Introduction: Precipitation is one of the most important atmospheric elements in any climate, and the world climate is categorized on the basis of this climatic element. In some climates, precipitation occurs in all seasons while, in others, precipitation only occurs in cold seasons; in yet other climates, it occurs during warm seasons. In most regions that are adjacent to sub- tropical high pressure systems, precipitation occurs only at specific periods of the year. All regions that are located to the North of this system have precipitation during cold seasons although, in practice, the beginning and end of precipitation is not fixed. Sometimes, periods of precipitation occur and, in some years, end much later or sooner than normal. Therefore, in some years, the precipitation period is very short. In the south and southwest of Iran, the period of precipitation and cultivation coincide. Therefore, in this climate region, periods of precipitation are used directly by plants and agriculture products. When the precipitation period is shorter than normal, a part of the plants’ water needs is not provided, and the water resources of the region are influenced intensively by this fact.Materials and Methods: In this research, first, the daily precipitation data of synoptic stations in South and Southwest Iran (including Provinces such as Khuzestan, Kohkilooye- boyerahmad, Lorestan, Busheher, Hormozghan, Chaharmahal va Bakhtiari, Fars and Ilam) over a 36-year period were extracted. In the next step, the start and finish of precipitation periods were determined according to agricultural years in Iran which begin in October. In order to determine the length of the precipitation period in the stations of the south and southwest of Iran the days between start and end of precipitation were calculated. Then, those years in which the precipitation period length was less than 160 days were analyzed as short periods of precipitation. Figure 2 shows the condition of the years investigated in respect of the shortest precipitation period length. Then maps at levels of 700 and 850 HPa were produced for all selected samples from ECMWF data with a resolution of 0.25*0.25 using a scrip in Grads software. The locations of daily cells of sub- tropical high pressure were identified in the selected sample and mapped using ARDGIS10.3 software. The basic component analysis method was used for identifying the pattern of the shortest precipitation period length. Applying basic components analysis to the sea level pressure data led to omission of the patterns with very low repeatability, and patterns having higher repeatability were classified. In this research, the first fifteen components of sea level pressure with 0.934 percent total variance were justified. Finally, the topographic maps an dsubtropical jet stream for the selected components at levels including 700, 850, 1000, 500, 250 and 300 were analyzed.Results and Discussion: The investigations were conducted on the central cores of the Saudi Arabia high pressure cell in November as the beginning month of precipitation, and March as the end month of precipitation in years with a short precipitation period; these showed that even in November, which was the second precipitation month in the region, the high pressure central core did not have suitable eastward and southward movements. This synoptic pattern caused a situation that even in the second precipitation month, the Saudi Arabian subtropical high pressure system prevents the entrance of Sudanese low pressure, as the most important precipitation system in the region, into the southern and south-western regions of Iran. Meanwhile, the westward movement of high pressure caused a situation where the Mediterranean trough did not extend to lower latitudes. Therefore, the Mediterranean system cannot enter the region. The location of the central core of Saudi Arabian subtropical high pressure showed that the high pressure central cores had earlier westward movement than in other years while, in March, the high pressure nucleus should be located in the East of Saudi Arabia and on the Arabian Sea and Sea of Oman. This westward movement caused a situation whereby Sudanese and Mediterranean low pressure exited the precipitation route of the region earlier than normal; in other words, the precipitation stopped sooner than usual. In these years, the main controlling system in the region was the Siberian high pressure system. During the short period precipitation years, the southern ridge of the Siberian high pressure system in combination with Saudi Arabian high pressure have had a significant southward extension, so that it is extended to the South of the Saudi Arabian peninsula and sometimes to Ethiopia at the lower levels of the atmosphere. In such situations, the Sudanese low cannot enter South and Southwest Iran through its normal routes. As a result, the Sudanese system moves to the West, and enters the eastern Mediterranean with a northward movement and, passing over Sudan and Egypt, and combines with Mediterranean cyclones. In this situation, precipitation occurs later than usual.Conclusion: Saudi Arabian subtropical high pressure plays a fundamental role in the beginning and ending of precipitation periods in the South and Southwest of Iran. With regard to the yearly movement of this high pressure (westward and northward movement during warm periods of the year, and southward and eastward movements during cold periods of the year), it plays a determining role in the beginning and ending of precipitation in this region. For the entrance of the Sudanese system into the south and southwest of Iran, this high pressure system should have a southward movement in order to leave this region and have an eastward movement to provide the necessary conditions for its entrance into this region. But it is observed that in years when the precipitation period in this region is short, the aforementioned system leaves Iran much later, and it has a low eastward movement.

