The analysis of Dust Storm Index (DSI), Frequency of dust events and horizontal visibility in the West Asian

The phenomenon of dust as one of the atmospheric hazards in many countries in arid and semi-arid regions of the world can have devastating effects on various aspects of human life, including health, ecology, transportation, industry, energy, etc. Iran is located in a part of the dust belt that stretches from the Sahara Desert to West and Central Asia, and every year dust storms occur in different parts of the country and cause a lot of damage. Recent droughts have also intensified internal dust sources in the Middle East and Iran. In this study, in order to investigate the temporal and spatial pattern of dust phenomenon in the Middle East, Dust Storm Index (DSI) and horizontal visibility in West Asia have been analyzed. In order to calculate the dust storm index and to study the horizontal visibility, the observed data of synoptic stations in the study area in the period 2009 to 2018 were used, which include Iran, Iraq, Syria, Turkey, Tajikistan, Uzbekistan, Turkmenistan, Armenia, Azerbaijan, Georgia, parts of Russia, Kazakhstan, Afghanistan, Pakistan, Yemen, Oman, UAE, Qatar, Bahrain, Saudi Arabia and Jordan. The monthly and annual DSI averages showed that this index is suitable for monitoring wind erosion on a large scale using meteorological records that calculate the frequency and intensity of dust storms in the region. This index is a combination of three indices of stormy days with local dust, moderate dust storm and severe dust storm, which are calculated based on the number of times the dust phenomenon codes reported in synoptic stations. The results showed that the changes of DSI in the whole study area have increased from 2009 to 2015; but from 2015 to 2018, this trend has been decreased. The DSI index provides a comprehensive index of the trend of change in the region, taking into account both local and non-local dust. Studies show that the DSI index in most countries in the study area is highly dependent on local dust storms and moderate or non-local dust storms have less impact on these countries. The study of the frequency of dust events shows that during this study period, based on the report of the frequency of the dust phenomenon of non-local origin and visibility of less than 5000 meters in spring, it was determined that the western and southwestern regions of the country are more affected by the phenomenon of non-local dust sources. The similar pattern prevails in spring for the number of dusty days with a decrease in visibility in the north, south and southeast, east and northeast and the center of the country. Meanwhile, the number of dusty days in the western and southwestern part of the country is higher in summer than in spring. In autumn, the frequency of dusty days with a less than 5,000 meters visibility reduction in all regions of the country has sharply decreased compared to spring and summer. The trend in the frequency of dust-related phenomena in winter follows the autumn regime and approximately with the same intensity in the areas of east and northeast, south and southeast, northwest and center. The study of the average monthly, seasonal and annual horizontal visibility showed that in the central and eastern regions of Saudi Arabia and southwestern Iran, more than 60 days with dust has led to a decrease in visibility to less than 800 meters. In addition, in the Makran coast in southern Pakistan and in the Sistan region, the highest number of dusty days with a decrease in visibility between 800 and 1500 meters has been reported. It was also observed that in summer in the Sistan region more than 100 days of dust with a decrease in visibility of less than 800 meters has been recorded. Also, in this season, in addition to the Sistan region, dust events on the coasts of Makran, southwestern Iran and eastern Saudi Arabia have reduced the horizontal visibility between 800 to 1500 meters. In general, the largest decrease in visibility in May is in the eastern part of Saudi Arabia near the shores of the Persian Gulf, in the southern part of Kuwait, as well as in the northern part of Saudi Arabia on the Iraqi border and in the central part of Iraq. In June and July, in the eastern parts of Iraq, western and southwestern Iran, as well as in part of northern Pakistan on the Afghanistan border, between 10 and 20 days of dust in the horizontal range of less than 800 meters were reported. Moreover, the annual study of the decrease in horizontal visibility due to dust events showed that the largest decrease in visibility was recorded in 2010, so that in the northern parts of Saudi Arabia on the Iraqi border and small parts of central and eastern Iraq on the border between Iran and southwestern Iran between 10-20 days and in the southwest of Pakistan on the coast of Makran between 50-60 days of dust events in the horizontal visibility range of 800 to 1500 meters has been reported.