نسیم حسین حمزه

Today, dust storms affect the lives of many people around the world and cause a lot of financial and human losses. Every year, Iran is affected by dust storms caused by internal sources and external dust sources located in neighboring countries. Semnan province has several dust-producing sources due to its geographical location, that affect not only this province, but also the neighboring provinces. The purpose of this study is to investigate the dust phenomenon of Semnan province and its monthly changes in the 15-year period, between 2003 and 2017. For this purpose, meteorological stations data of the province and the aerosol optical depth of satellite product have been investigated. Also, GOCART model has been used to estimate the amount of dust emissions from this region and to determine the vertical dust flux in Semnan province. The study of average dust flux showed that the highest values ​​of dust flux was in the southeast of Shahroud and then in the south of Shahroud, Damghan and Sorkheh. A monthly investigation showed that in all months, the average AOD in the southern half of Semnan province was higher than in the northern half. The highest levels of AOD were in May, April and June, and the highest amounts of dust flux were observed in March, June and July. Also, the prevailing wind direction was northeast and northeast, and this factor causes dust particles rising from this region and dust loading in the south of Semnan province and areas of Isfahan, Yazd and South Khorasan provinces.

The Great Khorasan in northeast Iran has a variety of surface structures and plains and high peaks, but due to its vicinity to the deserts of Afghanistan and Turkmenistan, it is affected by dust all the time, especially in summer. The purpose of this study was to simulate summer dust in this region by RegCM model. For this purpose, during the period 2000 to 2017, three extreme dust events were selected. The satellite image used to confirmed dust mass presence and then the synoptic structure was analyzed. Finally, the simulation results of RegCM 4.6 model were compared with the observational data including the horizontal visibility and aerosol optical depth (AOD) of Aqua satellite. The synoptic analysis showed that during the summer, low thermal pressures form in the southern Afghanistan and high pressure in the north. This structure lead to the development of north and northeast winds with speeds of 12 to 21 m / s and dust emission on the eastern border of Iran and western Afghanistan. Investigation of RegCM accuracy done by visibility, Aquas’s AOD showed that model performance in South Khorasan is better as Razavi Khorasan. The highest correlation coefficients of AOD of model and horizontal visibility were obtained at Khorasan central stations including Gonabad, Ferdows, Nahaband and Ghaen at -0.82, -0.77 and -0.44 respectively. RegCM model performed a better dust simulation in severe dust with a horizontal visibility down to less than 1000 m, high continuity and horizontal extension. Overall, the RegCM model underestimates the AOD value for the Aqua satellite algorithm.

On February 1-6, 2014, a large snowstorm occurred in Iran, especially in the northern regions of the country. Snowfall in northern Iran was unprecedented in the last 30 years and caused extensive damage. Synoptic study of snowstorm phenomenon showed that there is a surface high pressure system in the north and a dynamic low pressure system in the east and southeast of Iran in the sea level pressure map and a deep through at the level of 500 hPa. The 925hpa wind map shows that the north and northwest winds throughout Iran cause cold air to enter from higher elevations. At 300hpa, the core of a western jet is located in central Iran and there is a northern jet north of Iran's borders. Maps of sea surface pressure anomalies show an unprecedented increase in pressure in northern Iran, as well as surface temperature anomalies show a sharp decrease in temperature throughout Iran, especially in the northeast. Examination of cross-sectional area of wind, temperature and relative humidity showed that in addition to large-scale and thermodynamic factors, the presence of a cold front in eastern Iran is another reason for rainfall in this region. Comparison of the output of the WRF model with the maps related to the mentioned date shows that the model simulates the synoptic pattern of the region in this sample well, although the intensity of the systems is slightly less than the real value. Examining the precipitation output of the model, it can be concluded that the model has calculated the height of snow and the extent of the areas covered by it much less than the actual values, but the main error is in determining the type of precipitation.

In this study, First, the snowstorm from 1 February 1 to 6 February, 2014 is evaluated synoptically and thermodynamically by using ERA5 data with an accuracy of 0.5 degrees. The European Climate Forecast Center (ECMWF) presents its forecasts at 37 pressure levels from surface (1000 hPa) to 1 hPa. ERA-Interm re-analysis data was replaced by ERA4 data, which provides a new, higher-quality atmospheric quantity analysis (Di et al., 2011; Francis et al., 2019). In this study, the mean sea level pressure, geopotential height at the level of 500 hPa, 850 hPa and 925 hPa along with zonal and meridional winds at several levels with a spatial resolution of 0.5 degrees were used.Also, snowfall data, maximum and minimum temperatures of several synoptic stations located in northern Iran were obtained from their SYNOP reports, which well indicate the severity of the storm and its extent. Then, in order to simulate this event, the WRF model with a horizontal accuracy of 30 km and 30 vertical levels was implemented from 00 UTC on February 1 to 00 UTC on February 6, 2014. For the initial and boundary conditions of the model, GFS analysis data were used with an accuracy of 0.5 degrees.

