جستجوی مقالات مرتبط با کلیدواژه "کم فشار سودانی" در نشریات گروه "جغرافیا"

  Although precipitation is of great importance in all climates, it plays a vital role in the arid and semi-arid regions. The spatial and temporal distribution of precipitation in all climates is affected by special synoptic structures in which one or two systems play the controlling role. The southern Iran is located adjacent to two important climate systems whose spatial arrangements determine the timing and amount of precipitation in the mentioned region. Therefore, it is important to study the possibility of predicting the drought and wet years in this geographical region of Iran according to its strategic role in the ecology, agriculture, industry, transportation, and politics. The study was conducted on Hormozgan, Boushehr, Kohkilouyeh-va-Boyerahmad, Chahar-mahal-va-Bakhtiari, and Khuzestan Provinces in Iran. The daily precipitations in the selected stations were extracted, harmonized and arranged in a 30 year statistical period. Then, the situation of each station was determined from the viewpoint of drought and humidity using the SIPA criterion and DIP software. We have selected the years in which intense wet years were half of the selected stations, based on the mentioned criteria. These years have been selected as the samples of intense wet years. The atmospheric data of the mentioned years were extracted from the website http://www.esrl.noaa.gov, and the daily maps of these years were created at levels of 1000 and 500 hPa in the longitude of-40 degree west to 100 degree east and the latitude of zero (the equator) to 80 degree north using Grads software. The Arabian subtropical high pressure nuclei were determined for all days and their maps were created as the output maps using ArcGIS10.3 software. The data were reproduced in a matrix with the dimensions of 67×2145 based on the daily precipitation of more than 5 millimeters. The study area, located between the latitudes 0 to 80 degree north and -40 to 100 degrees east, has a number of days according to the spatial data resolution which was 2.5×2.5 geographical degree. Afterwards, justification of the data distribution according to the special values, variance percentage and accumulation variance was determined for analysis of the factors. Just 12 factors had the values larger than 1 in the primary analysis. The principle component analysis and Varimax rotation showed that concentrating on the correlation of 13 factors can explain 89.18 percent of the pattern’s behavior.
Finally, the dominant patterns in the selected intense wet year samples were extracted through studying the maps of 1000 and 500 hPa from the twelve extracted factors. Then, we have analyzed the maps of subtropical jet stream, divergent and convergent flux, special moisture, temperature blow, and etc. Moreover, the maps of different levels in all rainy days of the intense wet years were reviewed. Comparison of the repetitive patterns resulting from the review and the principle factor analysis provided similar results. Our study showed that in all the rainy days, the central nucleus of the Arabian anticyclone cell at all levels of 850 and 700 hPa was located in the east part of the longitude 45 degree E. When the anticyclone central nucleus is located in the east part of the longitude 55 degree E, the precipitations are more extensive, and cover all the regions from Khuzestan to Hormozgan. As it can be seen, in the rainy days, one or two divergent flux nuclei are located on Oman Sea or western Arab Sea and Gulf of Aden. In such condition, the streams in the lower levels of troposphere (from sea level to 850 hPa) are changed into eastern streams in northern Oman Sea and gradually on the Arab Sea. This condition is the most suitable mechanism for moisture advection towards the Sudan low pressure. In the same day, two strong nuclei of convergent moisture flux are dominated on Ethiopia and central Arabia which receive the moisture transmitted from the warm seas. The highest moisture advection towards the Sudan system is done under the layer of 850 hPa from Arab Sea, Oman Sea and the Gulf of Aden. Because of the topographic condition at levels higher than 850 hPa, this advection comes much lower, and the moisture transmitted from the transition branch may add to this moisture from the tropical convergence region. With the moisture advection in the lower levels, proper thermodynamic condition is provided for the development of convection clouds on the region. These clouds initially appear as mass clouds on Sudan and Red Sea, and then on Arabia, and then gradually move to Iran through the southern streams before trough at levels of 850 hPa and higher. Then, the clouds grow and extend through involving in the upward streams dominated on the front trough and under the subtropical jet stream, which are located on Red Sea and northwest Arabia. The proper temperature blow and diabatic warming resulting from condensation process provide a severe condition in the northern part of the jet. In order to have an intense wet year in the south and south west of Iran, the eastern movement of Arabian high pressure is considered as an important factor. With the eastern movement of this high pressure, a proper synoptic condition for advection of moisture toward the precipitation system is provided. A proper condition is also provided for the extension of the Mediterranean trough towards the southern latitudes on the south east part of African desert and cold advection on the region at the middle and upper layers of troposphere. The Arabian high pressure has very high ability for wet condition, especially at the lower layers, because of its dynamic structure. Therefore, the moisture moved through the divergent flux toward the southern systems is considerable and provides significant potential energy for the convection systems. The cold advection from the northern latitudes and the warm advection from the subtropical latitudes provide proper heat gradient for intensification of the subtropical streams in the northwest domain of the Arabian high pressure. These jet streams are formed in the limits of northeast Arabia and provide proper dynamic condition for intensification of intense convection streams in the Northern Arabia and Southern Iran. The convection clouds cause intense precipitations on the region because of the access it has to the moisture of the southern warm seas and also the moisture moved from the northern branch of the tropical convergence region