Introduction: According to the definition of World Meteorology Organization (WMO), as the speed of the air mass is more than 30 meters per second, the jetstream will arise. The core of the jetstream is with baroclinic atmosphere due to the large difference in temperature and wind speed. There are two west jetstreams in the northern hemisphere. The northern jet stream is called polar front jetstream and the southern is called subtropical jetstream. Polar Front Jet Stream produce intense temperature gradient by polar air mass and tropical Polar Front Jet Stream is produced by strong temperature gradient of polar and subtropical air. The subtropical jetstream is produced by temperature gradient in tropopause as limited to the upper troposphere.

Introduction: Zayanderood basin is one of the major internal river basins in the central Iran, where Zayanderood River is the only permanent river. Fluctuations in rainfall in recent years have affected different areas of Iran. Because of the dependence of the provinces of Isfahan, Yazd, and Kerman on water resources of this basin, changes and fluctuations in precipitation within this watershed have had many social and economic consequences with bad outcomes in regional, national and international dimensions. Therefore, knowledge of mechanisms governing the procurement and availability of water resources in the basin can be helpful to develop strategies to deal with social and economic stresses. This can happen with more confidence, planning and proper management of water resources in the basin.

Introduction 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. Materials and methods 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. Results and discussion 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. Conclusion 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.

Iran's location in the latitude of 25 to 40º N in the northern hemisphere has made the subtropical jet stream a factor in regulating humidity systems in Iran, making it easy for humidity systems to enter the country when this jet stream is in Southern Iran. But as it moves northward, its strength decreases. In recent years, it has been reported that the positioning of subtropical jet streams in the Northern Hemisphere is shifting. Therefore, the purpose of this study is to evaluate the position of subtropical jet stream location and its variability over Iran. In this study, data on zoning wind velocity ranged between 30 and 80º E in the Northern Hemisphere, at levels between 1000 and 10 hPa from NOAA, as well as outputs of circulation models including CanESM2 and GFDLCM3 for the historical period 1948 to 2005 and periods Future from 2006 to 2100 were received from IPCC in two scenarios RCP4. 5 and RCP8. 5. In this study, the main components of the jet stream include the central core velocity of the jet stream and its latitude position. Investigation of the position and velocity of the jet stream indicates that the subtropical jet position changes in Iran and its eastern regions are followed by significant incremental changes. Whereas in west Iran, there is a significant decline in jet stream velocity changes. The future of Jet Stream positioning in Iran based on the CanESM2 and GFDL-CM3 climate models in both rcp4. 5 and rcp8. 5 scenarios indicates that relative to the base period in both scenarios as well as the near and far future of its position to the north Moves.

Introduction: A season is a certain time of a year that is distinguished by alternation in timing and intensity of the solar radiation and atmospheric conditions (Zolfaghari, 2005). A natural season is a certain time of the year which can be separated by homogeneous weather type (Alsop, 1989)…

