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Ali Bayat, Mohammad Saligheh, Mehri Akbari,
Volume 4, Issue 2 (7-2017)
Volume 4, Issue 2 (7-2017)
Abstract
Cyclones as the most important factor in the turbulence of mid-latitude regions are low pressure centers that have one or more closed Isobaric curves. Mid-latitude cyclones are responsible for transfer of the heat and moisture between the tropics and the polar regions. Mediterranean cyclones have the most important role in Iran's rainfalls and get from different tracks to the country and affect Northwest, West and South West of Iran in the cold period of the year. Cyclones are the prevailing weather phenomenon in Mid-latitude region and studying the characteristics and effects of these phenomena has always been of interest to climatologists and meteorologists. Iran which is located in mid-latitude region, actually is located in the path of one of the world's largest cyclone formation and the Mediterranean Sea has a main impact on the rainfall occurrence in Iran. Regarding the importance of cyclones in rainfall occurrence in Iran, especially in cold season, studying this atmospheric event seems very necessary.
The aim of this study, is to identify, tracking and analysis of the winter season (December, January and February or DJF) rain producing cyclones using special database and algorithm during three recently decades (1979-2009).
In this study, in order to specify, tracking and statistical analysis of the DJF rain producing cyclones, relative Vorticity of 700 hPa were received from ECMWF data center with
spatial resolution and temporal resolutions of 6-hourly for a 30 years period (1979-2009). The study domain includes Mediterranean and Middle East, from 
E to 
E and 
N to 
N.
Cyclone identification will follow the approach presented in Blender et al. (1997) and Flaounas et al. (2014), including two important points and conditions: 1- A cyclone is considered to be a relative Vorticity extrema, thus the aim will be to look for the maximum value of relative Vorticity at the 700 hPa level in a box of
grid points. 2. For the intensity of the cyclone, the threshold of relative Vorticity, is considered equal or greater than 
. Then by 6-hour tracking the maximum amount points of Vorticity and connecting them to each other, identification and routing of cyclone is implemented. The track 
can be written as 
, where 
describes the cyclone position in sequential time steps t = 0, … , T, which are given at 00, 06, 12 and 18 GMT. The initial position is considered the first detected maximum of relative Vorticity. During the cyclone detection and tracking method, different threshold values are commonly applied to eliminate, for instance, the weakest centers and short living systems. For this reason, the threshold of equal or greater than 8 time steps (3 days and more) is considered based on Alijani (1366) to eliminate short living cyclones.
In this study, climatology of cyclones such as genesis and lysis centers, tracks and frequencies in the Mediterranean Basin and Middle East were analyzed. The results showed that, center and eastern part of the Mediterranean region include Aegean Sea, Cyprus, and northern borders of Mediterranean Sea, Adriatic Sea and also Red Sea, Sudan and Iraq, are most important cyclogenesis centers in winter season.
These cyclones which form in these cyclogenesis centers, move to the east with two eastward and northeastward distinguished directions. In northwest of Iran, the cyclones by passing over West and East Azerbaijan and Ardebil provinces in eastward and northeastward directions, move into western and southern shores of Caspian Sea. In southwest of the country, the cyclones are entered into Iran over Khozestan, Chaharmahal and Kohkilouyeh provinces. Part the cyclones are moved into center and northeast of the country with northeastward direction and other part by crossing over southern shores of the country move to the southeast of the country. In mid-west of the country, the cyclones are arrived into Iran in three different tracks. The most important path is Kordestan's path which the cyclones in the origin of Mediterranean are entered eastward into Iran by crossing over Zanjan and Hamadan provinces, under the Alborz ranges are moved into east and northeast of the country. Two other paths are observed in Ilam and Kermanshah Provinces which in these tracks also the cyclones are approximately moved eastward into center and northeast of the country. The results also showed that eastern part of the Mediterranean region specially Cyprus, Aegean sea, Adriatic sea, Red sea and Sudan and also Iraq are most important wintertime cyclogenesis centers of Iran. The main entrance paths of the cyclones includes northwest, mid- west and southwest. The averages cyclones lifetime are 4.5 days, movement speed is about 20 km/h and also travelling length is about 1700 km. Of the other notable results we can refer to decreasing of the rain producing cyclones during 3 last decades.
The aim of this study, is to identify, tracking and analysis of the winter season (December, January and February or DJF) rain producing cyclones using special database and algorithm during three recently decades (1979-2009).
