climatic analysis and routing of comprehensive dust storms in the south and center of iran

introduction dust storms, one type of dust event, are in most cases the result of turbulent winds, including convective haboobs, which raise large quantities of dust from desert surfaces and reduce visibility to less than 1 km. this dust reaches concentrations in excess of 6000 mg m3 in severe events. the major dust source regions are areas of arid and hyper-arid, with mean annual rainfalls of less than 100 mm. the temporal and spatial investigation of dust storms shows the interference of various factors in their occurrence and expansion. these factors is divided into two groups; environmental and human parameters. the climatic (such as low rainfalls, high evaporation, intense wind, drought and atmospheric general circulation) and geological (for example lithology such as quarts, clay, silicate and feldspar) factors are natural parameters, and factors such as over grassing and crop production, wastage of vegetation, increasing growth of population, battle and political problems are human parameters. much of the current interest in dust storms relates to their possible role in the systems and subsystems of earth. dust loadings may affect air temperatures through the absorption and scattering of solar radiation, may affect cloud formation, and convectional activity, influence sulphur and carbon dioxide levels in the atmosphere, and influence geo-chemistry cycle, soil evolution and desertification. dust storms are one of the destructive climatic phenomena which are affected by various climatic elements such as pressure, precipitation, wind, temperature and evaporation. these phenomena impose much damages to human health, 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. in recent decades, trend of dust storms has increased in iran, specially west and southwest areas. this factor is raised the necessity of dust study in these areas more than before. frequency and intensity of dust storms in west and south west parts of iran has made this phenomenon one of the most principal environmental hazards in these regions. according to the extent of dust storms and their vast effects on health, economy, environment and agriculture and their increasing trend has drew the attention of many researchers. the recent study is composed of data analysis of dust storm by iran meteorological organization earth database, synoptic analysis of weather and climate patterns causing and transmitting dust using middle and upper atmosphere data in shiraz, fasa and bushehr stations and tracking wind packages carrying dust particles using hysplit model by forward and backward methods.   methodology in the research, environmental and circulatory databases were used to be analyzed the synoptic systems of comprehensive dust storms in the south and center of iran. at first, the number of dust days was determined using climatic data of more than 30 synoptic station in a 9-year period, from 2002 to 2009. then, their circulation patterns were identified and plotted. also, the circulation patterns of extreme dust days was identified and plotted in the study area. though, meteorological data, the middle and upper atmosphere data such as the sea level pressure data, level of 500 hpa geo-potential height, wind direction, vertical velocity, surface temperature and the subtropical wind profiles were received from 100-1000 hpa height from ncep/ncar database and were drawn for 0-80 longitude and 10-70 latitude using grads software. then, visibility, wind direction and velocity, relative humidity and precipitation were extracted for each day. wrplot view version 5.3 software was used for wind rose drawing by wind storms causing direction and speed. hysplit model was used in order to recognize the origin of dust storms and wind flow routing in shiraz, fasa and bushehr stations.   results and discussion in recent decades, the global effectiveness of dust storms has become increasingly apparent, so that, major advances have occurred in sources, transport paths and their many impacts both on humans and the environment, their frequency at different time scales. earth database investigations shows that the year 2008 has one of the most extreme dust storms in terms of visibility reduce and durability of dust storm days in the study period that 2008 june 10 is the most extreme dust storm day with visibility of less than 500 meter in the stations. the mentioned dust storm as one of the most intense dust storms of the recent years can help having a good recognition of atmosphere patterns causing this phenomenon and routing winds transmitting aerosols. the results of the middle and upper atmospheric circulation base shows that west winds occurred in 2008 june 10 with a low pressure center in the east of iran and high pressure in the north of africa (figures 1 & 2). the condition is caused the entrance of dust to the study region.     fig. 1- map of 500 hpa at 10/6/2008 fig. 2- map of sea level pressure at 10/6/2008   the isothermal map of sea surface represents the ascendancy of 50 degrees curve on the center of iran and isothermal map of 42.5 degrees on the east saudi arabia. besides, the curves of 40 degrees have dominated on iraq and syria which causes temperature increase and drought air in the region. omega map of sea surface at 2008 june 10 indicates the -0.1 conture in central regions of iran which shows the downward air flow in the area. also, the central core of the upward flow, with 0.2 contour are dominate in the north of saudi arabia, iraq and syria, which illustrate an air mass with ascending movement in these areas. patterns related to humidity represent relative humidity of 10% in iran. also, specific humidity is very low and is equal to 0.004 which represent minimum rates of humidity and very high drought causing or increasing dust storms in the studied day. the results of wind flow routing by hysplit model showed that the origin of dust storm mass had been north west of iraq and east of syria which had entered iran with a north west-south east direction (figures 3 & 4).       fig. 3- map of backward trajectories ending at 10/6/2008 fig. 4- map of forward trajectories starting at 10/6/2008   conclusion the study of circulation patterns transferring dust storms in the south and center of iran have abundant importance. for example, the west region of iran isn’t a source of dust because of natural characteristics such as mountainous areas, high rainfalls and dense vegetation. but, dust storms of this region are non-local. this region is the transport route of climatic systems into iran, and accepts the most effectiveness from these systems. according to source regions of iran dust storms which are hyper-arid lands of iraq, saudi arabia, syria and north africa, this study is a connective step between large scale circulation patterns and temporal and spatial activities of dust storms in the south and center of iran. according to the results of this study which has examined the relationship between large scale synoptic patterns and variety of space-time dust storm activities in iran using synoptic maps of sea level pressure circulation patterns, 500 hpa geo-potential height, wind direction data, vertical velocity, surface temperature with geo data of dust storms in the studied stations and wind flow routing model, it can be concluded that dynamic models can be presented by consensus of various specialists for this destructive environmental phenomenon. although, using the capability of satellite imagery and dust storm investigating using them can be of great importance for future studies. in general, the results of this research shows that wind flow routing model and atmosphere studies have the suitable capability for recognition and monitoring of dust storm path.