نوع مقاله : مقاله پژوهشی
عنوان مقاله English
نویسندگان English
The purpose of this paper is to identify the patterns used by of spatial distribution of moisture flux in rainy days in the southwestern stations of Iran. For this purpose, the codes of the current weather conditions transmitted by the stations located in seven southwestern provinces of Iran during the period of 1986-2016 and the most important cases of daily rainfall were examined in the thirty-year period in the region and were obtained a classification based on Geographies distribution from the maximum daily precipitation. In this study, ten case maps, which are based on gridded data of temperature, sea level pressure, geopotential height (HGT), component result Orbital (U) and meridional (V) winds and arrangement of vertical velocity patterns (Omega) prepared at different atmospheric levels from the United States National Oceanic and Atmospheric Sciences Base (NCEP/NCAR) databases (1986–2016) has been analyzed. The results indicate that in patterns with south to southwesterly currents, the low-pressure surface center extends from the south of the Red Sea to southern Turkey and is associated with the mid-level trough, where the moisture fluxes converge in the south of the Red Sea, southwest/south of Iran, and east of the Mediterranean Sea. On the other hand, the spatial distribution patterns of heavy rains are directly related to the topographical factors in the atmospheric conditions of the humid westerlies. In this study for investigating moisture flux patterns, meteorological data at 45 synoptic stations were studied during recent 30 years. «First of all, occurrence of daily rain is classified and then genesis and severity conditions of them were considered from the synoptical view point. Then prediction of time and spatial occurrences of daily rain is the major object of this study. Then using circulation under environment approach, the most important of patterns which play key role over climate of southwest provinces of Iran were classified. In the first step the "mean daily sea level pressure data were selected from the NCEP reanalysis data encompassing the region from 20°-60°N latitude by 20°-80°E longitude, with a 2.5 spatial resolution and for the 30 years period 1986-2016 in moisture flux. Selected territory is located in a geographical situation that covers all the effecting systems on Iran. For classification and delineate synoptical patterns Principal Component Analysis (PCA) and clustering method were used"(Kianipour et al., 2022).
Results and Discussion Study of 10 selected moisture flux and rainfall samples show that extracted information presents general synoptical patterns. Any type pattern was studied at sea level pressure and 850 hPa level as follows. Results showed that in this region, "the moisture flux have three general circulation patterns. Analyzing these patterns show that there is a trough which restricted to 30-45°E longitude in 850 hPa chart, and at least one relatively strong low pressure over Arabian Peninsula in sea level pressure extending to the west, south west and the study region. This study shows that above aspects are the major specifications of moisture flux (Kianipour, 2022)." Based on the results, the presence of low pressure over the western Kazakhstan and high pressure over the Arabian Sea and low pressure in eastern Arabian Peninsula, play key roles in moisture penetration into Iran and increasing atmospheric moisture"(Golkar et al., 2016). This study has classified the synoptic patterns affecting Iran and the Middle East from 1986 to 2016, for all seasons months and days and with a higher accuracy using data with 0.5° resolution on a daily time scale. Then, composite maps of the mean sea level pressure (MSLP), and 500 hPa geopotential height (GPH) were prepared and analyzed. Previous studies have focused mainly on 500 hPa and MSLP parameters. «However, in the present study, the 850-hPa moisture flux, which was considered an important quantity for the precipitation occurrence, was calculated and analyzed in each of the patterns»(Raziei, 2007). One of the conditions in forming moisture flux is existence of a trough in 850 hPa charts, between 30 to 45°E longitudes that extends at least to 25°N latitude and lower. Because of cold advection in behind of the trough from mid-latitudes to the south, and warm advection in ahead of it from south latitudes to the north, increasing of pressure gradient and wind velocity at ground surface is observed. Existing at least one relatively strong low pressure on the north of Saudi Arabian Peninsula at sea surface charts, associated with a trough over red sea or west of Arabian Peninsula at 850 hPa level, and extending to the west, south west and the region under study in Iran. This situation is one of the most important specifications of moisture flux. This low pressure cause to increasing pressure gradient on the north of Saudi Arabian Peninsula and south west of Iran. Position of troughs at 850 hPa level are in a form that they can lead and send the air masses from deserts in north of Saudi Arabian Peninsula, Iraq, Syria and Jordan to Iran. Because these areas are at the front of the trough which is very unstable (Kianipour, 2022).
کلیدواژهها English