In February 2014, between 1 and 6 February, heavy snowfall affected most parts of Iran, especially the northern regions of the country. Investigation of the Earth's surface synoptic map showed that a high-pressure system is located in the north and a dynamic low-pressure system is located in the east and southeast of Iran, which over time strengthens the surface high pressure and its tabs extend to central Iran and push back the low pressure. Also, the existence of an occluded front is obvious in the north of Afghanistan, followed by a cold front in eastern Iran. At the level of 500 hPa, there is a deep trough that moved cold air from higher latitudes and high surface pressure that causes this cold air to fall to the ground. On the other hand, if the humidity has risen to the mid-levels of the atmosphere, the necessary moisture will Provide for rainfall. The 925hPa wind map shows the north and northwest winds blew throughout Iran, which cause that cold air entered from higher elevations.The existence of a cold front in the east of Iran was confirmed in the synoptic map of 850hPa level and the intersection of geopotential and isothermal height bands, as well as the cross-sectional area of the meridional wind, temperature and relative humidity components. The change of wind direction from south to north wind, extreme temperature gradient, slope of isothermal lines and extreme relative humidity gradient are all signs of the presence of the front that has been observed in eastern Iran. Therefore, it can be concluded that in addition to large-scale and thermodynamic factors, the presence of a cold front can also be another reason for existence of precipitation.Comparing the output of the WRF model with the maps related to the mentioned date shows that the model has simulated well the synoptic pattern of the region in this case. The output of the model in the ground map has obtained the surface high pressure in the north of the country and low pressure located in the eastern and southern half of the country and has shown the pressure gradient in the center of Iran well. The model also shows the low-altitude and high-altitude geopotential centers of different compression levels similar to the real maps, and the well has a level of 500hpa and even the tilt of its axis to the east. The comparison of the maps shows that the intensity of surface high pressure, high altitude of 850 hPa and 500 hPa is slightly less than the real value. The WRF model obtained the height of snow and the extent of the areas covered by it far less than the values reported from the stations. By examining the amount of rainfall obtained by it, it can be concluded that the main error of the model is in calculating the type of precipitation that maybe is in result of error in calculating the temperature

The dust has affected most parts of Khorasan Razavi in July 1, 2014. The purpose of this study is ‎to simulate this event by RegCM model and validate its results with observation and remote sensing ‎data. For this purpose, horizontal visibility were prepared from the Iran Meteorological ‎Organization and AOD of Aqua and Merra-2 satellites from Giovanni web.‎ The result shows that the development of low heat pressure on Pakistan along with north and ‎northeast winds of 20 m/s in the east of the Razavi province leads to the transfer of dust to the ‎region. The HYSPLIT model and the output of the RegCM model show the dust source in ‎Turkmenistan and western Afghanistan.‎ The optical depth of aerosol obtained from the RegCM model is in good agreement with the ‎visibility in the east and south stations of the Razavi province, So that the correlation coefficient in ‎Gonabad was -0.98, Torbat Jam -0.66 and Sarakhs -0.61. In addition, the correlation coefficient of ‎the RegCM model and the Merra-2 optical thickness of aerosol obtained in Sarakhs and Torbat-e ‎Jam located in in the east of the province, including, were -0.45 and -0.78, respectively.‎ The result of comparing the time series showed the model output in three-hour intervals is in good ‎consistency with the visibility, the Aqua optical depth aerosol, and the Merra-2 optical thickness ‎aerosol in the east and south stations of the Razavi province. However, the RegCM model does not ‎estimate dust and has a time delay of 6 to 9 hours in the central and western stations.‎

Dust storms caused by erodible surface sources, such as deserts, have been one of the major natural disasters in recent years, affecting countries in the Middle East, including Iran. One of the major factors to formation the dust storms is the strong wind flow that can be created in the presence of low pressure systems in the potential. Therefore, the study and identification the atmospheric patterns can be effective in predicting these storms. One way to study dust storms is to use numerical models that can be used even for times when appropriate data is not available and also to predict these types of storms.

