dust storms in the region. In 25% of the storms caused by dynamic cyclone, the cyclone formed in northwestern Iraq-northeastern Syria was the main cause of dust rise. In about 17% of cases, the cyclone formed in the southeast of Iraq-southwest of Iran-Kuwait has caused a dust storm. In the remaining 7%, dynamic cyclone in northern Iraq or northern Arabia caused a dust storm. In the storms caused by the occurrence of dynamic cyclone in the region, the most dust rise was from the western and southwestern centers in the country of Iraq. Around the central lakes and especially Tharthar lake, 40% of the dust storms have had this type of effect. In addition to the importance of the role of the pattern of the upper levels on the strengthening of the surface cyclone, the mechanism of dust rising in the surface swirl facilitates the entry of dust to the upper surfaces and results in more dust transfer through these surfaces.

In the thermal cycle, the maximum frequency was in 2008. In nearly 60% of cases, the thermal cyclone has formed in the southeast of Iraq, southwest of Iran, Kuwait, northeast of Arabia and the north of the Persian Gulf. Storms caused by thermal cyclones in the region have lower intensity compared to storms caused by dynamic cyclones. The sources located in the southern half of Iraq, northern Arabia, Kuwait, and southwestern Iran have been more effective in raising dust in these sources than other regions.