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Volume 3, Issue 2 (5-2016)
One of the most important components of the extent of pollutants mixing and air quality at near the Earth's surface is the height of boundary layer. Many variables involved in determining the height of the boundary layer of atmosphere. Although all of the troposphere (the lower ~10km of the atmosphere) is affected by surface conditions, most of it has a relatively slow response time. The lower part of the troposphere that is affected on a shorter time scale is commonly defined as the Planetary Boundary Layer (PBL). The depth of the mixed layer has a significant effect on the concentration of air pollution, which itself is dependent on the intensity and duration of solar radiation and wind speed. According to Stull, one can describe the planetary boundary layer as “that part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcing with a timescale of about an hour or less.” Surface temperature has a strong relationship with height of the PBL. As the surface cycles between daytime radiation and nighttime cooling the amount of convection taking place changes. When the temperature gradient is steep, more convection takes place to dissipate thermal energy in the most efficient manner. In other words, the greater the temperature difference between the surface and the lower troposphere, the higher convective eddies must reach to alleviate the gradient. Relating this to Stull’s definition of turbulence, it can be concluded that the height of the PBL varies with surface temperature. In fact, the spatial range of the PBL can vary from less than one hundred meters to several kilometers. The strong relationship between convective turbulence and height of the PBL is sometimes used to define the boundary layer and call it the Convective Boundary Layer (CBL). Analogous to the Stull’s definition but focusing on turbulence, Lloyd et all describe CBL as “a layer of air typically of order 1km in depth, well mixed by turbulence maintained by buoyancy due to heating at the ground. It is bounded above by stably stratified, no turbulent air and grows through the day. In this study we aimed to analysis the status of ABL in 3 dust period days in Khuzestan province of Iran.
The Data were used in this study includes: The Daily data of dust concentration during 27Jan to 1 Feb 2015, the daily height of ABL also were used. The daily data of ABL were given from ECMWF with 1/8 degree spatial resolution. We used the Pearson correlation and synoptic analysis to assessment the condition of boundary layer at the mentioned days. For analysis the characteristic of ABL the climatic data of Wyoming University were used to assessment the thermodynamics of atmosphere. The spatial distribution of ABL height at the dusty day also were used for 12 UTC.
The results indicated there is the direct relationship between the ABL height and the concentration of dust in the mentioned days. So that in the days that the concentration of dust reaches maximum we fund that the height of ABL reaches maximum simultaneously and vis versa. The spatial distribution of ABL height shown that the height of ABL in the 29Jan reaches maximum that the maximum concentration of dust related to this day. And also the minimum concentration of dust observed in 27Jan and 1Feb that the in this day the height of ABL was minimum. The synoptic analysis also reveals that locating the low pressure system at the 500hp level that the Khuzestan province has been locating at the front of this system lead to transport the dust to study area.
In this study we reveal that the height of ABL in the dust days of Khuzestan has a totally revers behavior in compare to the air pollution days in Tehran. In the pollution days in Tehran the lowing of ABL height and inversion lead to intensify the concentration of pollution while in the dust days of Khuzestan the height of ABL were increased in compared with non-dusty days.
Volume 7, Issue 1 (5-2020)
Farshad Pazhoh[1], PhD in Synoptic Climatology, Department of Natural Geography, Faculty of Geographical Sciences, University Kharazmi, Tehran, Iran
Every year, important parts of a large part of our country are affected by the climatic hazards of heavy precipitation and lots of damages are done to the country. If the generating circulation patterns of heavy precipitation waves will identify, its occurrence can be predicted from at least one or two days before the beginning of the sequence of patterns ending in floods (Alijani, 2006, 156). Occurrence of heavy precipitation, so that its amount is more than the soil penetration capacity, causes runoff and floods. Now, if these heavy precipitations occur in urban areas, it is associated with more dangers, because the permeability in urban areas is less than in out-of-town areas, and a significant amount of such precipitation in urban areas has turned into runoff and floods. Cause damages to places, buildings and urban facilities (Taheri Behbahani and Bozorgzadeh, 1996, 2).
Two sets of data were used to conduct this research. One is surface data and the other is high atmospheric data. For this purpose, in the first category, the related precipitation data of the cold season of 8 synoptic stations in the southern half of Iran (Table 2) in the period from December 1, 1970 to March 31, 2014 were obtained from the Meteorological Organization. To identify the occurrence of heavy precipitation leading to major floods in the study area, considering that heavy precipitation has covered more than 50% of stations and the precipitation of each station is more than 95% during the study period.
Considering the above two conditions, 61 heavy and pervasive precipitations were selected from the total precipitations above the percentile of 95% of the stations. In the second category, high atmospheric data obtained from the National Oceanic and Atmospheric Administration of the United States. The synoptic scale in order to tracking the troposphere synoptic patterns includes a longitude of 20 west degrees to 100 east degrees and a latitude of 0 to 80 north degrees. In the selected synoptic scale, 1790 cells are located; the distance between each cell is 2.5 by 2.5 arc degrees.
