Investigating the Role of Interaction between Cut Off Low coupled patterns and Polar Front Jet Stream in Heavy Rainfall Event of April 2019 in Lorestan Province

One of the most dangerous atmospheric hazards that can cause so much damage is the flood phenomenon. Heavy rainfall has been a risk factor for flooding. Floods are one of the most severe and destructive atmospheric hazards that have financial and physical harmful effects and affect humans and nature on a large scale. Heavy rainfall and thunderstorms are among the climatic features of the arid and semi-arid regions causing floods. Iran is also affected by the flood phenomenon because of this feature. Synoptic patterns, distances and proximity to sea, altitude, slope and direction of roughness are the most important backgrounds for the formation of precipitation events. Interaction of these factors can exacerbate this phenomenon and increase heavy rainfall. Studying the climatology of precipitation, while recognizing the characteristics and behavior of precipitation, it is possible to plan based on it. Therefore, the mechanism of precipitation, especially in critical conditions (such as drought and flood), has been of interest to researchers in Iran and worldwide. Many studies have been conducted in this regard with different attitudes and goals regarding the dynamic, thermodynamic and synoptic analysis of heavy rainfall. Therefore, despite the fact that floods are recognized as one of the natural hazards and their occurrence every year in Iran, not only the damage caused by this phenomenon has not diminished but it is also increasing. Therefore, planning and trying to reduce the damages caused by it is essential. Lorestan province in southwestern Iran, due to its geographical location and geological and geomorphologic conditions, is affected by devastating floods every year due to heavy rains and floods, which often cause a lot of casualties and financial losses. Therefore, understanding the regime and pattern of these types of precipitation and the synoptical-dynamical mechanisms that lead to their occurrence will help to identify and predict these types of precipitation.The purpose of this study was to investigate the synoptical - dynamical analysis of one of the heaviest rainfall in Lorestan province (April 2019), which was unprecedented in the last two hundred years, based on meteorological statistics.

Believing that heavy rainfall in each region is associated with high-altitude convective synoptic patterns and meteorological conditions in the region, assuming occasional heavy rainfall in the western half of Iran is affected by the Cut Off Low and Polar Front Jet Stream, Selected as the case study. For this purpose, four databases for a wide area focusing on Lorestan province include: daily atmospheric monitoring parameters such as maps of pressure, temperature, wind, precipitation, relative humidity, etc. at different levels from the National Center for Atmospheric Research and The National Center for Environmental Forecasting (NCEP / NCAR), GFS global numerical model overlay maps, TRMM 7 Level 3 satellite precipitation data, and weather data at the time of precipitation were used based on synoptic signs and meteorological codes (ww). In the final step, data are used with time steps (hourly and daily) and different locations (from 0.25 to 2.5 °) for different atmospheric levels (sea level, 850, 700 and 500 hPa) for a range of 0 to 70 East longitude and 10 to 60 degrees north latitude were extracted and processed using various statistical and graphical software.

Lorestan is a mountainous land that, with the exception of a number of alluvial valleys and a few small plains, has no smooth area, and almost all of it encompasses the central Zagros Mountains, which regularly extend northwest to southeast. These topographies associated with deep converging valleys are often compact, tall, and uneven, which increase the flood potential due to the high slope and the low permeability. The amount of rainfall that resulted from the floods of April 2019 was much higher than other flooding events in Lorestan province, according to unprecedented meteorological statistics.The results show the instability and ascension of humid air by two Cut Off Low (COL) atmospheric forcing and the Polar Fron Jet Stream (PFJ) and the convergence of moist currents from the three moisture centers of the northern Indian Ocean (Arabian Sea), the Red Sea and the eastern Mediterranean. These features provided the dynamical-synoptical conditions necessary for heavy rainfall occurrence in April 2019 in Lorestan province. However, other environmental factors such as saturation of the soil surface layer due to the high amount of precipitation in the days before the floods, etc. have also contributed to the intensification of the flood.

The results of the synoptic-dynamic analysis of the flood system (30 March to 1 April 2019) in Lorestan province showed that the precipitation was in the synoptic patterns middle level of the atmosphere and the occurrence of convective upward movements depend on the lower levels. In this rainfall, Cut Off Low formation in the vicinity of polar front Jet Stream provided the dynamic conditions necessary for the formation of significant Heavy rainfall in Lorestan province.In such conditions, the fusion of the unstable region due to the jet stream with the unstable region of the eastern part of the Cut Off Low has a great potential for destabilizing and uplifting the humid air below which the convergence of moisture flux originates from the foci of the northern Indian Ocean (Arabian Sea). Red and Mediterranean have increased its humidity to saturation. In other words, the aforementioned patterns provide the potential for sufficient and dynamical- synoptical potential for heavy rainfall events in the study area.On the other hand, Lorestan province is on the located of entry of numerous western rainy and continuous rainfall caused by these masses during most of the cold season (region saturation), mountainous region and topography with deep and converging Compact valleys, very low permeability of the region Due to its special lithology, reduction of surface runoff time due to deterioration of vegetation in the area and some other human factors has doubled the flood potential of Lorestan province.