Synoptic analysis of winter (DJF) extreme rainfall in western Iran

Rain and snow occurrences on Zagros topography in the winter, provide groundwater resources, soil moisture, and surface water current on slopes, valleys and plains. Western Iran is covered by Zagros topography. The Zagros Mountains are one of the highest mountains in Iran and Western Asia. The maximum height of Zagros Mountains in western Iran reaches 4000 meters. These mountains are located in the form of a barrier in the direction of the entrance of the dominant western winds to the region. The width of these mountains in western Iran is relatively wide and about 300 km. The Zagros topography is a series of high mountain ranges, folds, valleys and plains. In this study, the effects of severe winter rainfall in the Zagros topography in western Iran have been investigated.

In the present study, the extreme rainfalls for the three months of December, January and February as winter months (DJF) for 1996-2017 were reviewed from synoptic and statistical points of view. Three extreme rainfall patterns were determined for the three winter months (DJF). In this way, the rainfall days of each month were divided into 10 categories based on maximum rainfall of each day. But the frequency of winter precipitation systems in western Iran have led to the occurrence of extreme daily rainfalls over the course of a specified statistical period. Due to the role of these systems in the occurrence of daily winter precipitation, the arrangement of synoptic patterns, moisture content, temperature fluctuations and airflow paths were analyzed simultaneously, with the occurrence of the most severe extremes in the current study. The tenth category included the highest averages and first category included the lowest. Daily rainfall maximums in the tenth category were selected as extreme rainfall patterns of that month. These three patterns included December 21, 2002, January 9, 1999 and February 3, 2006.

Extreme is an intense and unusual event in observations that may exceed a certain threshold. The most severe of extreme rainfalls occurred in the region at the end of the winter season. In other words, the intensity of the extreme rainfalls were respectively at February, December and January. This trend has been observed for stations located in Kurdistan and Ilam provinces. While in Hamadan province stations, the reverse mode has formed, and the maximum rainfall occurred in December, January and February, respectively. This trend for the stations of Kermanshah province has changed to January, February and December, respectively. But in Lorestan province it was observed as December February and January.

The results of the synoptic analysis showed that during extreme daily precipitations, high pressure systems were weakened in Iran and Mediterranean and Sudanese pressures were deployed in western Iran. The synoptic arrangement of the December precipitation occurred in the form of a deepening of the Mediterranean Trough over the Red Sea at a level of 500 hPa and directing lower levels moisture flow as a convergence flow from the south and west winds to Iran. Whereas the deployment of a cut-off low blocking system in January and February played an important role in the event of low rainfall. During these months, the merger of two Mediterranean and Sudanese sea-level systems over western Iran has brought heavy rainfall to most stations. At 850 and 700 hPa, the warm and cold air flow resulting from the establishment of the Saudi anticyclonic system beyond the northeastern boundaries of Iran caused a deep cold northward wind flow, to the west of Iran. This resulted the cold precipitation of January. In February, the deployment of a blocking system on Turkey and Syria affected the northwestern parts of Iran, and the Red Sea moisture stream to north. Because of this, heavy precipitations formed due to proximity to the path of moisture in the western slopes of Zagros, such as Ilam, and the rest of the stations had little precipitation due to the long distance from the moisture path.