In order to explain the fundamental interaction of the atmosphere and surface environment and the discovery of the circulation patterns cause heavy precipitation in Khorasan Razavi 29/03/2007 AD precipitation index was chosen as one day. Output of cell-based database for data loss is the main focus of the day Aphrodite Dargaz East was estimated at 43.66 mm. In order to determine of the front synoptic patterns forming, functions: moisture flux, volubility, maps, ground level pressure and levels of 500, 700 and 850 for 6 hours H.p NCEP/NCAR base obtained were analyzed with environmental approach to circulating. The results showed that the pattern of occurrence heavy precipitation trough atmosphere when it landed deep in the Mediterranean axis in the East Mediterranean and the Red Sea also serves as the main source of precipitations. During the occurrence of such circumstances, a barrier system will be deployed on the northern Iran South branch system to the west of the Arabian Sea and the Persian Gulf to the western regions of the country to be drawn. Under these conditions the moisture in areas of Sudan and the Red Sea is high and how the wind is such that moisture is sucked from the Red Sea and Arab and western parts of Iran guides. The initial formation of the mentioned systems in the West - East, but after entering Iran will shift its path towards the North East. Mentioned is a confirmation of the mechanism of satellite imagery as well.
The general direction of the two mentioned systems formed at the beginning of the West - East, but after entering Iran will change their route to the North East. Satellite images, as well as confirmation of the mechanism are mentioned.

negative and harmful actions damaged by natural disasters caused by climate change has been struggling.
The most frequent and severe thunderstorms this type of risk is that a lot of products each year Agricultural production facility construction, many areas of the world, causing human casualties And every year in our country's strength and weakness that we experience with this phenomenon. Kohgiloyeh due to the mountainous area of opportunity for the intensification and expansion of thunder storms is. n this study, severe thunder storms to explore Kohgiluyeh Boyer Ahmad, the code related to the storm Tndrydr period 1990 to 2010 the country was extracted Meteorological Organization. The sea level pressure data, geopotential height, humidity and omega of databases NCEP / NCAR downloaded software were traced GRADS. Finally, 70 of thunderstorms to investigate the source of moisture and synoptic factors causing these storms were selected and evaluated. The most severe cases were selected according to the criteria described in full detail. The results showed that the cause of the Thunder, the Sudanese system. Due to potential high humidity (due to the formation of the next hot water on the equator) and receive ample moisture from the warm seas of Oman and the Arabian and Red Sea are high potential energy. As a result of the release of considerable heat Badrrvy severe thunder storms on the province. It should be noted that the topography of the region in strengthening and intensification of these storms has been ineffective.

