Temporal-spatial distribution and synoptic patterns of storms with horizontal visibility less than 200 meters in western Iran

Introduction:

Dust storms with visibility of less than 200 meters are dangerous storms for human life and activity. By knowing the behavioral characteristics and how these storms form and spread over the sources of dust production, it is possible to reduce its many damages or adapt to it by creating the necessary infrastructure.

According to the purpose of the research, to investigate the historical trend of the occurrence of severe storms and the synoptic patterns that cause such storms, a statistical period of 33 years (corresponding to the last three solar cycles) was selected. Based on this statistical basis, 16 synoptic stations were selected as sample stations. All codes with visibility less than 200 meters were extracted for all selected stations. The days in which code was reported in at least two observatories with visibility less than 200 meters were selected as a day with a storm. To select synoptic systems, the days when this phenomenon was reported in at least one third of the sample stations were selected as a dust system. In this way, 68 study samples were extracted. These samples were tested using the method of factor analysis and visual inspection to select the dominant patterns.

The highest number of dust storms with visibility less than 200 meters occurs in two turbulent seasons in terms of weather, i. e. winter (593 cases) and autumn (426 cases). In monthly terms, the highest number of dust storms of this type occurred in January (352 cases) and December (236 cases). In terms of historical trends, from the beginning of the period (1987) to 2007, the number of storms has been almost constant. Since 2008, the number of such storms has increased significantly and this process continued until 2012. Since 2012, the number of storms has decreased slightly, but it has not returned to the conditions before 2007. In terms of spatial distribution, the highest number of storms occurred in the cold period of the year, in the eastern stations of the region, on the contrary, in the warm period of the year, the highest number of storms occurred in the western part of the study area.

Two recurring patterns are the main cause of dust storms in the cold period of the year. In the African gyre pattern, the significant northward expansion of this anticyclone ridge and the cold advection of the subpolar latitudes over the western region of Asia creates a severe temperature and pressure gradient in the entire lower and middle layer of the troposphere. Such storms generally start after the end of rain in the west of Iran. In the Sudan low pattern, with the significant expansion of the Sudan low pressure tongue over the western region of Asia and the formation of a deep trough in the middle layers, it causes the instabilities to intensify over the dust sources of Syria and Iraq. Most summer dust storms have followed a general pattern. In these patterns, three low pressure systems, Saudi Arabia, Pakistan and sometimes Lut desert, are the main cause of dust storms in the lower layer (up to the level of 850hpa). The strong winds resulting from this surface instability are the main cause of dust rising from dust sources in the region. Meanwhile, in the middle layers, calm atmosphere generally dominates the western region of Iran.

The result of this research showed that storms with visibility of less than 200 meters are increasing in the western region of Iran. In terms of spatial distribution, the highest number of such storms occurs in the cold period of the year in the eastern part of the region. From a synoptic point of view, dust particles rise from dust sources in Syria and western Iraq and enter the eastern part of the study area along the atmospheric currents of the middle layer of the troposphere. That is, during the cold period of the year, highland areas are affected by severe dust storms. Meanwhile, the western part of the study area is affected by the highest number of dust storms in the hot period of the year. Dust storms this time of the year are not very deep.