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

Introduction
Heavy rains are one of the climate phenomena that leads to human and financial losses. Since Iran is located in a dry and low rainfall region, this type of rain leads to flooding in a short period and causes huge damages that cannot be compensated in the short term. Due to its mountainous nature, the western region of Iran is suitable 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 climate change phenomenon 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. Accordingly, the current research was done to reveal the dynamic patterns of heavy rains causing floods in Lorestan Province based on synoptic systems.
 
Materials and Methods
In this research, three types of data were used. The first category is the daily rainfall data of the synoptic stations from 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, wind, an orbital and meridional component of wind level 300 hectopascal for 5 March and 1 April 2019, 15 February 2006, 29 October 2015 from NCEP/NCAR. The third group of data is related to the data of the upper atmospheric station of Kermanshah which were obtained from the University of Wyoming database for selected days. using the 95^th percentile method, the heavy rainfalls of the studied stations were determined. In the following, the statistical features of rainfall of four selected heavy rainfall 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 indices, the thermodynamic status of the upper atmosphere of the region is investigated.
 
Results and Discussion
At the level of 500 millibars, which is the main level of observing the systems that produce the main atmospheric phenomena, in two rain waves on 25 March and 12 April 2018, a very strong low-altitude core with a central height of 5550 geopotential meters was closed over the eastern Mediterranean Sea and the western part of Iran is located in the front part of a very deep trough, which indicates the dominance of a very strong circulation system in the west of Iran. Low pressure was visible on the surface of the earth in the west of Iran. The moisture map of level 850 for 15 February 2014 showed that the moisture cores are completely located in Lorestan Province. One of the moisture cores from the Persian Gulf has strengthened with a curve of 5 to 9 gr and covers the south to the northeast and has provided the conditions for the creation and fall of heavy rains in the province. The moisture sources of the system are provided by the Black Sea and the Red Sea, as well as the Persian Gulf. The trough of the Red Sea is associated with the establishment in the middle level of the atmosphere, 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. In addition, another circulation center has formed over the Mediterranean Sea, which it sends the Mediterranean and Black Seas 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 Lorestan and Kermanshah provinces, and the atmospheric systems are fully nourished in terms of moisture.
 
Conclusion
The analysis of the co-occurrence patterns of extreme rainfall events showed that a similar co-occurrence pattern was the generator of heavy rainfall 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 provided the conditions for the ascent and entry of low-pressure systems for the west of the country. 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 Skew-T diagram indicated that a global synoptic system involved the entire region and the local convection factor did not play a critical role.