جستجوی مقالات مرتبط با کلیدواژه « synoptic systems » در نشریات گروه « جغرافیا »

The phenomenon of precipitation is the final product of several complex atmospheric processes that are different in terms of time and place, and it can be considered one of the climatic characteristics of each region. The diversity of origin, temporal-spatial changes of rainfall systems, ground conditions such as altitude and topography, and distance and proximity to moisture sources cause precipitation behaviors, including continuity, to have temporal-spatial changes. Heavy and continuous rains are significantly effective in supplying water, feeding the underground water table, and storing it in the soil. It can also cause landslides, destruction of vegetation of forests and pastures, severe soil erosion, destruction of water structures, and waste of high volume of surface water resources in arid and semi-arid areas; therefore, it has always interested climate and meteorology researchers. In Iran, heavy and moderate rainfalls have been investigated from different perspectives. Some researchers have investigated heavy rains leading to floods in different regions of the country, and others have zoned rains lasting several days using the clustering method. Despite the various analyses conducted by the researchers, due to the difference in the selection of rainfall amounts and the different behavior of synoptic systems, the factors affecting rainfall lasting several days remain in the veil of ambiguity. This research aims to identify the synoptic patterns of rainfall lasting more than 4 days and to investigate the horizontal flux of specific moisture so that by identifying the behavior of this variable and its related fluctuations, accurate and comprehensive planning can be carried out in the direction of managing the country's water resources. What distinguishes this research from other studies is the method of estimating the limited rainfall of the study stations and the selection of the statistical period leading to 2022.

Methodology (Materials and Methods):

This research used two series of data; the first series was the daily rainfall data of 120 synoptic stations of the country from its establishment until 2022, which was taken from the National Meteorological Organization. First, the total four-day rainfall of weather stations from October to March was calculated. Then, based on the Andersen-Darling index, the best probability distribution function was fitted to the obtained data, and precipitation values above 99% of the fitted curve were obtained, among which the heaviest precipitation was selected. Five distinct clusters were identified using the hierarchical clustering technique based on the height of the middle level of the atmosphere. From each cluster, a precipitation system was chosen for synoptic analysis based on three criteria - intensity, continuity, and comprehensiveness of precipitation.The second series of data used in this research, which was used for the case study of synoptic systems, was taken from the NCEP/NCAR archive. These data in a regular grid are limited to 0 to 80 degrees east longitude and 10 to 60 degrees north latitude with a grid step of two and a half degrees, including an area where synoptic waves can be detected. From this series of data, we can refer to the average sea level pressure, altitude, temperature, and relative humidity of the pressure level of 850 hPa, the wind field and specific humidity of the pressure levels of 925 and 700 hPa, the height and the vertical component of the wind level of 500 hPa and the wind field of the level of 200 hPa.

Five specific patterns were obtained in the hierarchical clustering of the middle level's height. In the four obtained patterns, mid-level troughs with different depths are located in different geographical positions, and in the other pattern, a cut low pressure with a cold core was revealed in the north of the Black Sea. From examining the uncovered troughs, it was seen that in two cases, in the eastern part of the trough, there was a strong pile of height, which prevented the eastward movement of the trough and made it last. In these cases, the stack becomes more profound as the ship's depth increases and the ship's movement becomes slower. In two cases, short and low-amplitude waves caused by the rotating movement of the ship periodically passed through the study area and caused the instability of the atmosphere. In the case of a low-pressure cut, a deep ridge has caused the system to last and continue to rain. Since the water vapor is more in the lower levels above the water sources, the upward movement caused by the eastern circulation of pressure systems has caused the water vapor to be transferred into it and increased the amount of precipitation. Therefore, the persistence and transfer of water vapor have caused the continuation of precipitation in the study area.

Investigations showed that the synoptic systems of mid-latitudes, when passing over the warm waters of the Mediterranean Sea, caused an upward movement due to the gyre circulation and transferred a large amount of moisture on the sea to higher altitudes. In addition, the eastern winds of the high-pressure southern part over the Oman Sea have transferred a large amount of special moisture to the south of the Red Sea, which has been transferred to the south, southwest, and western regions of Iran with the southern winds of the eastern part of the Red Sea pressure trough. As a result of the upward movement, these values have participated in the precipitation process and increased the amount of precipitation and the continuity of rain in the western half of Iran.