After statistical study of the phenomenon of "dust" and the factors affecting it in the Jazmourian basin, a severe and widespread event of "dust" in the Jazmourian basin is investigated. Then, to investigate the prevailing atmospheric currents in the region, the HYSPLIT model is implemented as a matrix and in a leading way. In the implementation of HYSPLIT model, GDAS meteorological data with horizontal separation of 0.5 degrees have been used. Using the output of this model, it is possible to study how dust particles are transferred from this area. Finally, each of the studied dust phenomena is simulated using the WRF-Chem model to determine how the dust is emitted and transmitted in the region.

Leading HYSPLIT model output for November 28, 2016 UTC12 clock which was executed as a matrix with GDAS data with 0.5 degree horizontal separation for 36 hours at a height of 100 meters. The results showed that the particles from this region were affected by northwestern currents and were transferred to Sistan-Baluchestan province and the border between Iran and Pakistan. Aerosols optical depth of the output particles of the WRF-Chem model at UTC12 on November 24, 2016. Values higher than 0.5 indicate a high amount of suspended particles in the atmosphere in the study area. At this time, the highest light depth of particles is in the Strait of Hormuz and Qeshm Island, but the concentration of particles in a large part of Jazmourian region is between 0.4 to 0.6, which can indicate the presence of dust particles in the atmosphere. The results also show that the concentration of "dust" on the Oman Sea and the Persian Gulf is higher than 5000 micrograms per cubic meter. Also, the concentration of "dust" in Jazmorian region is high and is higher than 1500 micrograms per cubic meter. Also in some areas, the concentration of "dust" is more than 5000 micrograms per cubic meter.

In the present study, using the outputs of the WRF_Chem model, the dust phenomenon and its characteristics in the southeastern region of Iran have been identified. According to the results, the WRF_Chem model provides a reasonable estimate of the weather in the study area in terms of scale and time changes. By producing dust particle concentration distribution maps, areas of the simulation basin that have the maximum particle concentration were identified as the main sources of particle emission. In general, the performance of the WRF / Chem numerical model in this study confirms the applicability of this model in modeling and forecasting air quality, especially for air vents produced from natural emission sources such as erodible areas of deserts. The results showed that the studied dust phenomenon (November 24-27, 2016) dust particles from Jazmourian basin, were affected by northwestern currents and were transferred to Sistan and Baluchestan province and the border between Iran and Pakistan.Keywords: simulation, Dust storm, WRF/Chem model, MODIS.

In the present study, using the outputs of the WRF_Chem model, the dust phenomenon and its characteristics in the southeastern region of Iran have been identified. According to the results, the WRF_Chem model provides a reasonable estimate of the weather in the study area in terms of scale and time changes. By producing dust particle concentration distribution maps, areas of the simulation basin that have the maximum particle concentration were identified as the main sources of particle emission. In general, the performance of the WRF / Chem numerical model in this study confirms the applicability of this model in modeling and forecasting air quality, especially for air vents produced from natural emission sources such as erodible areas of deserts. The results showed that the studied dust phenomenon (November 24-27, 2016) dust particles from Jazmourian basin, were affected by northwestern currents and were transferred to Sistan and Baluchestan province and the border between Iran and Pakistan.

Introduction :

Jazmourian basin in southeastern Iran includes parts of Kerman and Sistan, and Baluchestan provinces. This region is one of the sources of dust production in southeastern Iran. The maximum dust emissions in the Sistan region occur in eastern Iran, southwestern Afghanistan, and Pakistan in the summer, and these storms load dust from local scales to regions. Vegetation and its type also play an important role in the severity of dust. There is positive feedback between precipitation and dust emission from the surface; This means that the dust from desert areas reduces rainfall and decreases rainfall, causes the soil to dry out, and further increases dust. In addition to the effect on rainfall, the presence of dust is also effective in its spatial distribution.

 In this study, first, using the optical depth data of the MISR sensors of Terra satellite with a horizontal separation of 0.5 degrees, the monthly and seasonal average optical depth of the aerators in the Jazmourian basin is determined. In order to study the trend of AOD change, the average AOD regions of TODRA satellite MODIS sensor are shown annually in the period 2000 to 2020, and the regression line slope was calculated by age estimation method and Mann-Kendall method with 95% and 99% confidence levels. The amount of vegetation changes in the region has been studied using the annual average of the NDVI index in the Jazmourian region. Since precipitation is one of the factors affecting the occurrence of dust events, the average annual precipitation rates of the TRMM satellite have also been studied. Finally, to investigate the effect of vegetation change on dust, the correlation between the average annual data of AOD and NDVI data and the correlation between the average annual rainfall data and NDVI was calculated to investigate the effect of annual rainfall on vegetation.

