بررسی ویژگی‏ های رخداد پدیدة گردوخاک خراسان بزرگ در دورة گرم سال و شبیه‏ سازی مسیر آن توسط مدل HYSPLIT (دورة آماری 20۰۰-20۱۷)

Introduction
Dust and sand storms are meteorological phenomena that have harmful effects on the environment, society, public health and natural resources. Climate systems and land’s structure are the main factors in creating dust storms. In general, variations of physical and dynamical specification of atmospheric systems have very significant role on the production and transport of dust in these areas. Dust storms are most commonly caused by strong pressure gradients, which increase the wind velocity over a wide area. Since most of Iran (including the northeastern and eastern regions) has an arid and semi-arid climate, it is conducive to the occurrence of dust events. In recent years, the intensity and frequency of dust storms in the east and northeast of Iran has increased. Due to the importance of this issue, synoptic analyzes and temporal and spatial distributions of dust events in the northeastern and eastern region of Iran have been performed in separate studies such as Karkon Systani,2012; Omidvar et al.,2016; Boroghni et la., 2016; Doostan., 2017; Poorhashemi et al., 2019. Despite the several studies conducted by various researchers for this region, but a comprehensive study of atmospheric pressure patterns affecting on dust production and transportation, temporal and spatial distributions of dust, the trend of horizontal visibility changes, backward tracking and determining dust sources and determining the prevailing wind in spring and summer for a long period has not been done. Therefore, the main purpose of this article is to investigation these issues for the study area.

Materials and methods
The study area ( Great Khorasan) is located in the east and northeast of Iran. This region includes the provinces of Khorasan Razavi, North Khorasan and South Khorasan, which have 17, 7 and 11 synoptic stations, respectively.
In this study, horizontal visibility, 10-meter wind speed and weather codes related to dust (codes 06, 07 and 30 to 35) were investigated between 2000 to 2017 in 35 meteorological stations of Great Khorasan. This data set were obtained from Iran Meteorological Organization. Due to lack of data, only 5 stations including Sarakhas, Gonabad, Tabas, Birjan and Nehbandan were selected for more investigation.
In addition, the grid point data (Sea level Pressure and 500mb) were extracted from the Era-Interim reanalysis products. The grid data with 0.75°×0.75° resolution selected for the area between 40-70°E and 20-45°N.
Throughout this study, it is attempted to investigation of temporal and spatial distributions and trend of dust in Khorasan, and to determine characteristics of effective pressure patterns and main dust emission's sources by using backward trajectory technique. Accordingly, the study method consists of three main parts:
The first part of this study is statistical investigations and determination of temporal and spatial distributions of dust events using observation data in meteorological stations. The second part is the study and identification of circulations and pressure patterns affecting on production and transport of dust using grid data for selected cases. The third part is to determine the active dust sources for the studied cases using the backward trajectory technique by HYSPLIT model.

Results and discussion
The number of dusty days in Razavi and South Khorasans in 2003, 2008, 2013, 2014 and 2017 was almost the same. This implies the predominance of large-scale circulations and pressure patterns rather than small-scale local factors.
Monthly distributions showed that the highest number of dusty days in South Khorasan was in May, July, and June respectively and in Khorasan Razavi were in June and May. North Khorasan also had the highest number of dusty days in June. Since the South Khorasan stations are located on the western slopes of the north-south Mountains, they had more dust events than Khorasan Razavi province. Gonabad had fewer days of dust events than Sarakhs, Tabas, and Birjand, but the intensity of dust is higher in Gonabad station. The highest number of dust events occurred in 2008 and 2014, indicating that dust was widespread, but the least visibility recorded in 2012, that detected as the severe dusty year.

Conclusion
Most of the dusty days in the study area occurred in June. Strong north and northeast winds in summer have played an important role in the production and transfer of dust in the provinces of Razavi and South Khorasans, and therefore the dustiest days have occurred in this season. While in North Khorasan, the wind pattern was variable and most of the dusty days occurred in spring.
The results of long-term statistical study of changes in horizontal visibility in Razavi and South Khorasans indicated that the trend change patterns were different in these provinces. So that this trend increased from 2005 to 2016 in Khorasan Razavi, while in South Khorasan it decreased. The trend of horizontal visibility changes, in addition to being different seasonally, has not been the same in different stations. For example, the line trend slope of horizontal visibility at Nehbadan station in spring and summer were +0.34 and +0.27 but in Tabas -0.28 and -0.3. Most of the dusty days in the study area occurred in June. Strong north and northeast winds in summer have played an important role in the production and transfer of dust in the provinces of Razavi and South Khorasans, and therefore the dustiest days have occurred in this season. While in North Khorasan, the wind pattern was variable and most of the dusty days occurred in spring.
The results of synoptic analysis of the atmospheric pressure patterns which leading to the occurrence of intense and extensive dust events in spring and summer showed that depending on the pressure gradient created between the two pressure (low pressure and high pressure) systems, threshold wind speed, sources and transport path varied in the study area. In the spring of the development of low-pressure system on the eastern border and the high-pressure system on the western border of Iran leads to the creation of pressure gradients in the west-east direction in the central regions of Iran and as a result, the creation of westerly winds in the study area. The strong westerly winds on dusty sources create the favorable conditions for dust production. In summer, the development of heat low-pressure system and two high-pressure systems, one in northern Afghanistan and the other on the Caspian Sea, leads to north-south pressure gradients and creates northeasterly and northly winds with an average speed of 18 ms-1.
The dusty paths simulated by the HYSPLIT model are corresponded with the prevailing wind direction so that dust sources and transport can be determined. The results showed that in spring the central deserts of Iran were considered as dust sources of the study area and in summer the deserts of Turkmenistan and Afghanistan were considered as dust sources of Great Khorasan.