Revealing the dynamic patterns of heavy rains causing floods in Lorestan province based on synoptic systems

Heavy rains are one of the climate phenomena whose sudden occurrence leads to human and financial losses. Due to the fact that the country of Iran is located in a dry and low rainfall region, the occurrence of this type of rain leads to flooding in a short period of time and causes huge damages that cannot be compensated in the short term. Due to its mountainous nature, the western region of Iran is a suitable region for the intensification and expansion of rainfall and flooding. Because mountains play an important role in increasing atmospheric precipitation by trapping air humidity. Today, the general and expanding phenomenon of climate change has become one of the most important aspects of meteorology. Global warming is the driver of climate change and simply means an increase in the average temperature of the atmosphere. It has started continuously, creeping, and increasing since 1950 due to the accumulation of greenhouse gases in the atmosphere. One of the most important aspects of this large-scale general event (climate change) is the increase in entropy or anomaly of the earth's atmospheric system. Following the increase in atmospheric anomalies, some extreme events such as severe droughts, torrential rains, floods, extremely hot temperatures (heat waves) in summer, and late frosts in spring have increased significantly across the country. The behavior of the natural climate of the region tends towards disorder and abnormality in a significant way every year.

In this research, three categories of data were used. The first category is the precipitation data of the synoptic stations of Lorestan in the statistical period of 2000 to 2020, which was obtained from the meteorological organization of Lorestan province. The second set of data related to the factors of the middle and lower levels of the atmosphere, including geopotential height, sea level pressure, Omega, moisture level wind, an orbital and meridional component of wind level 300 hectopascal for days 5 and 12 April 1398, 15 February 1384, 7 November 1394 from NCEP/NCAR site taken. The third group of data is related to the data of the atmospheric station above the stations of Kermanshah and Ahvaz (because Lorestan province does not have an atmospheric station, the nearest stations were used), which were obtained from the University of Wyoming database for April 5, 1398, February 15, 1384, and November 7, 1394. became First, using the 95th percentile method, the heavy rainfalls of the studied stations were determined. In the following, the statistical features of rainfall of 4 rainfall waves 5 and 12 April 1398, 15 February 1384, and 7 November 1394 were analyzed. The synoptic conditions that produced these rains were investigated using the data of the middle and upper levels of the atmosphere of the ECMWF database version ERA-Intrim. To examine the patterns of the middle and upper levels of the atmosphere, the maps of the daily synoptic factors of these two precipitations are analyzed. Finally, using thermodynamic indicators, the thermodynamic status of the upper atmosphere of the region is investigated.

The analysis of the synoptic patterns of these extreme precipitation events showed that a similar synoptic pattern was the generator of these heavy precipitation waves in the region. In the studied days, the presence of a deep trough on the eastern Mediterranean Sea and the western part of Iran is located in the front part of a very deep trough, which has clearly provided the conditions for the ascent and entry of low-pressure systems for the west of the country. Low pressure was visible on the surface of the earth in the west of Iran. In accordance with the deepening of the western winds, in addition to the Mediterranean, the moisture sources of the system are provided by the Black Sea and the Red Sea, as well as the Persian Gulf. The descent of the Red Sea is associated with the establishment in the middle level of the atmosphere, and it has provided the possibility of deep ascent and the formation of clouds and precipitation, and it has brought heavy rains and floods in the west. In the humidity map, it can be seen that a circulation center has formed over the Oman Sea and the Persian Gulf, which directs moisture from the Oman and Arabian Seas to the south and southwest of Iran. Also, another circulation center has formed over the Mediterranean Sea, which sends the Mediterranean and Black to the west and southwest of Iran. This means that these two centers concentrate the moisture of the South, West, and North-West seas of the country on these provinces, and the atmospheric systems are fully nourished in terms of moisture. The instability indices of the upper atmosphere, which were analyzed with emphasis on the upper atmospheric stations of Kermanshah, did not confirm the existence of extreme instability in the region. The average instability in the lower levels of the atmosphere, which did not have the ability to extend to the upper level, the Skew-T diagram indicated that a global synoptic system involved the entire region and the local convection factor did not play a role.

The results showed that during the studied days, a deep trough was placed over the eastern Mediterranean Sea and the western part of Iran, which prepared the conditions for the ascent and entry of low-pressure systems for the western part of the country. At the surface of the earth, low pressure has prevailed on the surface of the earth in the west of Iran. According to the moisture flow map, the center of Wacherkhandi is formed on the Oman Sea and the Persian Gulf, which directs the moisture from the Oman and Arabian Seas to the south of Iran. Also, another circulation center has formed over the Mediterranean, which sends the moisture of the Mediterranean and Black Seas to the west and southwest of Iran. The average instability in the lower levels of the atmosphere indicated that a global synoptic system involved the entire region and the local convection factor did not play a role.