جستجوی مقالات مرتبط با کلیدواژه "جنوب غرب ایران" در نشریات گروه "زمین شناسی"

The formation and occurrence of any climatic phenomenon require conditions that, when the sum of these atmospheric and environmental conditions occur simultaneously, make it possible for the phenomenon to form and occur. Depending on the type of phenomenon, some of these atmospheric or environmental factors are formed in the atmosphere alone, while others are formed in the physical environment under it. A phenomenon like a dust storm is a combined atmospheric-environmental phenomenon. The existence of dry and highly fragmented soils provides favorable conditions for the rise of dust. For this reason, dry deserts, bare plains with soft and granular soils are always prone to this phenomenon as sources of dust production. Therefore, physical substrates with this feature should always be studied and investigated as potential places for dust phenomenon to occur. What is related to atmospheric conditions, severe instabilities without moisture or with little moisture are the basis of dust storms. This weather pattern is more likely to appear in arid and semi-arid climates. It is not possible to control or change the structure of atmospheric phenomena with current human technology. Therefore, the best way to reduce the effects of destructive phenomena such as dust storms in a geographical area is to first identify the source and path of dust storms.

The selected study area for this research is the southwest of Iran, an area that has experienced an increasing trend in the frequency and intensity of dust storms in recent decades. The study area includes the three provinces of Kohgiluyeh, Boyer Ahmad, Chaharmahal and Bakhtiari.In the first step of the research, all synoptic stations with complete statistics for 33 years (1986-2019) in the study area were identified and extracted. .In the second step, using the data (number of reports of dust phenomenon (nhz), number of reports with dust phenomenon (ndu), number of reports with sand and dust storm phenomenon (nbdu), wind speed and horizontal visibility less than 1000 meters and codes (06.07, 08.09, 30.31, 32.33, 34.35, 98) days with dust were extracted from the data of Meteorological Organization. In the next step, using the website [https://www.ready.noaa.gov/HYSPLIT, ↗] (https://www.ready.noaa.gov/HYSPLIT,) the origin of dust storms in the southwest of Iran was plotted using the backward method from the GDAS data system with a resolution of 0.5 degrees. This data is available from 2007. Therefore, for all the dust reports with the above characteristics from each of the sample stations, the origin of the storm has been determined.

Statistical analysis of dust storm reports from selected stations showed that 3027 cases of dust storm have been reported during the 33-year statistical period from a total of selected stations. In terms of monthly distribution, the months of January, February and December respectively have reported the highest number of dust storms. In terms of seasonal distribution, the highest number of pollen reports were from winter (1153 cases, 38.1%) and spring (711 cases, 23.5%) respectively. The highest number of dust storms was reported from the two dry years of 2008 (225 cases) and 2009 (243 cases). In terms of stations, the highest number of dust storms was reported from Ahvaz and Abadan stations. Based on the routing performed in this study, the input dust storms came from six areas:Iraq (from four parts), eastern Mediterranean countries, Saudi Arabia, central and northeast Africa, distant sources in eastern Europe, and local sources within the study area.The origin of 93.25% of the input dust storms to the study area was from sources in central Iraq, and about 20% of the dust storms originated from eastern Mediterranean countries, while 16% of the dust storms were from local sources in Khuzestan province. The origin of dust storms in high-altitude stations in the eastern part of the study area, such as Shahr-e Kord, Yasuj, and Kuhrang, was from distant sources in Syria and Jordan or northeastern Africa.

Given that, 84% of the dust storms entering the region originate from outside the border. Considering that the two sources of the west and the center of Iraq are the source of 35% of the storms entering the region. Diplomatic efforts for the cooperation of the country of Iraq to stabilize the soil in these resources are very necessary.

Autumn and winter rains play an important role in Iran's agricultural and livestock economy, as well as feeding underground water tables, especially in the western and southwestern regions of Iran. The continuous reduction of underground water reserves in Iran increases the importance of atmospheric precipitation especially during autumn and winter seasons. This importance is affected by the centrality of agriculture in Iran, as well as the supply of drinking water in small and large cities.
This sensitivity is more in the southern half of Iran and especially in the southwest, which has rivers full of water and it, is a region full of large dams.

In this research, the mechanism of rainfall in the two seasons of autumn and winter in southwestern Iran was investigated and compared. Then, the synoptic conditions of the longest and most intense rainfalls that occurred during the selected data period in the research area were identified. In this research, a rainy day was defined as a day when the weather station received at least half a millimeter of rain. Also, the duration of rainfall at each station was defined as the number of consecutive days of rainfall at that station. In first step, daily rainfall data were collected from 8 weather stations in the 30-year period (1987-2016) in southwest Iran. During the second step, separation of rainfalls according their duration using programming in MATLAB environment; it made it possible to select 40 precipitation samples. During the last step, synoptic patterns were designed from daily weather maps, including moisture flux, atmospheric height, and the position of jet-stream cores.

In this research, the analysis of patterns showed the following results 1) Sources of continuous and widespread rains in the autumn season, warm waters around Iran include the Red Sea, Gulf of Aden, Arabian Sea, Persian Gulf and Oman Sea. 2) The origin of the wet currents of these rains is from the central regions of tropical Africa. 3) During the winter season, these resources are generally limited to the Red Sea and the Gulf of Aden. 4)  Also, the analysis of the position patterns of negative omega cores at the level of 500 hPa indicates the conditions of inflammation and severe thermal gradient in the atmospheric column over the southwestern lands of Iran during the autumn season compared to the winter.

Based on the designed patterns, the western troughs in the eastern Mediterranean deepen during the winter season compared to other seasons and they provide the following conditions for rainfall in southwest Iran: 1) The path of precipitation systems also reaches the southwest of Iran. 2) Stronger cores are formed from jet-stream in the southwest of Iran. 3) More water areas such as the Mediterranean Sea, the Red Sea, the Gulf of Aden and the Arabian Sea contribute to the provision of rainfall moisture.

