عنوان مقاله [English]
On April 01th , 2019 Iranian calendar, heady damages was inflicted on Lorestan province due to the arising of the heavy rainfall and consequent destructive flood . Present study was carried out to investigate the synoptic mechanisms associated with the occurrence of this rainfall. At first, the data related to the daily rainfall in different research stations was received from the metrological organizations. Afterwards, data related to atmospheric levels including Sea Level Pressure (SLP), the altitude geopotential of 850 and 500 hectopascal levels, vertical atmosphere speed , wind stream in 1000, 850, and 500 hectopascal, specific humidity of 700 hectopascal level , and the river flow of the 250 hectopascal level during the studied days were prepared and received from National Center of the Environmental Prediction (NCEP) and National Center for Atmospheric Research(NCAR). The created maps were drawn and interpreted in Grads Software.
The results of the study indicated that, on April 01th when the flood hit the region, synoptic investigation of the Sea Level Press (SLP) shows a low-pressure system with a central core equals to 1206 hectopascal is located in Northern Europe with its braces covered the north of the Black Sea. Furthermore, the low-pressure system having central core of 1026 hectopascal covered wide regions surrounding Aral lake to North-east of Afghanistan. Accordingly, a low-pressure system having central core of 1008 hectopascal moved to eastern parts compared to previous day and dominated wide regions from Mediterranean through all investigated areas to Central Iran. Given that low pressure system spreads north-east Russia in line with northeast-southwest through Red Sea and East Africa, a high-pressure shave was created which was accompanied by instability and intense ascending of the weather on desired April 01th along with the arising of the rainfall in investigated region. Consequently, with the rise in the altitude and in 850 hectopascal level, low pressure system having central core equaling to 1300 hectopascal dominates North-Russia through lower latitudes. Subsequently, lower altitude closed system corresponding to 1420 geopotential meters resides Eastern Mediterranean and, compared to previous day headed eastwards and spread West and North-west of Iran including investigated region in the north-south direction.
Moreover, high altitude system with central core which equals 1520 geopotential meters has spread North Europe up to 48-degree latitudes, which, given to high altitude system located on Afghanistan and Pakistan, a more intense shave arose compared to previous day. Afterwards, at 500 hectopascal level, central core having 5540 geopotential meters places on East-Mediterranean Sea in its Northeast-southwest direction, which resulted in deeper and wider trough so much that central trough spreads west Iran. The placement of its positive vorticity covers wide investigated region and results in ascending motions. The deepening of the cyclone and penetration of trough to the lower latitudes can gives rise to the precipitation of cold weather.
Synoptic investigation of vertical atmospheric speed of 1000 hectopascal level revealed that maximum negative omega equals -0/2 pascal / second that is located on the North-west of Caspian Sea in Northeast-Southwest direction, and, on its movement toward Lorestan, it decreases in its intensity and reaches -0/05 pascal/ sec. Given that the second negative omega equaling to -0/15 pascal/sec is placed on Persian Gulf and omega is negative throughout the investigated region, the required conditions are really for the occurrence of instability, ascending of the weather, and rainfall. Subsequently, with the rise of altitude in the atmospheric levels, the maximum negative omega has increased and reached 0/3 pascal/sec. Within 500 hectopascal, the direction of negative omega changed from Red Sea towards investigated region which, accordingly, has been strengthened over Caspian Sea and spread the intended region in North-South direction. Given the increasing altitude of the level of the atmosphere, the maximum omega has risen and reached -0/4 pascal/sec, which accordingly intensified the conditions for atmospheric instability.
Synoptic investigation of specific humidity indicated that most of the humidity advection happened from the Southern water reserves to the investigated regions. Maximum humidity core is 24 g/kgs over Red Sea and is decreasing in extent toward Iran, which, subsequently, passed over Persian Gulf and reached 18 g/kgs in Oman Sea and spread intended area in Northwest-Southeast direction.
Humidity injection, particularly from southern water reserves, led to arising of the heavy rainfall during the specified day in investigated region. With the rise in the altitude at the level of 700 hectopascal, maximum specific humidity, which equal 10g/kgs, residing in East Africa and got the investigated region having passed over Caspian Sea. So , the sources for the heavy cloud rainfall on Farvardin 12th are Red Sea, Mediterranean Sea, Arab Sea, and Persian Gulf.
Key Words: Heavy rainfall, Geopotential, Omega, Synopsis, Lorestan.