Variability and changes in precipitation amounts, including extreme precipitation and droughts, trigger relevant societal and environmental impacts. Atmospheric circulation strongly determines precipitation variability over southern half of Iran. The influence of prevalent synoptic situations on the distribution of precipitation amounts needs to be understood, as well as their frequency changes to appraise the impact of atmospheric circulation on intra- and inter-annual (decadal) fluctuations of precipitation amounts. On the other hand, the atmospheric moisture budget plays important role in the hydrology of this region. The aridity in the southwest and south of Iran is due to subsidence scale caused by Hadley circulation and a location far from oceanic sources of moisture. However the moisture and other properties of atmospheric quantities are transferred by local circulation from another source to the given region. Many investigations have been carried out on this subject. The consideration of the precipitation and its source region of moisture flux over south and southwest of Iran, during 1970-74 has been shown that 23 percent of the total low pressure system which passed over this area are from the southwest of the Red Sea (Faraji, 1982). In the investigation of moisture flux over Iran, which is carried out by Alijani (1995) noted that the Mediterranean Sea is the main source of moisture for precipitation over Iran. Evans, Smith & Oglesby (2004) have shown that the Persian Golf and the Caspian Sea are the main sources of moisture for precipitation over the Alborz and the Zagros mountains. They have shown that the subtropical jet current brings the warm and moist air from the South part of Saudi Arabia and the Aden golf to the Middle East in the middle and higher troposphere (Dayan &Abramski, 1983). The Red sea inverted trough at the surface with regard to its amplitude, is the Major source that transfer warm and moist air from the Aden golf to the East and North of the Arabian peninsula and also southwest and south of Iran (Alpert, 2001). The low pressure system which is formed at southwest of the Red sea is associated with high potential acceptability of moisture and warm air due to its characteristics of dynamical and thermo dynamical pressure for heavy rainfall over the study area. Rurerde (2006) using specific humidity map and its extension from the Caspian Sea to the North part of Iran derived a value of moisture quantities from each source region of moisture to the precipitation over Iran. The principal objective of this research was to understand the characteristics of moisture flux from each region of moisture source and their contribution to the rainfall during the above period.
Study Area : The study area for this study is southern half of Iran, which lies approximately between 25N and 35N in latitude and between 44_E and 64_E in longitude. The study area of this research contains some provinces such as: Ilam, Khozestan, Lorestan, CharmahalBakhtiyari, Kohkiloye and Boyerahmad, Bushehr, Esfehan, Yazd, Kerman, Hormozgan, Fars, and Khorasan Jonobi. Based on the Koppen climate classification, most parts of this area are categorized as generally having arid (BW) and semi-arid (BS) climates. The important mountain of this area is the Zagros, which play an important role in nonuniform spatial and temporal distribution of precipitation.
Material and Examination of precipitation properties needs long and high quality records of data. In the present study, the time series of precipitation data at 183 stations for the period 1974-2004 were collected from the Islamic Republic of Iran meteorological organization (IRIMO) and were analyzed. Data homogeneity is assessed by IRIMO previously. The Empirical orthogonal function (EOF) was applied to detect and describe spatial and temporal change in the precipitation. In addition, the principal component (PC) was applied to detect dry and wet years. In addition, to identify the spatial and temporal variations of the precipitation, we used the NCEP-NCAR reanalysis monthly mean values of the temperature, Geopotential height and pressure in 1000, 850, 500 and 300 hPa levels. These data have been widely used by many researchers over the last few years in tropical climate research. Finally, we calculated 30 years anomaly for these levels in dry and wet years.
The maps of the wettest months have shown the synoptic situation over study and its adjacent area during the above period. These figures have show the high pressure over the Oman and the Arabian seas, deep trough over the East of the Mediterranean and the west of the Red sea. In this case, eastern Saudi Arabia received more moisture and latent heat fluxes than the other parts of the area. Two moisture source regions as observed from the divergent flux, which are located over the Aden golf and Red sea, supply moisture to study region. Southern half of Iran receives precipitation during this study, mainly from the west of the Indian Ocean (the Arabian Sea, the Oman Sea, The Aden golf), the Red sea and the some extent from the Mediterranean. The divergent moisture and latent heat flux during wet cases were observed over north part of the Arabian Sea, the Aden golf, the central part of the Red sea and convergence was observed over South, central of the Arabian Peninsula, southern half of Iran. These processes are indicative of the upward motion of moist air, which in turn may release latent heat due to condensation. This available heat energy may be the source of low-level latent heat instability. The large amount of moisture and latent heat flux over Saudi Arabia in is transported, by rotational wind from the Red sea and the Mediterranean as mentioned above. Conversely, in dry years due to weak trough of East Mediterranean, systems move from latitudes upper than 40ºN and the cold air does not transfer from the southeast Europe to the Northeast of Africa resulting development of Sudan low. Overall, decreasing transfer of cold air will lead to decreasing of moisture and latent heat flux from the Indian Ocean to the southern half of Iran.
Atmospheric circulation strongly modulates precipitation patterns. Precipitation is one of the most important atmospheric variables in the global hydrological cycle and plays a key role in the Earth’s energy balance.The investigation of synoptic maps in 1000, 850 and 500 hPa levels in four systems shows that in wet years due to deep trough of East Mediterranean and West of the Red Sea, the cold air transfers from the southeast Europe to the Northeast of Africa resulting development of Sudan low. This low transfer to the center and north of the Arabian Peninsula, then move to the Southern half of Iran. The development of this low is due to release of sensible heat from the Arabian Desert and latent heat flux which transfer from the Indian Ocean, to this area. This low affects the southern half of Iran with significant value of perceptible water and low- level latent instability. It is seen that the transfer of moisture flux in the low-layer from the Indian Ocean plays an important role in precipitation over study region. Conversely, in dry years due to weak trough of the East Mesiterranean, systems move from latitudes upper than 40ºN and the cold air does not transfer from the southeast Europe to the Northeast of Africa resulting development of Sudan low. Overall, decreasing transfer of cold air will lead to decreasing of moisture and latent heat flux from the Indian Ocean to the southern half of Iran. The findings of this study confirmed the results of the previous studies (Alijani, 1995; Farajzadeh, 2007;Milind, 2006; Mofidi&Zarin, 2005).

