عنوان مقاله [English]
Dust storms, one type of dust event, are in most cases the result of turbulent winds, including convective haboobs, which raise large quantities of dust from desert surfaces and reduce visibility to less than 1 km. This dust reaches concentrations in excess of 6000 mg m3 in severe events. The major dust source regions are areas of arid and hyper-arid, with mean annual rainfalls of less than 100 mm.
The temporal and spatial investigation of dust storms shows the interference of various factors in their occurrence and expansion. These factors is divided into two groups; environmental and human parameters. The climatic (such as low rainfalls, high evaporation, intense wind, drought and atmospheric general circulation) and geological (for example lithology such as Quarts, Clay, Silicate and feldspar),
factors are natural parameters, and factors such as over grassing and crop production, wastage of vegetation, increasing growth of population, battle and political problems are human parameters.
Much of the current interest in dust storms relates to their possible role in the systems and subsystems of Earth. Dust loadings may affect air temperatures through the absorption and scattering of solar radiation, may affect cloud formation, and convectional activity, influence sulphur and carbon dioxide levels in the atmosphere, and influence geo-chemistry cycle, soil evolution and desertification.
Dust storms are one of the destructive climatic phenomena which are affected by various climatic elements such as pressure, precipitation, wind, temperature and evaporation. These phenomena impose much damages to human health, industrial and agricultural installation, population centers and communication ways. The recognition of source regions, creation and expansion style of dust storms and their relation with atmospheric circulation patterns are fundamental factors in reduction of their damages. In recent decades, trend of dust storms has increased in Iran, specially west and southwest areas. This factor is raised the necessity of dust study in these areas more than before.
The aims of this study are the recognition and assessment of atmospheric circulation patterns transferring dust storms, expansion style and their temporal frequency using cluster analyses of 19-year data (from 1987 to 2005) in Ilam region. Therefore, hypotheses and questions of the study have concentrated on the investigation of transferring systems of dust and the description of their temporal and spatial distribution.
In this study, environmental and circulatory databases were used to be analyzed the synoptic systems of Ilam dust days. At first, the number of dust days was determined using climatic data of Ilam station in a 19-year period (from 1987/01/01 to 2005/12/31). Then, their circulation patterns were identified and plotted. Ilam Dust days are included 259 days that among them year 2000 with 38 days has maximum frequency, and year 1998 with 4 days has minimum frequency. Monthly maximum and minimum frequency of Ilam dust days belong to July (52 days) and December (one day) respectively.
The circulatory database includes sea level pressure, 500-hPa geopotential height, wind direction, omega, temperature, pressure gradient. The geographical territory of database is 0 to 70o latitude and 0 to 80o longitude. Finally, the database was formed as a 6940*1 matrix in MatLab software. Then, a Paygany cluster analysis was performed using the Vard integrated method and the Euclidean distance on matrix to determine circulation patterns.
Finally, for the study area, total and fractionated denderogram was drawn in MatLab software. According to dendrogram fractionation method, data variance, homogeneous groups and compare mean test, dendrogram was cut in 4 and 5 levels. Also, the circulation patterns of extreme dust days (visibility 100 meters and less) was identified and plotted in the study area.
The obtained results of cluster analysis upon pressure data in Ilam represent five circulation patterns for sea level pressure and four circulation patterns for the 500-hPa geopotential height. In 5-level, the dendrogram of SLP patterns is divided into five clusters, in which 3 and 4 patterns belong to cold period, and 1, 2 and 5 patterns belong to warm period (Fig. 4-9). Furthermore, In 4-level the dendrogram of the 500-hPa geopotential height patterns is divided into four clusters, in which 2 and 3 patterns belong to warm period, and 1and 4 patterns belong to cold period (Fig. 10-14).
According to horizontal visibility index of 100 meters and less, two days was realized for extreme dust days in study area. These days are 1999/7/22 (1378/4/31) and 2000/7/17 (1379/ 4/27). Their circulation patterns have been presented in figures (15) to (23).
In recent decades, the global effectiveness of dust storms has become increasingly apparent, so that, major advances have occurred in sources, transport paths and their many impacts both on humans and the environment, their frequency at different time scales. The study of circulation patterns transferring dust in west Iran has abundant importance. For example, Ilam region isn’t a source of dust because of natural characteristics such as mountainous areas, high rainfalls and dense vegetation. But, dust storms of this region are non-local. This region is the transport route of climatic systems into Iran, and accepts the most effectiveness from these systems. According to source regions of Iran dust storms which are hyper-arid lands of Iraq, Saudi Arabia, Syria and North Africa, this study is a connective step between large scale circulation patterns and temporal and spatial activities of dust storms in west and southwest Iran. Therefore, principal hypotheses have concentrated on synoptic circulation patterns in lower and meddle atmosphere creating and transporting dust in Ilam and temporal distribution and their frequency. The most important results are as following:
1- In sea level, Gang’s low pressure in warm season which its major core is located on India, Pakistan and Persian Gulf, in cold season Siberia and Black Sea high pressures and their integrated tongue in north Iran, and Sudan’s low pressure are the most important systems creating and transporting dust in study area.
2- in 500-hPa, Mediterranean high trough which is integrated with Red Sea high trough in cold period and is weakened in warm period, also, Azores sub tropical high altitude which is oscillated between north Iran and south Saudi Arabia in cold and warm period respectively, are the most important systems.
3- In the extreme dust days, the domination of Persian Gulf and Pakistan low pressures in the south and west of Iran, existence of trough on Mediterranean Sea with the formation of anticyclone secondary cell in Iran cause the maximum dust transportation to the study region.
4- Wind vector patterns in sea level show western and northwestern winds which have a high intensity. Also, omega patterns in sea level indicate ascending vertical motions in the West of Iran and East of Iraq, with maximum reduction 0.4 p/s.
It is possible, systems which cause rainfall, can lead to occurrence and transportation of dust storms to the study area due to the lack of moisture. It is probably which increasing trend of dust days in Ilam be the consequence of frequency increase of circulation patterns, drought and desertification. The latest hypotheses have the more probability than previous one.