In order to investigate the status of atmospheric synoptic systems during the occurrence of severe rainfall anomalies in eastern Iran, the rainfall statistics of 31 meteorological stations located in the region during the statistical period (from 1990 to 2020) were used. In this regard, precipitation anomaly index (RAI) was used to identify these periods. To explain the structure of the atmosphere at the time of its occurrence by referring to the website of the National Center for Environmental Prediction/Atmospheric Science (NCEP/NCAR) networked data on geopotential height, sea level pressure, vertical velocity component (omega), Relative humidity, orbital components and wind meridian were obtained. Studies have shown that the occurrence of severe rainfall anomalies in eastern Iran is associated with the formation of a strong ridge in the middle of the atmosphere on the east of the Middle East. This causes a recurring current, or high continuity, to dominate the area. On the other hand, the advent of cold weather and the dominance of the north current to the region, the conditions of climbing and instability have been minimized. The location of the region below the upper convergence core of Rudbad is also one of the main reasons for the occurrence and continuation of severe rainfall anomalies in the region. In order to investigate the relationship between the major cyclones for rainfall, the two regions of the Red Sea and the Mediterranean were examined. The results of the study of the correlation between omega indices and the meridional component of wind on the Red Sea and Mediterranean corridors with sea level pressure on the region also showed that with decreasing ascent values in these areas and increasing pressure on eastern Iran, a correlation Positive is established.

In this regard, the present study investigated the climatic of the Zahedan snow days since 1976 over a period of 40 years. To achieve this goal, two databases were used. First databases; the present weather codes are related to the snow phenomenon, which was collected daily from the Meteorological Organization. The second database, relates to the daily data of sea level pressure and geopotential height in the troposphere from National Center for Environmental Prediction (NCEP). With Environmental to Circulation Technic and cluster analysis, weather patterns causing snow days, identified. The results of the analysis showed that, snowfall in Zahedan is concentrated in autumn and winter seasons (December, January, February and March). At sea level pressure, integration patterns of Siberian-Caspian high-pressure migrants with 32.5% and Siberian-Caucasian highs with 20%, play the most roles in snowfall event. Also in the middle troposphere, Turkmenistan's ridge and Blocks (cut-off high) account for 42.5% and the European omega block pattern accounts for 25% of the snowfall frequency. It seems that the simultaneous activity of atmospheric disturbances in the immigrant high pressure systems of the Siberian, European, Caucasian, Russian and Caspian, deep ridge polar latitudes and its extension to the lower latitudes, It will exacerbate showers and temperatures, cold weather and snowfall.

Rainfall prediction plays an important role in flood management and flood alert. With rainfall information, it is possible to predict the occurrence of floods in a given area and take the necessary measures. Due to the fact that the three months of January, February and March are most floods and most precipitation is occurring this quarter, this study aimed to investigate the factors affecting precipitation and modeling of this quarter. For precipitation modeling, the monthly rainfall data of the Hamadid and Baranzadeh station in the statistical period (1984-2014) for 30 years as a dependent variable and climatic indexes, large-scale climatic signals including sea surface temperatures and 1000 millimeter temperatures Altitude of 500 milligrams, 200 milligrams of omega and climatic elements have been used as independent variables. Due to the nonlinear behavior of rainfall, artificial neural networks were used for modeling. Factor analysis was used to determine the best architecture for entering the neural network. For prediction of precipitation, the data that showed the most relationship with precipitation was used in four patterns, in January the fourth pattern with entropy error was 045/0, the number of input layers was 91, the best makeup was 15-1, and the correlation coefficient was 94% Was. In February, the third pattern with a correlation coefficient of 97%, entropy error, was 0.36. Percentage, number of input units was 8 units, and the best type of latency layout was 10-1. The precipitation of March with all patterns was high predictive coefficient. The first pattern with entropy error was 0.038, the number of input units was 67, the hidden layer arrangement was 17-1, the correlation coefficient was 98%. 

