| چکیده انگلیسی مقاله |
Introduction Pressure systems, including low and high pressures, are among the most important factors in shaping the general circulation of the atmosphere on Earth. These systems influence seasonal air circulation and wind patterns, and depending on the conditions of the water and land areas over which they blow, they determine the type of movable particles, especially floating particles (in this research, dust). Study area The land of research for this study with an environmental approach, encompasses the country of Iran in the southwest of Asia, covering an area of 1.65 million square kilometers. With a synoptic approach, it includes a region extending from the equator to 70 degrees north latitude and from 20 degrees west to 100 degrees east longitude. Materials and Methods In the current research, the hourly codes "06," representing the general concept of "visibility in meters," were collected from the dust code set for 38 cities in Iran over half a century (1970 to 2020) and related to the local time of 15:00 from the National Meteorological Organization. In this study, a dusty day was defined as a day with at least one hourly dust report. The selection of meteorological stations was conducted in a way that covers all over Iran. By arranging visibility values in daily tables and obtaining the frequency of stations with dust (abbreviated as "EDR"), 612 charts of the fluctuations in EDR frequency (one chart for each month) were drawn, and by applying two identifiers of "dust events based on visibility distance for at least during 3 consecutive days" and "dust reports from at least two adjacent stations," a total of 561 dust waves were identified. Therefore, each dust wave has three distinct elements, which include an increasing amplitude, a peak day, and then a decreasing amplitude of dust frequency. The aim of the continuous selection method of days for retrieving dust waves was to test the hypothesis of the impact of pressure systems on the arrangement and duration of the elements mentioned above in the form of positioning patterns of these systems. In the synoptic section, the loading of daily air-maps at the level of 1000 hPa from NCEP/ENCAR and the design of positioning patterns of pressure centers, both low and high pressure, during peak dust days was conducted in two time frames: the first time "distinguished by warm and cold periods of the year" and the second time "distinguished by two 25-year periods." For this purpose, the months of "June, July, August, and September" were considered representative of the warm period of the year, while the months of "December, January, February, and March" were considered representative of the cold period of the year. Results and Discussion The initial results showed that over the past half-century, among the 38 selected cities in Iran, three cities -Bandar Abbas, Bandar Genaveh, and Dezful—have been distinctly the dustiest in Iran. It was also determined that out of 561 dust wave events recorded in Iran, 189 occurred during the warm and 207 during the cold season. The trend in the frequency of dust waves indicates an increase in this dust hazard over the past half-century, especially during the warm season. In the synoptic section, the results indicated the northward advance of subtropical high-pressure centers over the past 25 years, particularly during the cold season. As a result, the desert belt in this part of the Earth has transformed into a buffer zone between generally low-pressure systems in the south and generally high-pressure systems in the north. This condition is also observed, albeit with less intensity, during the warm season. In this study, this supercontinent devoid of pressure systems during peak storm days of the cold season, extending from the deserts of Africa to the deserts of Asia, was referred to as "Afria" to indicate its orbital direction over the desert lands of these two continents, including the Sahara in Africa and the deserts of Hijaz, Iran, Turan, Tarim and Gobi in Asia. The deserts of this zone are the source of air currents or well-organized winds from the Hadley cell. Conclusion Comparison of the northerly positioning of subtropical high-pressure foci suggested the expansion of the Hadley cell in three zones of "African Desert", "Southern Mediterranean Waters" and "Hejaz". If it is determined in the future and with sufficient data that this expansion has also included the land of Iran, we should be more concerned than ever about the arrival of dust from the dusty deserts of Turkmenistan (Central Asia) to Sistan and Baloochestan in the east and southeast of Iran, and as a result, days darker than dust. |