Journal of Spatial Analysis

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Dust particles consist of important aerosols and resulting in blowing strong winds on the surface of desert areas. These particles enter the atmosphere under the influence of different factors including: weather condition (wind, precipitation and temperature), land surface (topography, humidity level, roughness and vegetation), soil features (texture, density, composition and land use (agriculture).

Today powerful dust storm destroys people lives and causes severe damages to their life and also causes financial problems in most regions of the world especially in west and southwest of Asia. Dust storm is one of the most important natural phenomena and also a kind of severe natural disaster that influence Iran and its west and southwest part. The location of Iran on the desert belt is accompanied by frequent increasing of sand and dust storm. Integral prediction of dust storm phenomena can be useful in decreasing damages caused by these storms. So synoptic-dynamic analysis of dust storms and their simulation play an important role in achieving to this goal.

In this research, we investigate severe dust storm in August 2005 that affected a large area of our country. Select of dusty days were based on minimum visibility and maximum durability of that dust storm. At first, we show the minimum of daily visibility table. These data has been provided by Meteorological Organization in 5 western cities. The synoptic maps were related to these phenomena derived from NOAA website and synoptic and dynamic interpretation has been done. We have got the data with resolution of 2.5 degree from NOAA website.

Then 700 hpa relative vorticity maps were drawn. We investigate MODIS images instrument on Aqua satellite and evaluate the amount of mass concentration of dust particles. Then the Lagrangian Integrated Trajectory Model has been used to determine the backward trajectory of dust particles. We run HYSPLIT model by GDAS data with a resolution of 0.5 degrees. At last we investigate the output of the WRE-CHEM model. This model was run to simulate dust storms in 7-10 August and FNL data with a resolution 1 degree use for initial and boundary conditions. WRF-CHEM is used to simulate dust condition and transmission. As a part of WRF model, its main application is the study of atmosphere chemistry.

At 500 hpa a very strong ridge entered Iran from the southwest. It covered all areas of our country which prevents the transference of dust to high levels of atmosphere. In 700 hpa relative vorticity maps show one day before dust storm reach to Iran a Positive voriticity is located in Iraq and Syria. So dust comes up to higher levels of the atmosphere and in dusty days in our country. There is a negative voriticity located in our country and because of downside movement of the air, dust storm happen in Iran.

Dust loading and friction velocity of outputs of the model has been drawn in dusty days. The time series of dust concentration of output models for Tabriz are compared with the concentration data of Environmental Organization of visibility data. Result show that a low pressure system is located over the Oman sea that its blaze has been extended to the northwest of Iran. On the other hand a high pressure center is located in the Europe that extended to the east of Mediterranean. So strong pressure gradient were in Iraq and Syria and they caused the creation of strong winds in their deserts which caused dust emission.

Friction velocity related to the model outputs show that the velocity of wind is high in dusty days in Iraq and Syria. So conditions are suitable for dust raining. Satellite images showed that WRF/CHEM model is simulated very well in emission, source, diffusion and the extent of the areas covered with dust. Comparing MP10 concentration of the model output with and Environment Organization data of Tabriz city show that WRF/CHEM model forecast daily changes well. But model underestimate significantly in quantity of concentration. This error may be due to a model considering only dust quantity but other pollutants affected on visibility. In general it can be said that in this event, dust concentration has been underestimated by WRF/CHEM model especially in maximum amount of PM10 concentration.

