جستجوی مقالات مرتبط با کلیدواژه "vorticity" در نشریات گروه "جغرافیا"

Synoptic systems have a major role in the earth’s climatic changes. These systems, especially cyclones and anti-cyclones, move to transport air currents, and thus, sensible and latent energy, thus alternatively causing instability and stability in one region. Cyclones are synoptic systems in which sea level pressure is low, the air current is ascending, and the wind direction in the northern hemisphere is anti-clockwise. The vertical air movements in cyclones create clouds, precipitation, and even thunderstorms. Considering the literature on the relationship between heavy precipitation and cyclones, the frequency, temporal and spatial distribution and main paths covered by them have been examined; however, it is thus noted that few studies have ever investigated the three-dimensional structure of cyclones and their structural changes during the distance covered. For this, this study aimed to examine the three-dimensional structure of Mediterranean cyclones entering Western and Northwestern Iran through an immediate set of atmospheric variables.

This study used two different databases to identify Mediterranean cyclones entering Western and Northwestern Iran in the cold spells of the year (October-May). The first database pertains to the daily precipitation data of 15 synoptic stations of Western and Northwestern Iran for 10 years (from 2004 to 2013), obtained from Iran’s Meteorological Organization. The second database also pertained to the variables of atmospheric temperature, specific humidity, geo-potential height, sea-level pressure, vorticity, wind meridional component, and wind zonal component, all of which were derived from the European Center for Medium-Range Weather Predictions website (https://www.ecmwf.int) in the form of daily observations for 10 years (from 2004 to 2013) for the cold spells of the year (October -May).After data were obtained, the following five-step algorithm was defined to investigate the three-dimensional structure of cyclones entering Western and Northwestern Iran with origin in the Mediterranean Sea.Step 1: Selecting the most pervasive precipitation days in Western and Northwestern Iran=Step 2: Detecting the time and place of formation of Mediterranean cyclones at 1000 Hpa levels Step 3: Tracking cyclones at 1000 Hpa levels during the cyclone lifespan Step 4: Identifying cyclones at various pressure levels and their bonding together Step 5: Investigating the three-dimensional changes of cyclones during their lifespan

The main goal of this study was to understand the three-dimensional structure of cyclones entering Western and Northwestern Iran with origin in the Mediterranean Sea. For this aim, and to determine Mediterranean cyclones and locate their formation in the Mediterranean Sea, a five-step process was introduced. According to this process, 93 cyclones causing pervasive precipitation in Western and Northwestern Iran in 10 years (from 2004 to 2013) were identified and their three-dimensional structure was determined. The most important results from investigating the three-dimensional structure of the cyclones were as follow:• Most cyclones causing pervasive precipitation in Western and Northwestern Iran had initially shallow and middle depths then developed into deep cyclones during their paths. Before their central cores were established on land, most cyclones caused pervasive precipitation in Western and Northwestern Iran.Except for the beginning days of the birth of cyclones causing pervasive precipitation in Western and Northwestern Iran, vorticity values, being negative at some levels and positive at some other levels, revealed at all levels positive values during [cyclones’] course of movement eastwards in the remaining days. On days with pervasive precipitation in Western and Northwestern Iran, vorticity in cyclonic centers increased, especially at upper atmospheric levels.When shallow cyclones are formed on European land, there is always a kind of weak temporal inversion at lower levels of the atmosphere, which can be a major orographic feature of cyclones. In the course of the Mediterranean cyclones’ movement eastwards, the temperature gradient in the central core of the cyclones involves many changes. When cyclone centers are established over the seas, the temperature gradient is low, and when established on land, it is high.The daily changes of cyclones’ central cores during their course of movement eastwards are low at the seas and high on land. Wind zonal component values in the center of cyclones causing pervasive precipitation in the west and northwest of Iran were greater than those of wind meridional components.