Precipitation is a favorable phenomenon for human due to the water resources for agriculture, drinking water and health purposes. Because of the importance of precipitation, understanding the mechanism of this phenomenon in synoptic, dynamics and climatology has always been of interest for researchers. Although heavy rainfall is useful for water supply, it has destructive effects, such as floods. Heavy rains show the mechanism of precipitation more clearly because the pressure generating systems are more clearly detectable.    In this study, mean sea level pressure pattern of severe precipitation in southwestern Iran over a period of 30 years was identified and classified. Moreover, the area and time of research were defined, and using the Climate Prediction Center (CPC) precipitation data, the 30-years extreme precipitation with 99th percentiles was determined. There were 64 cases of extreme precipitation from 1989 to 2018 with 99th percentiles.  Classification of extreme precipitation systems based on mean sea level pressure field was performed with a spatial resolution of 2.5° data from the NCEP-NCAR center. Extreme precipitation in southwestern Iran at warm time of year was classified using factor analysis technique. In this study, it was shown that the pressure systems that cause extreme precipitation in southwestern Iran during the warm season are classified into five main patterns: (A) The first dominant pattern is created by the Red Sea trough and the low-pressure center of Saudi Arabia. It should be noted that in this pattern, the role of the Red Sea trough is more prominent than the low-pressure Saudi Arabia. In some cases, Saudi Arabia has formed along the Red Sea trough, reflecting the influence of the Red Sea on creation and development of the Saudi Arabian low-pressure center. (B) The second dominant pattern is formed by the-low pressure of Saudi Arabia and the low-pressure center in eastern Iran. The low-pressure center in the eastern part of Iran has an important role in the precipitation of Iran. It is present in all patterns, but first one. The presence of this low-pressure center has created atmospheric fronts in Iran. The rainfall region of this pattern shows that the cold front activity of this low-pressure center causes precipitation. (C) The third pattern is similar to the second one, but a high-pressure center located in the north and northwest of Iran. The presence of this high-pressure center indicates the cold air advection behind the low-pressure cold front of eastern Iran. (D) The low-pressure center of the eastern part of Iran and the high-pressure center in the north and northwest of Iran exist in the fourth pattern. The difference between this pattern and the third one is elimination of Saudi low-pressure center in this pattern. Therefore, the dynamic effects are similar to the third pattern and the extreme precipitation caused by this pattern is due to the cold front of the low-pressure system of the eastern part of Iran. (E) The fifth pattern, like the third one, consists of low-pressure systems in eastern Iran and Saudi Arabia. The difference between these patterns is presence of low-pressure of Cyprus in the fifth pattern. The low-pressure of Cyprus is cause of classical precipitation pattern (formation of the occluded front) in this pattern. The occluded front in this pattern has expanded from low-pressure eastern of Iran to low-pressure in Cyprus and converted into a secondary cold front over several stages of the frontogenesis process.  In this research, the southwestern precipitation systems of Iran during the warm season are classified into five patterns. The most important factor of precipitation in the southwestern part of Iran is the Red Sea trough with 48% frequency of occurrence. Therefore, it can be concluded that almost half of the extreme precipitation of southwestern Iran during the warm season is related to this area that extends from the Red Sea trough to the west of Iran.

Three species of Spatangoid Echinoid belonging to genus Douvillaster: D. douvillei Gauthier, 1902; D. longesulcatus Cotteau and Gauthier, 1895; D. valamtarensis Gauthier, 1902, and a species of Neitheidae Bivalve belonging to subgenus Neithea (Neithea): Neithea (Neithea) coquandi Peron, 1877, were described from the Aptian deposits (Dariyan Formation) at the Mish anticline in the Zagros basin, southwest of Iran. Douvillaster was recorded from the Aptian to the Turonian of the Tethyan margins (Middle East, Africa, Madagascar and Europe). The first appearance of Neithea in the North Pacific is in the Berriasian, and was continuously present until the late Albian and reached their maximum diversity in the late Aptian (this coincides with the Late Aptian global warming phase). The Neithea appeared in the Berriasian in the Tethyan realm (like to North Pacific region), thrived in the Mid-Cretaceous (Cenomanian), then decreased in diversity and finally became extinct at the end of Cretaceous.

Statistical studies on Palynology contents of Late Cretaceous to Paleocene age Gurpi Formation in a surface section in Zagros Basin, SW Iran indicate changes in abundance, species diversity, ratio of Spiniferites to Cyclonephelium (S/C), palynological marine index (PMI) values and organic facies. These palynological variations clearly reflect fluctuations in relative sea-level and depositional environment and can be used for recognition and differentiation of systems tracts and key horizons such as flooding surfaces and sequence boundaries. Dramatic increase in PMI, dinoflagellate species diversity and S/C ratio associated with sedimentary facies parameters indicate a marine transgressive systems tract while, a reduced species diversity, lower S/C ratio and PMI value along with increase in abundance of phytoclasts and degraded land-plant materials are characteristics of marine regression. The variation trends of the palynological parameters when consecutive indicate a complete cycle of relative sea-level change. Abundance of Spiniferites, Achomosphaera, Areoligera, and Cleistosphaeridium species and presence of peridiniacean dinoflagellate cysts show a warm Tethyan, upper bathyal to middle shelf environment of deposition for the Gurpi Formation. The nine depositional sequences identified here can be correlated to the global third-order sea-level cycles. The main regional sequence surfaces like 180 (maximum flooding surface) in the Late Cretaceous sequences (68Ma) in northeast of Arabian Plate are recognizable here.