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Dust phenomenon is a natural occurrence that occurs widespread in arid and semi-arid regions of the world, especially in the sub-equatorial latitudes. This phenomenon is among the greatest environmental problems in the world. The release of this destructive climatic phenomenon in a scattered manner in the atmosphere varies in size, time and concentration. Since this phenomenon is influenced by the specific conditions of climate effects, its effects may continue to be as close as 16,000 kilometers from the source and cause abnormal environmental effects on the one hand, and numerous damage to agriculture, industry, transportation and telecommunication systems on the other hand. Dust storms, as an atmospheric destructive phenomenon, have created adverse environmental impacts for the west of Iran and caused many problems for the inhabitants of this region. Therefore, studying this phenomenon is necessary in order to achieve a comprehensive approach to deal with it. The present study was conducted with the aim of identifying the instantaneous atmospheric conditions, conduction and source of the dust storms with a synoptic modeling approach.
In this study, in order to investigate the dust storms structure in the southwest of Iran, the dust storm occurred on May 15, 2015 was selected. The reason for choosing the present day, based on reports from the Observatory and Monitoring Center of Ilam’s Environmental Protection Office, was the most polluted day of 2015, so the amount of aerosol recorded was 1200 µg/m3 in the air of Mehran City. To analyze the storm structure, a combination study was performed using NECP/NCAR reanalyzed digital data and output of dynamic and regional models. The first group consisted of three regional models of NAAPS, DREAM 8b and NMMB/BSC, and the second group included HYSPLIT dynamic model with backward method. NECP / NCAR data are also used in the synoptic analysis of the storm.
The average slope of air pressure in the sea level at the time of the dust storm in the west of Iran has increased and a high pressure difference of 20 hPa is observed between east and west of Iran, which is accompanied by a high pressure difference and severe winds in the southwestern borders of Iran. Also, the surface moisture flux of the soil has fallen sharply for the day of the storm occurrence in the study area. High advection in the Western part of Iran has been accompanied by a change in the density and mass of the air with heat, resulting in very rapid and intense air rotational movements around the Earth's surface; on the other hand, the coincidence of the positive and negative vorticity in a single significant amount in the formation of the lower level jet has caused the emergence of the dust storm to occur in the mentioned day. On the day of the dust storm, the orbital component of the wind speed was Western, and its velocity was more than 5 meters per second on the western borders of the country. The meridian component of the wind speed was also Southern. Therefore, the effect of present pattern on west of Iran during the day of storm dust has played a significant role. The optical depth index and surface dust concentration index in the NAAPS model have shown that dust concentrations ranged from 640 to 1260 µg/m3 to the west. Besides, the amount of sulfate in the region was estimated to be between 1 and 2 µg/m3. Comparison of the output of DREAM Bb and NMMB / BSC models showed an increase in concentration values per Dust surface unit on the day of storm occurrence. Based on the results of two models of DREAM Bb and NMMB / BSC in the case of western dust in Iran, it can be concluded that the effect of local factors and close proximity to the centers of the dust source have a significant role in the occurrence of present phenomena for western Iran. The simulation of the Dust storm direction with the HYSPLIT dynamic model and the backward method has shown two routes of dust entering the west of the country; a) Northwest - Southeast; b) West-East direction. The main origins of the first route, the northwest of Iraq and the east of Syria, and the second route were the center of Iraq.
Keywords: Spring dust storms, Regional modeling, HYSPLIT model, particles optical depth, West Iran
 

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In the study of the trend of dust storm index, the results showed that the study period of 2003-2007 in Semnan province has an increasing trend and has shown significant changes in the 95% confidence range, but the lack of significant changes in the last decade shows the effects of various events. In cross-cutting decisions in the field of dust in the region. The zoning of the DSI index changes in different regions of the province in a 15-year statistical period indicates that from the west to the east of the province due to the increase in the frequency of stormy days with moderate dust (MDS), dust has increased. The correlation between drought and DSI index in Semnan province showed that although DSI index increased during the period under analysis with increasing drought intensity and its correlation with drought during the 15-year period was not significant, but the pattern of DSI index is consistent with It is the pattern of the drought process. According to the results, it can be acknowledged that the dust situation has always been affected by climate, but the relationship between drought and the DSI index has always fluctuated with respect to droughts and wetlands. However, different climatic parameters are different and their impact is different. In addition to human activities, the main role of wind in the amount of dust or the existence of another source of dust should be considered.