An investigation of the most important changes of atmospheric variables (e.g., geopotential height, vorticity, temperature, humidity, and wind meridional and zonal components) in two horizontal and vertical dimensions during the life span of cyclones causing pervasive precipitation in Western and Northwestern Iran determined that thickness (vertical spreading), vorticity, temperature, humidity, and wind zonal and meridional components underwent large scale changes. The thickness of cyclones during their life span saw an increase, with some shallow cyclones changing into deep cyclones. Vorticity also saw positive values at all levels on all days, except for the beginning days when it held negative values at some levels, and positive values at some levels. Also, the temperature gradient in the central cyclonic cores saw changes. When deployed on the seas, cyclone cores had a low-temperature gradient, and when placed on land, they had a high-temperature gradient. During the cyclone’s life span, the specific humidity value by the time of pervasive precipitation in Western and Northwestern Iran increased at all levels, with the wind zonal component value being greater than the wind meridional component value at most levels.

The aim of this study was to analyze the mechanism is precipitation Comprehensive Iran. For this purpose the daily precipitation data of 483 synoptic and climatology stations arranged. In this study, a comprehensive annual rainfall is said to have a minimum rainfall and above, 50% sequence coverage and have at least two days. Winter surround Iran on the condition of rainy days were extracted and examined. Then, to review and analyze the mechanism of atmospheric precipitation comprehensive synoptic and dynamic parameters such as moisture flux, vortices, ground level pressure, Geopotential, meridional and zonal wind component for the levels of 1000, 850, 700 and 500 HP studied and analyzed was. The results of this study showed that the widespread mechanism of dynamic and synoptic Winter country most affected by the composition of the atmosphere patterns such as the Mediterranean low pressure - low pressure core Persian Gulf, Iran, Central High East Europe closed low pressure, low pressure Urals - the Middle East, high pressure, low pressure Saudi Arabia - High pressure belt Europe and Siberia - Iran's low-pressure center. However most of the winter precipitation of moisture flux feed barley middle-Level interaction, particularly levels of 850 and 700 HP respectively. It was while change 500 hPa atmospheric dynamical mechanism is an important role in Iran's winter inclusive.

The role of tropospheric vertical anomalies in rainfall solid Case study: the hazard of hail in Kermanshah
Climate risks is one of the Types of hazards that damages human communities such as the phenomenon of hail, in the micro-scale, it causes financial losses and casualties. Hail is associated to the atmospheric elements and geo-location factors. Whenever weather conditions and appropriate physical processes are combined with geo-location creates and intensifies this phenomenon.
Losses resulted from hail has been more effective in the agricultural sector and in the effect of damaging the crops When growth and budding. However, it disorders in other sectors such as, blemishing residential buildings, Losing large and small animals also, damaging to the aircraft flight and its components. Hail considerable damage in Kermanshah province every year so that Farmers insure their crops against this Phenomenon and the government will incur heavy costs for damage that is inflicted on the sector of activity.
Research The current weather data has been used with 3-hour intervals in the statistical period of 65 years (1951 to 2016) from synoptic stations of Kermanshah Province that includes the stations of Kermanshah, West Islamabad, Ravansar, Kangavar, West Gilan, and Sar-e-Pole-Zahab.
Among the 100 present weather code, Codes 99, 96, 91, 90, 89, 87 and 27 have been considered that including hail phenomenon by varying intensities and includes any appearance of this phenomenon in Hours scout and three hours earlier. Then, based on the above code, Were coded in Excel to identifies Codes 96, 91, 90, 89, 87 and 27 When entering from the Meteorological Data To the desired program among Group VII of the data, And when the written code, were identified, Hail days were marked.
Given that in this study Hail is studied regarding the synoptic conditions and temperature anomalies. Therefore, for the synoptic situation, Pressure data, vorticity, Special moisture, Components U and V, Omega transverse profile And outgoing longwave radiation, And for the temperature anomaly, Temperature and isothermal anomalies components Were getting from esrl.noaa.gov/psd site And using the software Grads were drawn maps for a selected day To determine the formation of hail.
Commentaries The frequency of occurrence of hail has reached 187 in the period 65 years in Kermanshah province. This phenomenon generally occurs from mid-September to mid-June. The most number has been in Kermanshah station and the Least in Sar-Pol-Zahab station.
April has had the highest number of hail frequencies in Kermanshah province and the greatest losses in the month related to the agricultural sector. Therefore, Select System hail seems essential to examine how the temperature anomalies and the formation of hail in the month.
On the day of the event, trough hail has been formed in the East Mediterranean.Wrying the trough axis From North East to South West resulted in cold air from high latitudes to the East of the Mediterranean.
The establishment of trough in the middle and low pressure level in sea level and its following Convergence in the balance has created positive omega until balance of 200 hPa and most serious it is at the level of 400 hPa. Negative omega has maintained its association from ground surface until High levels in the study area.
The airflow of vorticity balance 1000 and 500 Hpa Suggests vorticity positive settlement area on the case study. Establishment of short wave in the vicinity of the study area and intensifying ascending conditions also Prolong Positive trough conditions from surface of Earth until 500hpa balance have been The necessary dynamic conditions for Hail in this day.
Special moisture and wind Vector with 700hpa balance of Moisture transfer has been done by two opposite vorticity system. Trough rotary motion Based on the Mediterranean and along the Red Sea on the one hand and Moving anticyclone over the Arabian Sea And the Persian Gulf and Oman Sea on the other, have conveyed Moisture of all moisture sources from The seas around to The study area.
Also OLR anomalies for the hail event day indicates being Negative in the study area and the sharp decline of Outgoing longwave in this day Compared to its long-term average And hence the conditions of cloudiness and the formation and intensification of convection has been provided.
1000 hpa positive anomaly 2 ° is representative the Higher than the average temperature conditions and in the 500hpa anomaly balance Minus 2 degrees Celsius is representative Lower than normal temperatures in the balance. These factors aggravate the vertical temperature gradient in the study area these days. 20 degrees Celsius the Isothermal curve and -20 ° C. Respectively, the levels of 1000 and 500 Drawn to the area of study And has created a large temperature difference Between the upper and lower levels.

