بررسی نقش گـردش منطقه‌ای جو بر روی خـاورمیانه در وقـوع تـوفان‌های

1- Introduction In middle and lower troposphere, the main control factor of weather and climate over the Middle East is a local to regional scale thermal forcing including forcing from high mountains. In this regard, regional scale atmospheric circulation associated with surface forcing has an important role in the occurrence of summer dust storms over the area. Southwest of Iran experiences a large number of dust storms every year. Many of them occur in warm period when a series of semi-permanent synoptic systems in lower troposphere control weather and climate in regional scale. In spite of large number of works which have been done during two to three last decades, the main governing mechanisms on the occurrence of dust events over the Middle East are not much clear. Therefore, this study aims to clarify how dust storms occur over southwest of Iran during warm period of the year. In this regard, the research tries to answer to the following questions: What are the most important synoptic patterns of dust storms occurrence in southwest of Iran? What are the main sources of dust for each synoptic pattern? and how regional atmospheric circulation can cause and develop the dust storm events over the area? 2- Methodology In this research, the structure of regional atmospheric circulation is investigated to explain the causes of summertime dust storms occurrence in southwest of Iran as well as their main sources of dust. The hourly visibility data of eight synoptic stations located in the Khuzestan and Ilam provinces is building and finally the French school outlines a new discipline combining the study of the built environment with critical assessment of design theories. They all claim that understanding the built environment is possible through three fundamental dimensions: form, time and scale. The English geographical urban morphology has not dealt in detail with individual building types geographical, as opposed to architectural, influence has dominated. There is no parallel to the detailed evolutionary typological research that has developed in , for example, Italy and its close links with planning and architectural practice (Larkham,1998:167).Although the three schools differ in their socio-cultural roots and use different intellectual subjects and methods, but all together they provide a serious and useful tools in the study of urban forms and built landscapes , backing up the researches in the field of practical design. They should be reread and refined carefully by designers and planners in order to derive creative methods and ideas for managing and designing the built environments.