عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The western area of Iran is subjected often to dust, which reduces horizontal visibility to 5 km, and sometimes even to less than 1 km. In this study, dust datasets are developed based on the visibility less than 5 km. for each 3-hour spell through 22 years (1992-2013). Dust data comprised monthly frequency for the period, in 6 stations across the west and south west of Iran. It consists of monthly dust frequency (06-code of synoptic reports) in the 22-year period from the meteorological synoptic stations. Then monthly and seasonal pressure patterns and their anomalies were studied for the months that had minimum or maximum of dust frequency during 22 years (1992-2013). Thus, in this case study the annual and interannual occurrences of dust events based on 22 years of 3-hours visibility. An analysis of large-scale circulations over the lower and middle of troposphere, for the two months (July of 2004 and 2009), was done using NCEP-NCAR reanalysis data to link their roles in and contributions to the raising frequency of dust across the western Iran. For this rezone we investigated the main factors to describe the relationship between effective Large Scale Atmospheric Circulation (LSAC) and dust frequency over western Iran, using the available meteorological data from synoptic meteorological stations and the data set includes monthly sea-level pressure, and 850, 700 and 500mb geopotential heights, wind, Temperature fields and their anomalies relative to long term mean (1981-2010). Finally the relationships of dust frequency with precipitation and North Atlantic Oscillation (NAO) have been investigated.
Surveys showed that in recent years the event frequency, intensity and the extent of the region affected by dust have been increasing. The monthly data indicate that the dust season starts at February and ends in September, with a maximum in June and July. More than 70% of the total annual dust was recorded between March and July, the 'high dust season'. The monthly and annual total dust frequencies vary considerably from year to year. In the period study, the maximum and minimum dust frequencies have been occurred in 2009 and 2004 years respectively, where the ratio of dust frequency in 2009 to 2004 was more than 31 times.
During in the years that precipitation decreases, the occurrence of dust event increases; thus, there was a meaningful relation between precipitation and the decreasing of the frequency of dust event. A highly significant negative correlation (more than -0.50) was found between dust event and precipitation in the west and south west of Iran (Table 3). Also, a negative correlation was found between dust event and the intensity of the NAO in the some stations (Table 3), which modulates the cyclonic activity over Europe and the northern Mediterranean.
Maximum numbers of dust events usually occurred in the summer. Because of strengthening the subtropical high pressure from north east Africa to Iran, the waves of Mediterranean have been delivered to northern latitudes, and there will be no possibility of moisture feeding and no precipitation. While low pressure system moves to the east, the conditions for high pressure system's deployment located in the west of upper trough, by convergence of upper flows, leads to its creation and strengthen of high pressure. So, intensive pressure gradient has created between low and high pressure systems which leads to creation of strong winds and causes dust storm over Iraq and east of Syrian. However, geographic condition might have an effective role in creating strong winds. Some potential predictors are found through detecting the atmospheric circulation factors in qualitative analyses. These are as following order:
1- In poor dust (rich dust) periods, ridge (trough) and positive (negative) geopotential height anomalies over east of Mediterranean Sea, were relatively strong compared to those in normal years, while over the west of Mediterranean Sea was standing or developing Vice versa conditions.
2- Another reason for rich dust periods in the west of Iran was developing of heat low pressure in the central of Iran and this region that is usually at the same time along with developing low pressure on north of Iraq. In poor dust periods, the situation was reversed, especially, strengthening high pressure system over the Caspian Sea and developing ridge of high pressure in the central of Iran.
3- Variation of precipitation in the region: The scavenging of summer dust particles by winter precipitation is a major process of dust removal on the region. The results showed the negative correlations between the winter precipitations and summer dust levels for the all stations. A high correlation was found between them with an R2 value of -0.71. Therefore, variation of winter precipitation in the region could affect summer dust levels.
4- A negative correlation was found between dust event and the intensity of NAO in the some stations, which modulates the cyclonic activity over Europe and the northern Mediterranean.