The Recognition of behaviors of the atmospheric circulation patterns and effective climatic elements in occurrence of flood is very important، especially because abnormal climate changes such as global warming in recent years tend to change in atmospheric circulation patterns and outbreak climatic inelegances in many areas in the world. Intensive and torrential precipitations in anomalous time and space is very important for a country like Iran with specific climatic characteristics and is always among the factors which in different geographical regions has some irrecoverable effects on humans life and finance and has affected human beings and nature widely. The south-west region of Iran that has been located in the windward slops of Zagros mountains and in the path of westerlies and rainbearing air masses، is among the regions which due to different atmospheric circulation patterns and receives heavy precipitations and is exposed to destructive floods. That flood of 20 November 2011 which has occurred in Khuzestan and Kohkilou-e-va-buyer-Ahmad provinces is obvious among them. This flood imposed many death tolls and caused a big mass of financial damages to those regions. Coordination before occurring accidents and hazards is the key factor in crisis management. Therefore identification of flood causing patterns and their punctual prediction can facilitate better management of crises and hazards and minimize mortality and financial damages caused by these type of natural disasters. This flood in the study area created by an extreme storm in western and southwestern regions of Iran. In order to analyze the synoptic pattern of this flood، the data of sea level pressures، surface wind directions، surface temperatures، geopotential heights of 500 Hpa and specific humidity and wind directions of 850 and 700 hpa and data representing omega values from NCEP/NCAR website has been collected and then synoptic maps were drawn by using GrADS software which were related to 2 days before flood until a day after that. Then the causes of flood were revealed by synoptic analysis of the maps. Analysis of synoptic maps reveals role of some systems in creating the flood. By positioning the pressure center of Sudan low between European high pressure and Arabian subtropical high pressure has been resulted to form a convergence belt over the Sudan and the Red sea. European high pressure caused the advection of cold air from high latitudes and in other hand، Arabian subtropical high pressure caused to advection of warm and humid air from low latitudes into the Sudan low - Red sea’s convergence belt. Convergence of northern cold air and southern warm air in this belt، tend to intensify the belt. Gradually، with intensification of the system and its NE move، the convergence belt lied over SW regions of Iran. Pressure map of 500 hp shows a deep trough in North Africa. This trough intensifies temperature gradient in upper levels of the atmosphere and following of this event، the trough becomes deeper and its axis lies over the Red sea. Analysis of specific humidity and wind direction maps of 850 and 700 hpa indicated that the humidity sources for the rainfall systems were the Red sea، Arabian Sea and Persian Gulf in 850 hp level and Red sea and Persian Gulf in 700 hp level. Locating Sudan low pressure between Arabian subtropical high pressure and European high pressure tends to create an air convergence belt with SW-NE trend from Sudan to north of Arabian Peninsula. Cold Advection from European high pressure and warm advection from Arabian high pressure finally intensify temperature and pressure gradients in the convergence belt and the transformed the system into a dynamic system. In result of advection of cold air to back part of the system، its eastward movement became faster and pull the system over SW areas of Iran. Movement of cold air in back part of the system caused an extreme updraft of air and generated a severe rainfall. The Arabian subtropical high pressure provided required humidity for this rainfall from warm waters of Red sea and، Arabian Sea and Persian Gulf into the system and eventually resulted in a heavy rainfall that in a short time generated the disastrous flood with abundant damages in study area.

