Synoptic Analysis of the Conditions for Occurrence of Mesoscale Convective Systems

I
Introduction
Convective systems are atmospheric events that are associated with hazardous consequences, such as strong wind drafts, lightning, heavy rainfall, hail, or even tornado. They are manifested in the atmosphere within a broad range of spatial and temporal scales.
A convection cell is composed of two distinct regions: the actual convective part which consists of the coldest intense vertically extended cores and the stratiform region characterized by a more uniform texture with lighter precipitation. The stratiform area is partly produced by dissipation of older convective cells and partly by ascent of a broader sloping mesoscale layer. Typically, these systems can extend over hundred kilometers in one direction, and can last from a few hours up to several days.
Convective Systems (CSs) are very important from two aspects in the southwest of Iran. First, they produce lightning, heavy rainfall, hail and strong winds which can have very hazardous consequences and second, they provide beneficial rainfall for important hydrologic and agricultural needs of this region, because they are maily contributed to the total precipitation accumulation, this is while there are not any CSs database in this region. This study is intended to determine predominant synoptic conditions for CSs occurrence in the study area.
 
Data and methodology
Event days for the study have been selected using a set of storm report and precipitation criteria across the study area (Southwest of Iran: the provinces of Khozestan, Chaharmahal va Bakhtiari and Kohkiloyeh-va-Boyerahmad). If, at least, one station reported 6 hours of total precipitation more than 10 mm, that could be related to convection (storm, lightning, or shower). Pressure patterns of CSs occurrence have been classified using correlation and principal component analysis approaches. Results of correlation approach in further analysis were used because of its better outcome to determine percent of convective systems occurrence in each pressure pattern.
 
Results and discussion
Results indicate that occurrence of CSs depends extremely upon outspread of Sudan Low in southwest of Iran. Most of them are initiated in Red Sea convergence zone (southeast of Iraq, Kuwait, Northeast of Saudi Arabia) and some of them are initiated by cyclone or trough formed between west of Iran and Mediterranean Sea. The most frequently flow patterns are induced to CSs initiation at 850 and 500 hPa, respectively. The moist and warm air was transmitted by anticyclone dominated over Arabian Sea or southeast of Saudi Arabia (Saudi Arabia anticyclone) both in surface and at 850 hPa level. In surface, cold air was transmitted either by Siberian high-pressure (which acts independently or integrated with Azores high pressure or North Africa and Azores high-pressure. These flows were spread out of Sudan Low Pressure toward the South-West of Iran because of the pressure gradient strengthening. While at 850 hPa Azores and North Africa high pressure played the most important role in cold air transmitting to the region. The local high pressure located in north of Iran, north and south of Mediterranean Sea or Black Sea was sometimes responsible for this cold air. Under these conditions, Red Sea convergence zone outspread toward the west of Persian Gulf or South-West of Iran. In all flow patterns at 500 hPa, South west of Iran was located in east of a trough which its axis was placed on the east of Mediterranean Sea or between Mediterranean Sea and Iran. It is remarkable that there was no difference between the flow patterns induced by CSs initiation in different months.
 
Conclusion
In this research pressure patterns on occurrence of Convective Systems (with precipitation more than 10 mm) has been classified using correlation and principal component analysis approaches in the southwest of Iran. Results of correlation approach were used in further analysis, because of its better outcome and percent of CSs in each pressure pattern.
Results indicate that the occurrence of Convective Systems depends extremely upon outspread of Sudan Low in southwest of Iran. Most of them are initiated in Red Sea convergence zone (southeast of Iraq, Kuwait, Northeast of Saudi Arabia) and some of them are initiated by cyclone or the trough formed between west of Iran and Mediterranean Sea.