تحلیل الگوی دینامیکی و همدیدی بارش‌های سنگین در جنوب‎غرب خزر و غرب ایران (مطالعه موردی: بارش مورخه 14/8/1383)

Introduction
Understanding mechanisms and atmospheric circulation patterns which act on the heavy rainfalls is very important, especially because such rainfalls can create catastrophic floods causing damage. Various studies have been conducted on the effects of the atmospheric circulation on the rainfall occurrence all over the world some of which are reviewed here. For example, Habtemicheal and Pedgley (1974) conducted a synoptic study of spring rainfalls in Eritrea. Also, Nicholson (1981) studied the relationship between rainfalls with the atmospheric circulation during periods of drought and wetness in West Africa. The synoptic analysis of monthly and seasonal rainfalls along with the atmospheric circulation in Sri Lanka has been done by Suppiah (1987). Dayan, at al. (2001) described the conditions of synoptic systems causing a severe autumn storm over the Middle East. Kutiel at al. (2001) examined air patterns that create dry and wet periods in Turkey. Littmann (2001) identified the East Mediterranean air patterns by using the geographical location of Azores (an important high-pressure system).
In Iran, several studies have been performed in this field. The following can be noted. Sabzi Parver (1991) has done a synoptic review of systems that create floods in the southwest of Iran mentioning that the main factor of causing floods and heavy rainfalls in the southwest of Iran is the presence of a deep trough in the upper levels so that the axis of this trough crosses the south of the Red Sea. Najar Salighe (2001) studied the monsoon and Arabian rainfalls related to the summer rainfalls in Iran. Moradi (2001) carried out a synoptic study of floods on 11, Nov., 1996 floods in the central regions of Mazandaran province. He notes in this regard that the main factors causing rainfalls are the high-pressure on the ground surface and the presence of trough at higher levels. Mofidi and Zarin(2004) conducted a synoptic analysis of the effect of Sudanese low pressure systems on floods and heavy rainfalls occurring in Iran. They are noted that the main role to create flooding rainfall is polar vorticity stretching in 50Hp level. Jahanbakhsh and Zolfaghari (2001) investigated the synoptic models of daily rainfalls in the West of Iran. Omidvar's (2007) survey revealed synoptic and thermodynamic rainfall conditions in the occurrence of rainfalls in Shirkoo Region. Azizi and samadi (2007) studied the synoptic model of the floods on 20, Oct. 2003 in Gilan. They note that this flood was caused by the deep trough in the North-East of Europe.

Methodology
In this study, the dynamic and synoptic conditions of the heavy rainfalls 14/8/83 (4, Nov., 2004) were studied. On this day, at the number of stations in the country a very heavy rainfall has occurred. The highest rate was reported 147 mm by the Astara station, 115 mm in Bandar Anzali, 52 mm in Mehran, etc. Based on the drawn maps for this day, most rainfall occurred in the southwest of the Caspian Sea and then in the west of Iran. To carry out this synoptic and dynamic analysis, first, the data of the temperature, geo-potential height, humidity, wind and sea level pressures in 2004 for all standard levels were derived from the site www.cdc.noaa.gov. Then, a suitable network that can reasonably analyze the events of the meteorological systems was designed using the Grads in which synoptic systems can be shown. Maps were drawn for the same day and the days before and after the rain for a one- week period.

Results and Discussion
The results of the study showed that on 4, Nov.2004, there were a low-pressure system on the ground, a trough at the middle levels of the atmosphere, and the advection of cold air in the inspected area which were along with the axis of the jet stream in the north of the Caspian Sea. Due to its meridional component of the wind, the role of components related to changes in the vorticity of the horizontal meridional wind led to increasing the horizontal gradient pressure. The horizontal convergence of the temperature caused the vorticity to be positive and this resulted in intensifying the wind’s vertical speed; consequently, the cyclonic component of the wind was reinforced. Intensified baroclinic situation increases Ascending streams, cloud formation and rainfalls. The next day the Baroclinic instability is weaked. This day expanding high-pressure on the Caspian Sea created the force which rise moisture from ground’s surface to high levels and this has been caused high rainfall, especially in the West Caspian Sea. On the fifteenth’s day in the trough and the cold advection disappear in the West of country and this shows that the unstable system in the region has gone and rainfall is finished.

Conclusion
The results show that during the occurrence of this rainfall there is a high-pressure system with 1030 Hp in the centre is over the Black Sea and North Eastern Europe. Also at 500 and 700 Hp levels a deep trough is formed and extended from the northwest of the Caspian Sea to the eastern of Mediterranean Sea and to the North East of Red Sea. In the northern Black Sea a high ridge is associated with a high-pressure surface. The analysis of wind maps of the 300 hp level demonstrates that jet stream axis is located on the North Caspian Sea. Also in the study area and the East Mediterranean, winds are fractured and their directions in the West of Iran were turned from north-west to south-east. All these features that were described above caused the area to be highly unstable from the point of view dynamical aspects so that this instability extends from the ground surface to high altitudes, which finally leads to intensity in the rainfalls in the South West if Caspian Sea and west if Iran.