In this study, in order to specify, tracking and statistical analysis of the DJF rain producing cyclones, relative Vorticity of 700 hPa were received from ECMWF data center with
spatial resolution and temporal resolutions of 6-hourly for a 30 years period (1979-2009). The study domain includes Mediterranean and Middle East, from E to E and N to N.Cyclone identification will follow the approach presented in Blender et al. (1997) and Flaounas et al. (2014), including two important points and conditions: 1- A cyclone is considered to be a relative Vorticity extrema, thus the aim will be to look for the maximum value of relative Vorticity at the 700 hPa level in a box of
grid points. 2. For the intensity of the cyclone, the threshold of relative Vorticity, is considered equal or greater than . Then by 6-hour tracking the maximum amount points of Vorticity and connecting them to each other, identification and routing of cyclone is implemented. The track can be written as , where describes the cyclone position in sequential time steps t = 0, … , T, which are given at 00, 06, 12 and 18 GMT. The initial position is considered the first detected maximum of relative Vorticity. During the cyclone detection and tracking method, different threshold values are commonly applied to eliminate, for instance, the weakest centers and short living systems. For this reason, the threshold of equal or greater than 8 time steps (3 days and more) is considered based on Alijani (1366) to eliminate short living cyclones.In this study, climatology of cyclones such as genesis and lysis centers, tracks and frequencies in the Mediterranean Basin and Middle East were analyzed. The results showed that, center and eastern part of the Mediterranean region include Aegean Sea, Cyprus, and northern borders of Mediterranean Sea, Adriatic Sea and also Red Sea, Sudan and Iraq, are most important cyclogenesis centers in winter season.
These cyclones which form in these cyclogenesis centers, move to the east with two eastward and northeastward distinguished directions. In northwest of Iran, the cyclones by passing over West and East Azerbaijan and Ardebil provinces in eastward and northeastward directions, move into western and southern shores of Caspian Sea. In southwest of the country, the cyclones are entered into Iran over Khozestan, Chaharmahal and Kohkilouyeh provinces. Part the cyclones are moved into center and northeast of the country with northeastward direction and other part by crossing over southern shores of the country move to the southeast of the country. In mid-west of the country, the cyclones are arrived into Iran in three different tracks. The most important path is Kordestan's path which the cyclones in the origin of Mediterranean are entered eastward into Iran by crossing over Zanjan and Hamadan provinces, under the Alborz ranges are moved into east and northeast of the country. Two other paths are observed in Ilam and Kermanshah Provinces which in these tracks also the cyclones are approximately moved eastward into center and northeast of the country. The results also showed that eastern part of the Mediterranean region specially Cyprus, Aegean sea, Adriatic sea, Red sea and Sudan and also Iraq are most important wintertime cyclogenesis centers of Iran. The main entrance paths of the cyclones includes northwest, mid- west and southwest. The averages cyclones lifetime are 4.5 days, movement speed is about 20 km/h and also travelling length is about 1700 km. Of the other notable results we can refer to decreasing of the rain producing cyclones during 3 last decades.
Dr Hasan Lashkari, Mrs Mahnaz Jafari,
Volume 8, Issue 1 (5-2021)
Volume 8, Issue 1 (5-2021)
Abstract
Synoptic Patterns that Determine the Trajectory of Precipitation Systems of Sudanese Originntroduction
Introduction
Precipitation as an important climatic element has many irregularities and fluctuations. Iran, especially its southern half, has significant precipitation fluctuations. Several atmospheric systems are involved in the formation of precipitation in this region from of Iran. Sudanese system is one of the most important precipitation systems in Iran. This system, in different synoptic conditions, enters Iran from different input sources and passes through Iran in different ways.
The important and influential role of Sudan's low pressure on precipitation in Iran, especially in the southern part of the country, has been repeatedly demonstrated in numerous studies. But the formation and its expansion have received little attention. These reasons have led to the consideration of the position of Sudan's low-pressure synoptic expansion as an influential factor in the southern half of Iran precipitation. Therefore, the position of the expansion of this important climatic system has been investigated separately in the precipitation of the three regions south west, south middle and south east.
Materials and Methods
Two categories of data were used for this study. These data include daily precipitation data from the Iranian Meteorological Organization and the ERA interim gridded data include Sea Level Pressure (SLP) and the Geopotential Height of the 700 HP atmospheric level of the ECMWF. Second category data with horizontal resolution of 0.5 × 0.5° degrees during 1997-2017 statistical period were prepared.
To achieve the purpose of the study, the southern half of Iran was first divided into three regions: South-West, South-Mid and South-East. After extracting daily precipitation of the selected stations in all three geographic regions, a total of 142 precipitation systems was identified by applying the required criteria. From this number of precipitation systems, respectively, were obtained in the south west 107, south middle 19 and southeast 16, respectively. Then, the source of precipitation systems was extracted using the atmospheric lower level maps. Subsequently, the central core and zone of the first closed curve around the Sudanese low pressure were extracted separately for each group. The main axis of the Sudanese low-pressure trough are also drawn on all rainy day. Finally, the model or pattern of atmospheric circulation in the precipitation systems of the regions is presented separately.
Results and Discussion
The purpose of this study was to determine the position of the central core and the pattern of expansion of the first closed curve around the Sudanese system and the Sudanese system trough in precipitation in each of the three regions of the southern half of Iran. Since the arrangement of precipitation systems may vary in different months of the year, depending on the general atmosphere of the atmosphere, the position of the core, the pattern of expansion of the low-pressure trough and the trough of 700-hPa atmospheric level is analyzed separately each month.