In order to identify the jet stream patterns, first the factor analysis method with Varimax rotation was applied on the geo potential height data of 500 hPa during the selected 61 days of heavy and pervasive precipitations and found that the first 12 factors explain more than 90% data’s diffraction. The first factor accounts for about 32% of geo potential height data diffraction (Table 4). In the next step, in order to reduce the data volume and identify the synoptic patterns, the cluster analysis method was performed on the scores of the first 12 factors by the integration method and 4 synoptic patterns affecting the arrangement of the winds were extracted. Then, for each of the identified patterns, a representative day that had the highest correlation with the desired pattern determined (Table 3) and appropriate maps for the representative days of the patterns were drawn and analysed.
The results showed that the merged jet stream patterns (subtropical-sub polar), tropical jet stream (ridge-trough), orbital subtropical jet stream and meridian subtropical jet stream were effective in the occurrence of heavy precipitation, which meridian subtropical jet patterns and merged have played the most important role. In the first pattern, the merged jet stream plays role in 16 days and 26.3% of the precipitation days. The merged jet streams core is generally located on the Red Sea, and the subtropical jet stream penetrates from North Africa, and after crossing the Red Sea and northern Saudi Arabia, the left half of the jet stream’s exit covers the whole of the southern and central half of the country. The sub polar jet stream in a northwest-southeast direction from central and the west of the Europe from the centre and west of Europe penetrate to the lower latitudes and from central and eastern part of the Mediterranean and at the entrance part of the left side merge with subtropical jet stream. In the merger pattern, the sub polar jet stream corresponding to the western half of the trough of the middle-level of troposphere plays the role of cold air Advection and transferring the western winds to the lower latitudes, and the subtropical jet stream, corresponds to the eastern half of the trough, play the role of the discharge and divergence of warm and southern humid air on the southern half of the country’s atmosphere. In the second pattern, the subtropical jet stream (ridge-trough structure) with 13 days and 21.3%, generally in Northeast Africa, the subtropical jet stream with a huge ridge structure in direction of northwest to south east extends to the centre of the Red Sea and Saudi Arabia and also the trough structure of jet stream stretches from north of Iraq to the centre of the Red Sea. This trough structure’s sinkhole of jet stream this subtropical sinkhole has caused the left half of the jet stream's outlet with meridian curvature cover the whole of the southern half and most of the country after crossing Saudi Arabia and the Persian Gulf. But in the third pattern of the orbital subtropical jet stream, which plays a role in 14 days and 23.4% of heavy and pervasive precipitation days, the jet stream core has the most stretching and range, mainly from the eastern Mediterranean and north of Saudi Arabia to the western half of Iran, and the jet stream structure is completely formed west to east with least meridian structure. The intensity and pervasiveness of precipitations in this pattern such as the second one is weaker than the other patter. However, in the fourth pattern, the meridian sub-tropical jet stream is present as the most frequent pattern with 18 days and 29% of the selected precipitations days. In this pattern the jet stream has a southwest to northeast direction and the jet stream's core, like the third pattern, generally extends from north of Saudi Arabia to centre of Iran and sometimes to northeast of Iran. The locating of this jet stream with a suitable curvature on the important water resources of the south of the country and corresponding to the north eastern half of the trough from north eastern of Africa to north eastern of Iran after the merged pattern, has caused the most pervasive and intensive precipitations days in the south of the country.
Keywords: Heavy and Pervasive precipitation, Cluster Analysis, Subtropical Jet stream, Low Pressure, Trough, Southern half of Iran
Volume 8, Issue 1 (5-2021)
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.
Volume 8, Issue 4 (1-2021)
Iran is a country with low rainfall and high-intensity rainfall that is affected by various synoptic systems, the most important of these systems is Sudan low pressure, Therefore, recognizing the low pressures of the Sudan region is of particular importance, The purpose of this study is to gather a complete and comprehensive knowledge of the set of studies conducted about this low pressure, structure and formation and its effects on the surrounding climate. The present study was conducted using the library method and a search for authoritative scientific and research sources in connection with research on low pressure in Sudan and no data processing was performed in it. Thus, it has studied and analyzed the temporal and spatial changes of Sudan's low pressure over several years and its effect on the climate of the surrounding areas, especially Iran. In general, the results of this study can be divided into several categories, including studies on the recognition and study of Sudan low pressure, its structure and formation over time, pressure patterns affecting it at different atmospheric levels, and its effects on the climate of surrounding areas, especially Iran. Has been studied, The effect of this low pressure on seasonal and spring rainfall in Iran, snow and hail, floods, thunderstorms and also the effect of remote connection patterns on this low-pressure system have been studied, and finally, the analysis of these findings has been studied. It can be concluded that the Sudanese low-pressure system is a Low-pressure reverse in the region of Northeast Africa and southwest of the Middle East, which is strengthened and displaced in the upper levels of the Mediterranean and Subtropical jet stream and in the lower surface moisture injection from the Arabian Sea and Oman through high pressure. Saudi Arabia is inwardly the cause of severe instability in Iran and a major cause of heavy rainfall in various parts of the country.
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