تحلیل اقلیمی و بررسی طوفان ‏های گرد و غبار در خراسان رضوی

Climate analysis and study of dust storms in Khorasan Razavi

Introduction
Dust and air pollution from dust are of important social and everyday issues in society. According to the Earth Observatory website dust storms are considered natural hazards, which affect ecosystems for short time intervals ranging from a few hours to a few days. Dust outbreaks have a significant impact on climate, human health and ecosystems, and numerous studies have been conducted worldwide with different instrumentation and techniques to investigate of such events. In addition to human health problems, these phenomena impose much damages to 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 this has affected major decision-makings and policies. Dust particles enter the atmosphere under the influence of various factors such as weather conditions, ground surface characteristics such as topography, surface moisture, roughness and rough length and vegetation and soil characteristics (texture, density and composition) and land use (agriculture). Dust particles enter the atmosphere from the soil surface, rocks, volcanic lavas or environmental pollution and can lead to reduced evaporation, lowering the surface temperature and affect the precipitation process. Dust storms are one of the destructive climatic phenomena which are affected by various climatic elements such as pressure, precipitation, wind, temperature and evaporation. Dust storms are more common in arid and semi-arid regions and are far more important. Khorasan Razavi province has an arid and semi-arid climate with several dust storms occurring on its surface every year.
Materials and methods
Statistical study of dust storms and tracking and revealing the path of these storms is important. In this study, statistical analysis of dust storms was performed using data from Khorasan Razavi Synoptic Meteorological Station. According to the World Meteorological Organization (WMO) guidelines, when wind speeds exceed 30 knots per station and horizontal visibility is less than one kilometer due to the dust phenomenon, a dust storm is reported and, the WW = 30-35 codes related to dust or sand storms are introduced, the occurrence of these codes in Khorasan Razavi synoptic stations was investigated. In the next step, the source of the dust storm event was investigated as a case study on 10/17/2017 using the image of MODIS satellite in Mashhad. Monitoring of dust source region, transport pathways and plume characteristics is only possible from satellites because ground-based measurements are very limited in space and time. Therefore, it is important to identify, also for prognostic purposes, the atmospheric circulation patterns facilitating the transport of dust particles from their source regions over distances of thousands of kilometers downwind. Compared to ground-based measurements, satellite observations offer a more efficient way of determining key characteristics of aerosols at temporal and spatial scales that are needed to study and monitor aerosol impacts upon the climate system.
The Modis sensor was used to detect the path of the phenomenon and to examine satellite images. Compared to other sensors, MODIS measures the entire earth's surface in 36 bands, covering from the visible band (0.415 micrometers) to the thermal infrared (14235 micrometers). The Modis sensor is a high radiometric resolution (12-bit) device which is carried by two American satellites, Terra and Aqua. The crossing time of the two Terra and Aqua satellites along the equator is at 10:30 and 13:30 local time. In this study, images of the MODIS visible True color band with a resolution of one kilometer on the date under study in the area were received from NASA. Aerosol Optical Depth (AOD) is one of the most important parameters in the study of dust. The Aerosol Optical Depth actually refers to the distribution of dust aerosols in the atmosphere. This wavelength-dependent quantity is defined as the decrease in light per unit length on a given path Aerosol Optical Depth (AOD) is the degree to which aerosol particles prevent the transmission of light. It is defined as the integrated extinction coefficient over a vertical column of unit cross-section. It is an indirect measurement of the size and number of concentrations of aerosol particles present in a given column of air. The spectral dependency of AOD contains information about the dominance of fine and coarse mode particles, the aerosol source regions, the modeling of aerosol radiative effects, the air quality (through monitoring of particulate matter), and the correction for aerosol effects in satellite remote sensing of the Earth’s surface. Aerosol Optical Depth (AOD) maps were obtained with a spatial resolution of 0.1 by 0.1 degree and were received at three-hour time intervals from the Barcelona Forecast Center for the study. After the dust storm was detected and the source areas were identified, the tracing of the dust particles to Mashhad was determined using the HYSPLIT mode. The HYSPLIT model, which is a dual model, was used to calculate the dust trajectory, dispersion, and simulate it. HYSPLIT is a complete model for computing trajectories, complex dispersion, and deposition simulations using either puff or particle approaches. It is plausible to detect transport pathways through monitoring the dust source region in HYSPLIT. Developed by National Oceanic and Atmospheric Administration (NOAA), it is a Lagrangian model. This model is widely used for air parcel dispersion, transportation, and deposition simulation.
Results and discussion
The results showed that the highest amount of dust storms is associated with Sabzevar with 136 mild storms and 79 severe storms and Sarakhs and Gonabad were in the next ranks. The dust phenomenon was observed case-by-case by detection and tracking that, this phenomenon has been formed in Turkmenistan in the early hours and affected Mashhad by penetrating the eastern borders of the country. According to satellite imagery, the origin of the dust storm in history has been in parts of Turkmenistan northeast of Mashhad. The output of HYSPILT maps shows good overlap with satellite imagery. This path has also been overlapped in AOD and Trajectory_Wind survey. In general, it can be stated that initially the primary dust cores were formed in the Turkmen desert and the density of dust increases as it moves west, and it has penetrated to the west and on the eastern borders of Iran and then to the city of Mashhad.
Conclusion
Dust storms are observed and recorded at meteorological stations as a weather phenomenon that is categorized according to the degree of visibility deterioration. In this study, the days with dust storm (observation code 30 to 35, present weather), the synoptic analysis of wind speed, horizontal visibility and weather conditions have been adopted. The results showed that the highest amount of dust storms is associated with Sabzevar with 79 severe storms and Sarakhs and Gonabad were in the next ranks. Studieswith the various methods have shown that initially the primary dust cores were formed in the Turkmen desert and the density of dust increases as it moves west, and it has penetrated to the west and on the eastern borders of Iran and then to the city of Mashhad.
Keywords: Dust storm, Khorasan Razavi, MODIS, HYSPLIT.