نوع مقاله : مقاله پژوهشی
نویسندگان
1 دانش آموخته کارشناسی ارشد هواشناسی/ مؤسسه ژئوفیزیک دانشگاه تهران
2 دانشیار/هیئت علمی مؤسسه ژئوفیزیک
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Extreme precipitation refers to intense rainfall events in which the amount of precipitation far exceeds normal levels and shows significant statistical anomalies. These events often lead to destructive consequences such as flash floods, soil erosion, and severe damage to civil infrastructure. The southwest region of Iran, due to its natural structures and the presence of major constructions such as dams, is highly exposed to the hazards caused by these rainfall events. Therefore, understanding the synoptic and dynamic factors influencing the formation of such rainfall events can be of great importance. In this study, extreme precipitation events were identified using daily data from the Global Precipitation Climatology Project (GPCP) with a horizontal resolution of 1°×1° for the western and southwestern regions of Iran over the ten year period from 2013 to 2022. By setting the 95th percentile as the threshold for extreme precipitation, three case studies were selected based on the highest frequency and spatial coverage of grid points: November 23–25, 2014 (first), November 14–16, 2018 (second), and November 11–13, 2020 (third). In the next step, to conduct the synoptic and dynamic analysis of the three case studies, ERA5 reanalysis data with a horizontal resolution of 0.25°×0.25° were used. For the synoptic analysis, maps of mean sea level pressure (MSLP), temperature, specific humidity, and wind vectors at the 700 hPa level, geopotential height along with wind vectors at the 500 hPa level, and wind vectors at the 200 hPa level were plotted. The dynamic analysis was carried out using the quasi-geostrophic omega equation in the form of the Q vector, which includes three forcings for upward motions: Q vector divergence, diabatic heating, and the beta effect. The effects of these forcings on vertical velocity at different atmospheric levels, including 700, 500, and 200 hPa, were analyzed.
The results of the synoptic analysis indicated that three elements—a deep cold trough, moisture transport from the south, and the strengthening of the subtropical jet stream—were present during the occurrence of extreme precipitation in all three case studies. Nevertheless, their synoptic patterns exhibit differences, particularly in the mechanisms of initiation. In the first and second case studies, the simultaneous intensification of surface low pressure and the upper-tropospheric jet acted as the main driver, whereas in the third case study, the removal of blocking through the breakdown of a surface high-pressure ridge served as the primary prerequisite for the integration of activating factors. Finally, in all three case studies, the breakdown stage of the system occurs through the rapid eastward displacement of activating factors and the disconnection from the moisture source. The results of the dynamic analysis also showed that at the 700 hPa level, for the first and second case studies, the diabatic heating term was dominant and played a fundamental role in dynamic ascent over southwestern Iran. At the 500 hPa level, strong ascent continued in all three case studies, with the roles of Q vector divergence and diabatic heating being evident; however, in the third case study, the effect of Q vector divergence was dominant.
کلیدواژهها [English]
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