Instability of Synoptic severe along with abundant moisture has a major role in flood event. At the present study, the synoptic situation of the Sarbaz flood basin has been investigated. For this purpose, two databases were needed. First one contains daily discharge data of Pirdan hydrometric station and daily rainfall census in neighbor stations of basin and second database include maps of ground surface and 500hp level in confine of latitude 10° till 60° N and longitude 10° till 90° E that are prepared from the National Oceanic and Atmospheric Administration (NOAA). Because of the flood type of this watershed in the summer and winter season, strongest floods for each season are selected. Thus, provided maps are belonging to two days before flood, start day of flood and maximum of flood. Results showed that in the ground surface of summer type, establishment of two low pressure centers on Persian Gulf and Pakistan, spread of Persian Gulf low pressure flow to south-east of Iran and entrance of its warm and wet air into the low pressure of Pakistan, deepening of west troughs of Caspian sea and presence of low pressure pitch of Pakistan in front of this troughs were causing instability. establishment of high system of Iran and spread of its prongs, as rich, to north Caspian Sea and forming of deep troughs between high systems of Iran and low pressure in the west of India at 500hp level has caused instability in south-east of Iran that is located in front of troughs. In the ground surface of winter type, continuum of low pressure from east Africa to north-east of Pakistan with southwest-northeast direction was formed. Troughs that are emanated from the Aral Lake high pressure and prongs of high pressure in west of Caspian Sea have main roles in instability and control of this continuum path. In 500hp level, establishment of deep trough in west and center of Iran and presence of low pressure systems of ground surface of southeast of Iran in front of this trough have intensified instability.

Thunderstorms are sever weather phenomena which occur repeatedly in transitional seasons - fall and spring - in middle latitudes. These storms are among sever weather hazards which cause a lot of damage to agricultural fields, gardens, etc. Understanding the mechanism, genesis and development of these storms helps us in combatting or controlling them. This study looks at severe thunderstorms in Isfahan over a 19-years period (1990 -2008). At first the codes of thunderstorms which caused over 10 mm rains were extracted and then based on the data collected from NOAA NCEP-NCAR dataset - sea level pressure and 850 hPa and 500 mb geopotential height on days of rain - the required maps were drawn using Grads software and the precipitation patterns were identified. In this study, two general patterns were identified for thundershower in Isfahan. The first pattern (12 November 1993) at sea level pressure, anticyclone tongue that extends from the East through the Northern Sea of Oman and the Persian Gulf, caused moisture Advection in the study area and at pressure of 850 hPa level, cyclone with the contour 1500 geopotential meters Located on the region. At 500 hPa level, the study area is located completely in the deep trough western that in addition of providing moisture, has intensified instability to higher levels. In the second pattern (26 April 2006) a large low pressure center with 1008 hpa is located on the country. The flows direction in the system is such that flow moist air into the country and at the 850 hpa level, the study area is located fully in front of the cyclone whit contour curve 1470 geopotential meters that in addition to moisture advection of Persian Gulf into the country, has intensified instability in this level and the 500 hpa level, so the deep troughs western, passing through the country and the study area, has provided instability to the upper levels.

The changes in behaviour of climate and meteorological parameters are closely related to changes in atmospheric circulation. The analysis of historical atmospheric troposphere circulation is critically important to global and regional climate change and extremes with regard to its dynamical features. The circulation changes are manifested by a gradual reduction in high-latitude sea-level pressure, and an increase in mid-latitude sea-level pressure associated with one phase of the Arctic Oscillation (a hemisphere-scale version of the North Atlantic Oscillation. Recent observations have found that the tropical belt running around the equator has grown wider, and has expanded by around 2° to 5° latitude and into the adjacent subtropical regions since 1979. Global greenhouse gas emissions contribute to expansion of the tropics (about 0.05° per decade)in the northern hemisphere tropics. The effect of black carbon and troposphere ozone emissions are about twice the size of those due to greenhouse gases alone (about 0.07° to 0.12° per decade). The aim of this study is investigation of variability in the intensity of the monthly geo-potential height, vorticity and sea level pressure over synoptic circulation patterns impacting Iran. In order to accomplish this research, daily grid data with spatial resolution of 2.5􀵈2.5 degree during 1/1/1960 to 31/12/2012 have been extracted from NCEP/NCAR database. The monthly average sea level pressure and geo-potential height in level of 1000 hectopascal is calculated to detect the action of spatial kernel of each synoptic system. The for this investigation is including daily zonal and meridian wind components (Uwnd and Vwnd) data, geopotential height data for each levels of 1000, 850, 700 and 500 hectopascal levels, and sea level pressure. Vorticity is calculated by Uwnd and Vwnd components. Then monthly mean is also calculated for each levels separately. Although some circulation patterns can operate in special months and seasons of year but trend analysis has been evaluated for each circulation patterns in whole months and seasons. To detect time series trend, nonparametric Mann Kendal statistic test has been applied. The trend has been tested in 95% confidence level. Sen Estimator is used to calculate trend slope rate. Siberian anticyclone, Sudan cyclone, Mediterranean region cyclones (East Mediterranean, Mediterranean and Black sea) and Monsoon and Persian Gulf cyclones have been selected according to the maps of sea level pressure and geopotential monthly means. In total vorticity, geopotential height in different levels and sea level pressure have significant trend in 95% confidence level. The highest variation is observed for high levels at 700 and 500 hectopascal. In winter season, sea level pressure, vorticity and geopotential in 1000, 850, 700 and 500 hectopascal levels don’t show significant trend. While in spring and summer the trend of vorticity in four selected levels is significant and negative, geopotential height and sea level pressure have positive significant trend. Over cyclonic systems in Mediterranean region, the sea level pressure and geopotential height show increasing trend especially in winter season. The slope positive trend on the Sudan is significant in winter. The trend of geopotential height infour selected levels in winter is positive and significant. In other seasons, the trend is positive except in 1000 hectopascal level. Over Monsoon cyclone system in all seasons, geopotential height is increasing from 1000 to 500 hectopascal levels. Vorticity trend in low levels at 1000 and 850 hectopascal is positive. The increasing geopotential height over Persian Gulf is smaller than other atmospheric circulation patterns especially in low levels. Vorticity rate contrary to other systems is increasing. Increasing sea level pressure and geopotential height over cyclonic circulation patterns of Sudan and Mediterranean regions especially in cold seasons of year (winter and autumn) result in a decrease in systems cyclonic action, sea level gradient pressure, instability and precipitation in region. In warm seasons of year (spring and summer) negative trend of vorticity and positive trend of sea level pressure and geopotential height result in decrease in warm seasons precipitation and increase in stability. The results of this study agree with findings of some other researchers about increase in warming of troposphere, changes in synoptic systems intensity,instability increase and negative vorticity increase in north hemisphere.

