عنوان مقاله [English]
1-Introduction One of the factors that affect the climate of arid and semi-arid areas is dust storm. Numerical models are new methods for evaluation of dust storms which can also be used for forecasting dust storms. The goal of this research was to utilize air mass back trajectory and mesoscale meteorological model analyses to investigate the wind patterns in Iran leading to synoptic-scale dust outbreaks impacting the west and center of this region. Back trajectories from a receptor site should assist in determining the air masses and dust transport patterns into the area of study to help better understand and forecast the formation and direction of motion of dust plumes. Synoptic-scale desert dust outbreaks are initiated by speciïc weather conditions that depend on the nature and strength of large-scale wind systems over the region. Weather patterns that lead to dust outbreaks can be simulated using computer models that support a wide range of simulations related to the long-range transport, dispersion, and deposition of aerosols. Also, identify the source of infection is the first step in the process of determining an effective strategy for controlling pollution. One way to find the sources of pollution is back trajectory this means that the back trajectories from the receiver site can be used to specify the source locations. A coupling of meteorological and trajectory models is common method in studies of dust storms. In this study, we evaluated one off intense dust event that affected most of Iran. We used two numerical models, Weather Research and Forecasting (WRF) model and HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. WRF model analyses were used to investigate meteorological conditions in the center of Iran and HYSPLIT back-trajectory analyses were used to investigate the wind patterns that led to dust outbreaks by evaluating air transport pathways reaching Iran during dusty days. 2- Methodology In this study numerical models are used to investigate and forecast of dust storms. The Advanced Research version of WRF (ARW) was used to produce atmospheric fields at a high resolution over the study region. WRF model analyses were used to investigate meteorological conditions in region. WRF is a limited-area, non-hydrostatic, terrain-following sigma-coordinate model designed to simulate or predict mesoscale atmospheric circulation. HYSPLIT model was used to compute simple air parcel trajectories as well as dispersion and deposition simulations. 1 nested domains of 30 km horizontal resolutions and 28 vertical levels was defined in WRF model. The model was integrated continuously for 192 h starting from 00UTC of 30 Jun 2009. Initial and boundary conditions were adopted from National Centers for Environmental Prediction Final Analyses (NCEP FNL) data available at 1° horizontal resolution. Boundary conditions were updated at 6-h intervals during the period of model integration. Then WRF outputs converted to format of input data for HYSPLIT model and this model was run to investigate the source of dust storm using calculation of back trajectories from receptor site. The back trajectories provide the Lagrangian path of the air parcels in the chosen time scale, which will be useful to identify the source locations of the pollutant that fall in the track of the back trajectories. Also for reduce uncertainty of back trajectories, trajectories were calculated at different altitudes (100 m, 500 m and 1000 m). 3– Discussion On the sixth day of July, 2009, when the storms come to Tehran, horizontal visibility is less than 100 meters. Many cities and human activities were suspended. Shamal Wind is the prominent stream. From late May to early July, One alternate weather phenomena in the central region of the Middle East, is a summer wind that is of great significance in enhancing surface dust flux. The storm occurred on the sixth of July 2009, the present of a low-thermal pressure on Iran creates a strong pressure gradient in a semi-permanent high-pressure zone on Saudi Arabia. Synoptic conditions simulated with WRF showed that the formation mechanism of a summer shamal wind on the Tigris and Euphrates basin set on the Mediterranean and a frontal cyclone with remarkable high level forcing is made. With strengthen the System, A heavy postfrontal dust storm has formed that by passing Zagros mountain transport this dust to wide range. Convergence of location paths using the HYSPLT model, seen in the West Country and the Iran-Iraq border that indicate the possible locations of springs in the area behind this point. Behind the convergence point is located parts of Iraq and Syria, where is included arid and dry land and could be source of this dust storm. 4– Conclusion In this study, we try to test use of numerical models to investigate the conditions governing the formation of dust storms and also measure the performance of numerical models in forecasting of dust storms. For this purpose, one of the severe dust storms that affected most of Iran was selected and WRF model was used to simulate the atmospheric conditions governing it. WRF model output results forecasted Shamal wind that was main factor governing the formation of the storm. Then, back and forward trajectories of particles scattered on windy days (18-12 July 2009) by the HYSPLIT model indicate that wind in these days came from northern Syria and Iraq. Using the results obtained in this study it can be said that the coupling atmospheric forecasting model (WRF) of diffusion model (HYSPLIT) can be well evaluated Weather conditions and trajectories and forecast dust storms.