Narrow, rapidly flowing currents of air located near the tropopause are known as jet streams. These jets, often found nearly girdling the globe while exhibiting large meridional meanders, are among the most ubiquitous structural characteristics of Earth’s atmosphere and are known to play a substantial role in the production of sensible weather in the mid-latitudes. Jet streams are classified into two different types subtropical jet and polar-front jet streams. Subtropical jets are driven by angular momentum transport from the tropics and are centered at the poleward boundaries of the Hadley cell and polar-front jets are driven by baroclinic eddies and are associated with weather and climate events, such as precipitation and cold wave processes.To study whether jet streams have been changing in the past decades, we used the historical data of NCEP/NCAR from the National Centers for Environmental Protection and the National Center for Atmospheric Research, covering 1948 to 2023.We have used the daily means of u, v components, temperature and geopotential height, at 6 levels from 400 to 100 hPa and daily means temperature at 4 levels from 1000 to 700 hPa.In this paper, we firstly calculated the wind speed index, pressure index and latitude index that were defined by Archer and Caldeira (2008) to characterize the strength, the pressure level, and the latitudinal position of a jet stream, respectively. Then after that the variability of the characteristics of jet streams of both hemispheres in warm and cold seasons were investigated.In addition, the temperature trend in the upper and lower troposphere was determined by calculating the average seasonal temperatures over latitudinal zones 0°-15°, 15°-30°, 30°-45°, 45°-60°, 60°-75°, 75°-90° in northern and southern hemisphere at upper levels of 400, 300, 250, 200, 150, 100 hPa and lower levels of 1000, 925, 850, 700 hPa.We found that, in general, the jet streams have moved poleward in both hemispheres so that the shift of subtropical jets in the northern hemisphere was 0.061 (0.019) °/decade in DJF poleward (JJA) and in the southern hemisphere was 0.307 (0.223) °/decade in DJF (JJA) for 1948-2023. Also in the southern hemisphere, the subtropical jet in winter and summer is weakening but the polar front jet is strengthening. In the northern hemisphere, in both warm and cold seasons the trend of wind speed is increasing in the poleward side of the jet axis, and is decreasing in the southern side of its axis, while the trend of the maximum wind speed is upward. One of the important factors of these changes can be related to the upward temperature trend in the tropical and subtropical regions of both hemispheres.In this statistical period, the smallness of the slope of the zonal mean of seasonal temperature trend of the lower levels of the tropical region, compared to the slope of the average temperature trend of the upper levels of the subtropical region, has caused strengthening of the Hadley cell convection which is less than the weakening of its meridional circulation range. This could be one of the factors that lowered the trend of the jet streams speed in both hemispheres. Also, the results show that the mass-flux weighted pressure in the DJF season is minimum around the equator and is minimum in the JJA season around the orbit of 12°N from the west of Mauritania to the southeast of China.

in this research, their effects on the flight of airplanes were investigated. The study area is the country of Iran, and the flight routes of Kermanshah, Ahvaz and BandarAbbas to Tehran. The research data includes maps of the Vertical Transect (profile) of the jet stream, the daily average of the Zonal wind (U-Wind) and meridional wind (V-Wind) components for the winter period of 2018 through NOAA/NCEP environmental databases. Also, flight route information was received from FlightRadar24 and Flightaware systems. First, by using Vertical Transect maps, the days containing strong U-Wind were extracted, and the average position of the core of the Jet Streams in the Zonal and meridional wind components, the Tropospheric level of 200 HP was detected. The list of flights was prepared, and the Zonal Wind maps were produced. Finally, the height of the flights was compared with the level of the currents of the Jet Streams, and the influence or lack of influence of the Jet Streams on the flights was studied. According to the results of the research, all the Jet Streams caused turbulence for all flights, and they caused a decrease in the speed of flights between Ahvaz and BandarAbbas to Tehran and an increase in the speed of flights between Kermanshah and Tehran according to the direction and type of Jet Streams. It was also found that all the Jet Streams had a speed of more than 90 knots, so the capacity to create tension and turbulence such as CAT was seen in them