In order to investigate synoptic patterns of heavy precipitations in Sistan va Baluchestan province, 24 year (1987-2010) daily data of 6 synoptic stations was retrieved from meteorology organization. Moreover, data like sea level pressure, geo-potential elevation of 500 and 850 milibars were exploited from NCAR/NCEP database and then required maps were prepared in Grads software. Generally, two patters have resulted in the heavy precipitations in this province. In the first pattern which resulted in precipitations of December12, 1995, a cyclone with contour plot of 1017.5 and 1020 milibars crossed the Arabian Sea, 24 hours before the precipitation and caused humidity spreading toward the area. In 850 milibars, a cyclone above the country results in cold weather flow and a low pressure system with a 1500 geopotenial meter contour plot passed the Bangla Gulf and the Arabian Sea and supply the necessary humidity conditions for rising in this equilibrium level. In the second pattern which resulted in precipitations of June 5, 2010, a large low pressure system is formed 24 hours before the precipitation over southern part of Asia, which also influence south eastern and southern parts of Iran. 24 hours before the precipitation, a low altitude center covers the area under study and supply humidity and instability. On the day of precipitations, study area in 850 and 500 milibars are affected by a trough of 1475 and 5850 geopotenial meter contour plot and results in precipitation of this pattern.

The effective synoptic systems on dust events was investigated in the west of Iran based on observed dusty days at 50 meteorological stations using principal component analysis (PCA) and geographical information system (GIS). Results showed that the first 5 components explained 69. 11% of spatial variability of dust event variance. For detecting of affective synoptic systems، the computed correlation coefficient of stations with each component (Rotated Component Matrix) was transferred to the GIS environment، and synoptic patterns that affected spatial variability of dust events were simulated through Kriging interpolation method. It was shown that Azores high pressure system has the most effective role in frequency of days with dust in the west of Iran through the creation of surface thermal low pressures.

This research has been carried out in the southern coasts of Caspian Sea. Thisarea consists of about 150 catchments. Most of the rivers have small basin, covered byvegetation and confined by sea and mountains.In order Shafarood for the 1961-98 period were analysed. This period has nearlyexperienced 558 flood from which from which 59% belong to talar and Shafaroodcatchments, respectively. To determine the synoptic patterns of the floods, the surfaceand 500hP daily maps of 36 floods were analyzed. The result of the study showed that:The flood of Shafarood occurred mostly in the September to November periodand Talar floods in March and April.11% of the floods occurred simultaneously in both basins. The synoptic patternrespolsible fore these floods was the west of the Caspian Sea.The other floods were occurred when the through axis was moved over CaspianSea or to its east coasts. In These conditions Surface cyclones and anticyclones werevery important.The main moisture sources of the floods were the Medeaterian Sea for tha talarBasin and the Caspian Sea for the Shafarood Basin.