In this research large scale dynamic and thermodynamic anomalies during storm events resulted from cyclogenesis in the Mediterranean Sea and Sudan low-pressure systems، are investigated. 5 severe storms that happened in KarunBasin (1998-2008) were selected and using Japanese Re-analysis data (JRA25)، anomalies of dynamic and thermodynamic indices، 2 days prior to the start of the storm until the end of the storm were analyzed. The selected indices in this research are potential vorticity، convergence and divergence، vertical velocity، absolute vorticity advection، specific humidity، moisture flux، potential temperature and equivalent potential temperature. According to the results and comparing 6-hourly recorded rainfall amounts، it was found out that in the reference events، before the start of the storm، geopotential height values، in the under-study region decreased and at the time of maximum rainfalls، the geopotential height reached to its lowest values and by end of the storm، the values started increasing، whereas parameters related to convergence and vertical movements، such as potential vorticity، vorticity advection، moisture flux، convergence of moisture and specific humidity amounts corresponded to the same trend of rainfall from the beginning to the end. It is obvious that none of these indices can individually cause the occurrence of a storm، but by analyzing trends and regressions، it seems that there are meaningful relationships between geopotential height، moisture advection and potential vorticity and rainfall amounts which can be used in forecasting future rainfall events. To verify the results obtained from the reference events، 2 days without rainfall at least 2 days before and after the selected days، were also selected and studied. The results verify considerable changes of the selected dynamic and thermodynamic indices during stormy days compared to the days without any rainfall in the region.

This study aimed to identify the cause of intense rainfall occurred on April 30, 1994 in Kermanshah, The maps of surface pressure in the upper atmosphere Sudan and low pressure systems result in precipitation patterns were identified The role of this system as one of the major systems of the West was to climax. Maps and atmospheric pressure at the upper and lower levels, it was determined that Starting with the establishment of precipitation over the Mediterranean trough Turkey and Iraq and the location of the study area at the front of the upper levels, Sudan is associated with low pressure at the surface occurs. There is a high pressure system over the Arabian Peninsula and North-West Indian Ocean This system leads to amplification. Omega plans and volubility of the story of instabilities weather is rainy day. The moisture content of the maps, Most moisture levels 850 hPa and the Mediterranean Sea to be supplied the region is imported from the West. But the 700 hPa level is the main source of water that flows from the Red Sea to the south west of the study area appears.

Heavy rains are one of the features of arid and semi arid climates which result in flood. This kind of rainfall originates from environmental and synoptic conditions. Mediterranean cyclones are the major factor in heavy rainfall in Iran، but these cyclones do not happen in some parts of Iran such as Southern and Southeastern areas. In this study، it has been tried to pinpoint the synoptic reasons of heavy rainfall in Isfahan through the analysis of the relationship between this rainfall in Isfahan and atmospheric system over Iran and the areas around it. The findings of this study show that the major factor is the arrival of Sudanese low pressure system in this region from the southwest، of course if the ascent local conditions such as heat occur، the heaviest rains happen in Isfahan. In fact، this kind of rainfall in Isfahan has a Sudanese origin and if it is accompanied by Mediterranean system، heavier rain falls.