Results :

 The monthly average values of the optical depth of air vents were in the period 2000 to 2020. In January, AOD values in the center of the Jazmourian Basin, located on the border of Sistan and Baluchestan and Kerman provinces, are slightly higher than in other parts. From February to July, an increase in AOD is observed in the region so that in large parts of the Jazmourian Basin, the average optical depth of air vents in this month has reached about 0.6. Since August, AOD values have gradually decreased until December; only in the central areas of the basin, small amounts of AOD are observed. As expected, the maximum AOD in this region is observed in summer, and then in spring is in second place. The amount of AOD has decreased significantly in autumn and winter. According to the results, NDVI values were low in years when the mean optical depth of the particles was high. For example, in 2008 and 2012, when AOD values were higher than in other years, the average annual NDVI values decreased. Interestingly, the average annual values of optical particle depth in 2020 and 2017 were lower than in other years, but in the same years, the average annual NDVI values are higher than in neighboring years. The results show that in the years when there is an increase in rainfall in the region, an increase in vegetation and a relative decrease in AOD compared to previous years are also observed. It should be noted that in some years, with increasing rainfall and vegetation, the amount of AOD has not decreased much that it can be said that the amount of AOD, in addition to the activity of local dust sources, is due to dust particles from other areas to the study area and AOD changes only Not due to changes in rainfall and vegetation in the area.

The average monthly light depth values of air vents in the period from 2000 to 2020 show that in January, the AOD values in the center of Jazmourian Basin, located on the border of Sistan and Baluchestan and Kerman provinces, are slightly higher than other sectors. From February to July, an increase in AOD is observed in the region so that in large parts of the Jazmourian Basin, the average optical depth of air vents in this month has reached about 0.6. From August, when small amounts of AOD were observed only in the central areas of the basin, AOD values gradually decreased until December. Also, the average monthly AOD values in the region in the period 2000 to 2020 show that the highest AOD values in July are related to 2003 and 2001, and in June are related to 2008, which is significantly different from other years. Examination of the seasonal average values of the optical depth of air vents in the period under study shows that the maximum amount of AOD in this region is observed in summer and then spring, and AOD values in the autumn and winter seasons have decreased significantly.

Dust storms are one of important natural disasters that affect vast areas in the world and makes damage to people lives and causes many financial problems. Recently, change in precipitation and temperature pattern happened in the Persian Gulf countries due to climate change. Every year, dust storms causes many problems in different regions of Iran especially in Khuzestan province and this phenomenon frequently disrupt social, economic and official mechanism of the province. The areas are affected by Syria and Saudi Arabia dust particles. So analysis and simulation of dust storms will be helpful to investigate the potential sources and transport mechanism of dust particles in this area.In this research, 11 synoptic weather stations in Khuzestan province were investigated in 25years duration. Also results were investigated by AOD and NDVI indexes in all areas of the province. Ahvaz, Bostan, Abadan stations had the most dust storms frequency and Behbahan and Ramhormoz had the least. The average dusty days was 37.3 days per year and 2008,2009 and 2011 years had the most number of dusty days. The maximum amount of AOD and minimum amount of NDVI were in 2008 and 2009 that is in good agreement with number of dusty days in these years.

In this study, the trend of monthly and yearly dusty day frequency investigated in some stations in Khuzestan province in 25years duration (1993 to 2017). Then The Mann-Kendall Test is used for all of them. For this purpose, dust codes (06 and 07) separated in Khuzestan weather stations in 25years duration. Furthermore, reported visibility must be 5000m or less in dusty days. In the next steps, the MODIS Terra AOD was used to investigate Aerosol optical depth in the atmosphere. AOD is available daily and at a spatial resolution of 1 degree. Finally, the normalized difference vegetation index (NDVI) was investigated in the study area. The index investigates vegetation coverage changes in the study area and it used red and near-infrared channels.

In this case study, dust storms were investigated climatically in 25years duration (1993 to 2017). Monthly average number for dusty days showed that the maximum temperature and the minimum precipitation happened in June and July in Khuzestan province. Also the maximum frequency of dusty days happened in that two months and the minimum of dust frequency happened in November.The maximum amount of monthly AOD was in June from 2000 to 2018 in Khuzestan province that it is in good agreement with maximum average number of dusty days. Ahvaz, Bostan, Abadan weather stations had the maximum frequency of dusty days, but Behbahan and ramhormoz stations had the minimum frequency of dusty days.The seasonal average number of dusty days showed that the maximum frequency of dusty days was in spring and summer and the minimum frequency was in autumn. Also the maximum number of dusty days happened in 2008, 2009 and 2011. The maximum amount of AOD was in 2008 and 2009 in 19years duration (2000 to 2018). The mean annual NDVI index was the least amount in 2008 and 2009 that it is in good agreement with AOD and dusty days in the province.Totally, annual trend coefficient of dusty days was 1.39 in Khuzestan province which shows and increasing trend. Investigation of Mann Kendall Test in dusty days of Khuzestan weather stations confirms 1 to 10 percent confidence levels of the observation trend at 7 stations, but it was meaningless in 4 weather stations.  Seasonal analysis showed that the trend of number of dusty days was significant at 5% confidence level in the spring. This trend was not significant in summer but it was significant at 10% confidence level in autumn and 1% in winter.

