Spatial Analysis of Dust Storms in Iran based on Climatic and Vegetation Characteristics

Dust storms are one of the harmful climatic hazards that occur as a result of extensive ecological imbalances. Geographical location of Iran, i.e., the arid and semi-arid belt of the world, provides proper conditions for the occurrence of dust storm hazards. The present study aims to identify and spatially analyze the sources of Iran dust storms by using the National Centers for Environmental Prediction (NCEP/DOE); European Centre for Medium-Range Weather Forecasts operational (ECMWF) ERA-Interim reanalysis datasets and the records of 52 synoptic stations from 1984 to 2016. Since Dust storms occur in areas with disrupted ecological balance One-dimensional radiative-convective model (RCM) implemented to calculate the heat fluxes and heat balance at the ground. Finally, eight environmental parameters have been selected to identify dust sources in Iran. Combining the Analytic Hierarchy Process (AHP) and Index-Overlay (IO) method, the dust sources were detected and evaluated through the land-use maps and the Normalized Difference Vegetation Index (NDVI). The results indicated that before dust storm, the stored energy of the surface is high due to the increase in surface temperature and dry air. Therefore, the released energy through the surface instability forms severe winds in the stormy areas. After energy discharge, the amount of stored energy at the surface decreases drastically on the dusty day. In recent years, due to higher NDVI variations and exacerbation of soil erosion, higher potential dust sources have been found in the southern, southeastern and southwestern parts of Iran. In addition, considering the fundamental changes in NDVI, around the Lake Urmia in the northwest of Iran and eastern Caspian Sea, these areas also have more potential dust sources.
Extended Abstract
1-Introduction
The dust cycle, including dust emission, transportation, transformation, deposition, and stabilization, depends on the energy balance of the Earth system. As an aerosol, dust significantly affects the energy balance of the Earth system through the absorption and scattering of the radiation in the atmosphere.
Iran is located downwind of the major sources of dust, including the alluvial plain of Tigris- Euphrates and the Zubair Desert in Iraq, the Syrian Desert that occupies much of western Iraq and three remarkable desert areas of An-Nafud, Ad-Dahna and Rub al-Khali in the Arabian Peninsula (Comet 2003). The summer Shamal^[1] winds also influence Iran (Abdi Vishkaee et al. 2012). These conditions have caused the dust transported from such sources to affect the west and southwest of Iran (Zaitchik et al. 2007). Using MODIS Satellite Data and the combination of the dust origin maps with NDVI, topography and geology maps, the east of Syria, Iraq, and Saudi deserts are recognized as the main sources of dust storms flown to Iran. Besides, it is determined that the concentration of dust particles increases and influences the west and southwest of Iran (Karimi et al. 2011; Azizi et al. 2012; Ranjbar and Azizi 2012; Taheri et al. 2015). The deserts in Iran, Afghanistan, Pakistan and India are also among the most significant sources of dust storms in southwest Asia, which contribute to high dust load over the Arabian Sea in winter and spring (Pease et al. 1998). The primary analysis of dust storms in this region shows that the highest frequencies of dust storms occur at the common borders between Iran, Pakistan and Afghanistan (Sistan Basin, the Registan Desert, and northwestern Baluchistan and Makran Desert) (Middleton 1986 and Prospero et al. 2002). Wilkerson (1991) studied the dust sources using NOAA satellite images. He identified 14 points as the main dust sources in the region, including 2 points in the west and southwest of Iran. These areas are characterized by quaternary sediments, alluvial, playa, loess and old dry lakebeds, and streambeds (Gerivani et al. 2011). Most of the studies conducted on Iran’s dust storm sources indicate that the frequency and intensity of dust storms have significantly increased during the past years (Alizadeh et al. 2006).
2-Materials and Methods
52 synoptic stations distributed in Iran as well as daily metrological data records (from 1980 to 2016) were used to identify and analyze dust storms in Iran. The meteorological data include wind speed and direction, air temperature, dew point temperature, relative humidity, precipitation, evaporation, cloudiness, and ground temperature. The information required to identify dust storms was extracted using 10 codes related to dust storms and wind erosion based on the present study. Also, the daily and monthly reanalysis data relative to NCEP/DOE and ERA-Interim were used for the period 1980-2016.
The correlation between horizontal visibility and each variable was calculated using the Pearson correlation coefficient at 0.05 significant level. The RCM was used to calculate the heat fluxes and heat balance at the ground. By combining IO and AHP methods on the maps of each parameter, the first locating map was prepared. The results from locating dust sources were compared with the NDVI maps created based on Modis images.
3-Results and Discussion
Generally, the geographical distribution of dust storms occurrence in Iran indicates that from the north to the south, the number of days of dust has increased. In fact, more than 50 percent of dust storms during the statistical period occurred in the central and southern areas. Among all the parameters, eight parameters with highest contribution factor are selected to determine the dust sources. The selected parameters have a significant relation with the horizontal visibility as indicated by their correlation coefficients (95%). Using AHP method each parameter compared with the others based on the priority of the effect level. By comparing the results of site selection with the map of the frequency of dust storm in the statistical period, the most dispersed sources can be observed in the maximum dust frequency areas. Generally, dust sources are more frequent in the east, southeast, center, south, and southwest of Iran. The identified sources are more frequent in Sistan and Balouchestan, Hormozgan, Semnan, Kerman, Yazd, in central-eastern Iran and the south and west of Khuzestan province in southwestern Iran. Dust sources are less scattered in Markazi, Khorasan, west provinces and north of Golestan province. Potential dust sources have also been detected around Urmia Lake. The comparison of high potential areas with land-use maps indicates that these areas are free of any vegetation or include poor pastures and fallow, which are mostly located in deserts, sand dunes, saline lands, and ephemeral rivers (masils). Bare lands, fallow lands, poor pastures, sand dunes, deserts, ravines, saline lands, plateaus, and plains are more susceptible to form dust sources that are observable in the deserts of central arid areas of Iran. The comparison of the NDVI variations from late May until mid-June during the years 2000 to 2016 shows that the climatic conditions have caused severe changes in the relative values of vegetation in the extensive areas of Iran. 
4-Conclusion
This study analyzed meteorological components for the occurrence of dust storms and the day before it, in 52 synoptic stations during (1980-2016) and calculated the climatic/radiation conditions as well as the heat fluxes for them. The results indicated that before dust storm, the stored energy of the surface is high due to increase in surface temperature and dry air. Therefore, the released energy through the surface instability forms severe winds in the stormy areas. After energy discharge, the amount of stored energy at the surface decreases drastically on the dusty day. The southern, southeastern, eastern and the central regions of Iran have high potential to become fine springs. In recent years, higher potential dust sources have been found in the southern, southeastern and southwestern parts of Iran due to higher NDVI variations and exacerbation of soil erosion. In addition, considering the fundamental changes in NDVI, around the Lake Urmia in the northwest of Iran and eastern Caspian Sea, these areas also have more potential dust sources.