Sudan Systems are low-pressure systems which originate from the Red Sea and their low-pressure isoheights are detectable around south of the Red Sea، Sudan and Ethiopia. The system، in cold seasons passes Ethiopia، Sudan and Red Sea and reaches to west of Iran and results in severe rainfalls in northwest، west and southwest of Iran. In all these synoptic-dynamic systems، low-pressure isobars or low isoheights are considerable. 11 severe storms which happened in southwest of Iran (1998-2008) are selected and using Japanese ReAnalysises data (JRA25)، anomalies of dynamic and thermodynamic indices، 2 days prior to the start of storm and during stormy days have been analyzed. The selected indices in this research are relative vorticity، convergence and divergence، vorticity advection، specific humidity and moisture advection. According to the results of this research and analyzing 6-hourly recorded rainfall amounts، it is understood that in the reference events، before the start of the storm، geopotential height values in the under-studied region decrease and at the time of maximum rainfalls، the geopotential height reaches its lowest values and by end of the storm، the values start increasing.; whereas parameters which are related to convergence and vertical movements، such as relative vorticity، moisture advection، convergence of moisture and specific humidity amounts correspond to the same trend of rainfall from the beginning to the end.

Precipitation is one of the most complicated atmospheric elements having a lot of spatial and temporal variability. Specifying its characteristics and humidity sources, makes it possible to predict and understand this atmospheric element in a better and deeper way. The Red Sea trough is a surface low pressure which is extended over the Red Sea. This trough is usually associated with dry and warm weather. Simultaneous existence of a trough in middle level with surface trough can cause heavy precipitations and showery weather in east of Mediterranean and also in Iran. To verify the effect of the Red Sea trough on the precipitation of Iran, firstly the Iran precipitation data for a specific period (1/1/1340 to 10/10/1383 solar) were prepared. Secondly the Red sea trough for this period was also identified. Cluster analysis of the red sea trough showed that this trough has three main patterns. Then precipitation increment percent for each month of the year for every pattern of the Red sea trough was calculated and their maps were drawn too. This work was done for each day of the year as well.

Heavy rains are one of the features of arid and semi-arid climates which result in flood. This kind of rainfall originates from environmental and synoptic conditions. Mediterranean cyclones are the major factors in heavy rainfall in Iran, but these cyclones do not happen in some parts of Iran such as Southern and Southeastern areas. In this study, it has been tried to pinpoint the synoptic reasons of heavy rainfall in Southern regions of Bushehr province through the analysis of the relationship between this rainfall in Bushehr province and atmospheric system over Iran and the areas around it. The findings of this study show that the major cause of heavy and torrential rains is the arrival of Sudanese low pressure system in this region from the South and Southwest. Albeit, if the ascent local conditions such as heat occur, the heaviest rains happen in Southern regions of Bushehr province. In fact, this kind of rainfall in this area has a Sudanese origin and if it is accompanied by Mediterranean system, heavier rain falls.

In order to investigate some synoptic aspects of Sudan low systems such asupper and lower level tropospheric circulation pattern and moisture source and theiradvection way, the synoptic pattern on the occurrence time of December 2001 stormover Iran has been noticed. Therefore the maps of surface, 500, 200, 50hpa levelsbefore and after storm were studied. In order to understand the situation of subtropicaljet stream and how it strengthens, maps of zonal wind component and velocitypotential (x) of 200hpa level have investigated. Also, satellite images of cloud form ofstorm have been studied.The results showed that although like previous studies, there was low cycle indexand existence of a ridge in mid-troposphere over the west Mediterranean and a deeptrough over the east Mediterranean had important role on the formation and evolutionof Sudan low; but the main role belonged to the stretching of polar vortex in lowerstratosphere as the main source of vorticity for abnormal circulation and strengtheningof Hadley cell and Subtropical jet stream over Mediterranean and north of Africa.The results indicated that the suitable position of Subtropical jet stream core overMiddle East associated with the settlement and strengthening of a ridge in middle andlower troposphere over the Arabian Sea are necessary for the evolution and developingof Sudan lows.However, the position and speed of subtropical jet stream core over the MiddleEast and it’s axis direction in the upper troposphere control both the mid-troposphericcirculation pattern and the tracks of incoming Sudan lows to Iran.The consequences indicated that settlements and strengthening of ridge/high over theArabian Sea, while making a set positive feedback and subsequently intensify Hadleycell and strengthening subtropical jet stream over Red Sea region, lend to directtransportation of moisture from the Arabian Sea into Sudan lows.Moreover, it was found that the main sources of moisture of Sudan systems aretropical Eastern Africa and southwest sector of Arabian Sea which strengthening whenthey pass over the Read Sea and Persian Gulf.