Climate change affects all regions around the world and increase some extreme events. Also climate change threatens the Middle East so much. Severe droughts in this region with anthropogenic changes cause heavy sand and dust storms. In this study, dust phenomenon has been investigated in The Middle East during 10 years (from 2009 to 2018) from different perspectives. GOCART model dust flux output shows that the maximum amount of dust flux was in the southeast of Saudi Arabia, southeast of Iraq, region on the borders between Iraq and Syria, western half of Turkmenistan, parts of Kazakhstan and Uzbekistan. Also dust flux was seen in the north coast of Persian Gulf, Oman Sea and borders between Iran, Afghanistan and Pakistan. Also AI pattern is very similar to dust flux pattern and Index EVI was low in a vast areas of The Middle East that shows deficiency of vegetation cover in that areas. In the other hand, mean annual cumulative precipitation is less than 250 mm in the vast parts of study areas.

storm is one of the most important natural phenomena and a kind of natural disasters that influence many parts of the world. Dust storms cause many problems in different regions of Iran especially Khuzestan province every year. This province is affected by internal and external dust sources because there are some arid and semi-arid areas inside and outside of Khuzestan province especially in Iraq, Syria and Saudi Arabia countries. So a large number of studies have been done in order to analysis and simulation of dust storms. A severe dust storm has occurred in the west and south west of Iran on 7-10 Bahman 1395. The dust particles were drenched on the power transmission lines in Ahwaz city because of low precipitation on the 8th Bahman. So the electricity was cut off in the city. This event made many problems for residents of Ahwaz city. Synoptic analysis showed that, the existence of a cold front in the southeast of Iraq caused severe winds in the area on Bahman 8th. In Khuzestan province, ahead of the cold front, the south easterly winds triggered the activation of local dust hotspots located in the south of the province. The baroclinicity in the front area was quite apparent at the 850hPa level, and there was a deep height trough at the 500hPa. The output of HYSPLIT model in Khuzestan region near the surface showed the south-eastly wind flow. Also, the change in wind direction in the front area was clearly seen in the model output at 500m level. Present weather codes reported from Ahvaz synoptic station showed local and non-local dust storm and precipitation altogether. The visibility reduced under 100 meters in Ahvaz on 8 Bahman. WRF-Chem model with GOCART emission scheme was implemented for simulation of the dust event. The model outputs showed that the emission of dust particles from Iraq and south of Khuzestan was correctly simulated, and the they were in agreement with the satellite images; although the dust mass was not detected well, due to cloudiness in the region. The PM10 concentration of the model output and the measured values ​​at the air pollution monitoring stations showed that the model has a satisfactory performance in the estimation of the PM10 concentration. The model simulated the dust concentration trend correctly, however some errors in the output were seen. These errors were expected due to the complex topography of Khuzestan province. The model had a better performance in simulation of wind direction and rainfall. The model well simulated existence of the front and the sudden change of wind direction.

Satellite images show that WRF/CHEM model has simulated emission, source, diffusion as well as the extent of the areas covered with dust very well. Comparing PM10 concentration of the model outputs with Environment Organization data, show that WRF/CHEM model forecast daily changes very well. But model underestimate significantly in quantity of concentration. These errors may be are due to a model considering only dust quantity but other pollutants have affects on visibility. EURAD model has wrong prediction in dust storm sources and it simulates it in central of our country. 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 and EURAD model does not simulate dust concentration correctly in amounts and dispersion patterns.

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 parts. The location of Iran on the desert belt is accompanied by frequent increasing of sand and dust storms. 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 plays an important role in achieving to this goal.
In this research, we investigate 4 severe dust storms in August 2005, September 2008, July 2009 and June 2012 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 outputs of the WRE-CHEM model. This model was run to simulate dust storms by 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 storms 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.
The time series of dust concentration of output models for Ahvaz are compared with the concentration data of Environmental Organization and visibility data. Results 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.
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 PM10 concentration of the model outputs with Environment Organization data, show that WRF/CHEM model forecast daily changes well. But model underestimate significantly in quantity of concentration. These errors may be are 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.

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.