Introduction One of the main elements of the general circulation in mid latitudes are the fast and narrow flow maxmia called jet streams whose speed is usually more than 30 meters per second. They are one of the dominant features of upper level weather maps but changing through time, space and layers of atmosphere. The jetstreams cause vertical motion underneath through which produce stability and instability over the earth surface. It should be mentioned that they are much known for their instability production. Jet cores are one of the main components of the general circulation and their location and displacement are controlled by the elements of the circulation such as the Arctic Oscillation. According to the researches, the location of the jet streams is very important in climate events. Therefore, this research tried to identify and present the location and speed of jet streams in Middle East. There was no comprehensive study in this scale so far. Materials and methods In order to study the jet streams, the six hourly (00, 06, 12 and 18 GMT) speed of U and V components of the winds at the 700, 600, 500, 400, and 300 hPa levels were obtained for the window of 0 to 120 E and 0 to 80 N during 1965-2014 period. In total, 20 time series were produced from the combination of these hour and level scales for each pixel with the size of 2. 5 by 2. 5 degrees. In each time series the wind maxima of 30 m/s and higher were extracted. At the final stage, the mean monthly speed and monthly frequency of jet streams of all pixels were mapped for the study area. Results and discussion The frequency and speed of jet cores were mapped and are described here in monthly, seasonal and annual scales. According to annual frequency of jet core locations, the highest speed maxima in 300 hPa level are located over the North Africa in 40 percent of times, depicting the main track of the jet cores. Given the fact that jet cores enter the study region only in the cold period of year, this temporal frequency of 40 percent is not a low value. The location of speed maxima is the same in 500 hPa level but in about 20 percent of the time. This low value is reasonable for this level. As we know, the synoptic systems of this level control most of the time the weather and climate of the surface and these surface systems are not very frequent during a normal year. In this level, the bifurcation of westerlies is obvious. The southern branch is very influential in the Middle East. This bifurcation indicates the presence of a blocking high in the region, which most of the times prevent the entering of jet cores and hence an unstable conditions over the region. There are no speed maxima in the region in 700 hpa level. Since in the annual scale only in the 500 and 300 hPa levels showed the jet cores, in the seasonal scale we look only for these levels. The frequency of jet cores in winter is more than the other seasons and show two separate belts. The jet core affecting the climate of Iran is passing over the Persian Gulf which extends from North Africa towards China. Jet cores pass through this belt all of the winter season. However, their frequency decreases toward the north over Iran and from there extends toward the other maxima over the latitude 50 N to 60 N. The jet frequencies have decreased in spring reaching to about 52 percent over the area (Figure 3B). This rate of decrease indicates the sudden and rapid change from winter to the spring conditions over Iran. Its northward shift is also obvious over Iran. In this season, the jet maxima travel in a ridge like track over Iran. It indicates an unstable weather over the west part of Iran while dominating the stable conditions over the east part of the country. Conclusion Jet streams are very important instability factors in the atmosphere. Their spatial location and speeds control the tracks of pressure systems and surface climate. For this reason, this study tried to understand their speed and spatial variations in the Middle East. Their speed and frequencies were studied at the pressure levels from 700 hPa to 300 hPa in the monthly, seasonal and annual scales. The result showed that in all levels and scales, two speed maxima tracks were established over the latitude belts of 20N – 30N and 50N – 60N which is confirmed by findings of Li et al (2004); Zhang et al ( 2006); Li and Wettstein( 2012) and Pena-Ortiz et al (2013). The most important findings of the research is the coincidence of speed maxima with frequency maxima of jet cores in these latitude belts. During the warm period, both the frequency and speed of the jet cores decreased over the Middle East. The jet cores were absent at the levels lower than 500 hPa, and in 300 hPa all winds experienced speeds higher than jet threshold which is in accordance with Geer et al (1996). In brief, we can conclude that through the fall season the jet cores move southward to dominate over the Middle East and Iran during winter. Through the spring season, they began backward movement to the northern latitudes so that in May no jet core could be found in the area. This research demonstrated the importance of the wind patterns of 500 hPa on the climate and weather conditions of the Middle East as well as Iran.