In the synoptic pattern of systems, entering from the south west of Iran, the Arabian Subtropical High Pressure with the southwest-northeast direction is located in the eastern half of the Arabian Peninsula and west of the Oman Sea. In this pattern, the troughs are generally north-south. As a result, the rainfall intensity and intensity of precipitation systems, entering the south west of Iran are higher than the other two routes. The focal point of troughs this route is between 30 to 40° east (Eastern Mediterranean). In systems with South-Mid route, the Arabian Subtropical High Pressure has slightly shifted southward and found a northeast-southwest axis. In this pattern, the Mediterranean troughs are generally northeast-southwest. This pattern causes precipitation in the eastern half of the Iran. Or at least no precipitation in the northwest and west of the Iran.
The synoptic pattern of precipitation systems that enter Iran from the southeast is somewhat more complex. In this pattern, the Arabian Subtropical High Pressure has an unusual eastward shift. So that it is based in India. The troughs of this path showed two completely opposite patterns. In some systems, the troughs in the southwest-northeast direction with the orbital inclination, covers the whole of Saudi Arabia and southern Iran. On the contrary, in some systems the troughs stretch quite opposite to the first group, the northwest-southeast direction.
This asymmetry in the expansion of the troughs should be traced to the general topography of the Tibetan Plateau and the circulation pattern of caused by the presence of the Tibetan anticyclone. Basically Mediterranean troughs are disrupted in their usual eastward displacement after a longitude of 60 degrees. As you can see, the Sudanese low-pressure troughs for the South-East Route lack structural discipline and coordination.
Conclusion
The results of this study show that the location and pattern of expansion of the first closed curve around low pressure in different precipitation months and systems of the three zones do not differ significantly in location. Rather, it is the most important system in determining the direction of Sudanese systems, the Arabian Subtropical High Pressure and the pattern of expansion of the eastern Mediterranean trough. In the synoptic pattern of systems, entering from the south west of Iran, the Arabian Subtropical High Pressure with the southwest-northeast direction is located in the eastern half of the Arabian Peninsula and west of the Oman Sea. In this pattern, the troughs are generally north-south. In systems with South-Mid route, the Arabian Subtropical High Pressure has slightly shifted southward and found a northeast-southwest axis. In this pattern, the Mediterranean troughs are generally northeast-southwest. The synoptic pattern of precipitation systems that enter Iran from the southeast is somewhat more complex. In this pattern, the Arabian Subtropical High Pressure has an unusual eastward shift. So that it is based in India. The Sudanese low-pressure troughs for the South-East Route lack structural discipline and coordination. This asymmetry in the expansion of the troughs should be traced to the general topography of the Tibetan Plateau and the circulation pattern of caused by the presence of the Tibetan anticyclone.
Keywords: Synoptic Patterns, Sudanese Low Pressure system, Eastern Mediterranean Trough, Southern Half of Iran, Arabian Subtropical High Pressure.
Halimeh Kargar, , Mohammad Saligheh, Mehry Akbary,
Volume 8, Issue 4 (1-2021)
Volume 8, Issue 4 (1-2021)
Abstract
Conclusion
The results showed that the length of the precipitation period in the study area is 8 months (out of 213 systems, 60 systems were recorded in January and 50 systems were recorded in December). June, July, August and December did not have a comprehensive rainfall system. The difference in height between the Mediterranean Cyclone and the Arabian Anticyclone, i.e. the MAI index, was identified in 5 categories. The results showed that the higher the value of MAI index, the less rainfall systems and the higher the rainfall intensity. Most rainfall systems occur when the MAI index reaches 100 to 150 geopotential meters. In addition, the concentration of precipitation occurred in February when the MAI index reached 200 geopotential meters and above. In the continuation of the research, the difference between the Sudan Cyclone and Arabian Anticyclone, i.e. the SAI index, was also identified in 5 categories. The results showed that most of the systems occurred in the third category with a value of SAI index between 100 and 150 geopotential meters. Moreover, the results showed that the higher the value of the SAI index (, the lower the number of rainfall systems and the higher the average of rainfall systems.
Roya Poorkarim, Hossein Asakereh, Abdollah Faraji, Mahmood Khosravi,
Volume 9, Issue 4 (3-2023)
Volume 9, Issue 4 (3-2023)
Abstract
In the present study, the data of the ECMWF for a period of 1979 to 2018 was adopted to analyze the long term changes (trends) of the number of cyclones centers of the Mediterranean Sea.There are many methods (e.g. parametric and non- parametric) for examining changes and trends in a given dataset. The linear regression method is of parametric category and the most common nonparametric method is Mann-Kendall test. By fitting the Mann-kendall model and the linear regression model, the frequency of the cyclone centers of the Mediterranean basin was evaluated in seasonal and annual time scales. Analyzing the trend of changes of the number of cyclone centers on a seasonal scale showed that the five-day duration have had a significant trend in spring, autumn and summer. Whilest on an annual scale, there was no significant trend in any of the duration. By fitting the regression model on seasonal and annual scale, one- and two-day duration have a positive regression line slop.
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