The purpose of this study is assessing the spring precipitation and outgoing long wave radiation using Reviews Climate maps at different levels of the atmosphere, surface and Earth mapping reflecting long-wavelength radiation. For these data synoptic stations of the North West (15 April 2016) was obtained from the meteorological. The data from the induction by NCEP-resolution grid of horizontal 5 degrees of length and width of geographic and through the application Grads, maps of sea level, the 500 hpa daily, Vorticity 700 hpa, map convergence and divergence flow of moisture , long wavelength reflectance map of the earth's surface, atmosphere and map omega vertical advection map queries. Reviews maps indicate that the main cause of precipitation North West of Iran on 26 April, entry system, the rainfall from Eastern Europe and accompanying Western air mass with this system, and the effect of convection locally in the morning because of high levels of outgoing long wave earth is very minimal, but in the afternoon the existence of the local convection has intensified and the highest precipitation occurred in the study

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.

Hot days are considered as one of the manifestations of extreme temperature. Hot days are very important atmospheric events in terms of losing water resources، large demand for water and energy، and its effect on human comfort. Accordingly، these events could have physical، economic and social consequences. As Bonsal et al (2001) has stated this atmospheric event and the related atmospheric systems might emerge and occur during every month. These kinds of temperature anomalies especially in large scales are in relation with given synoptic systems. Although many investigations have been carried out on synoptic analyses of hot days around the world، it seems they have been neglected in Iran. For example Nasrallah et al (2004) studied hot waves in warm season over Kuwait during 1958-2000. They assume northward transfer of subtropical jet stream and a ridge emergence in 500 hp are synchronized with hot days. In Iran the studies primarily underlined hot days consequences. For example Brati and Mosavi (2007) studied hot days trend and Farjzadeh and Darand (2010) investigated the relationship between hot days and mortality rate. Yazdan Panah and Alizadeh (2012) had an investigation into probability occurrence of hot days based on Markov chain model. Study Area: In this study، atmospheric conditions during the most pervasive hot day over Iran were investigated; in addition، a new method for hot day identification was applied. To this end، Iran and also an area between 10W to 120E and 0 to 80N were taken as our field study. In the present study، in order to investigate synoptic of the most pervasive hot days in Iran، the circulation to environment approach was utilized (Masodian 2005). Accordingly، the following databases were used: 1-The maximum of daily mean database with 15*15 kilometer resolution during 1963-2009 was used. This database was obtained from 664 synoptic and climatology stations using Krigging interpolation method. Therefore، the database contains 17166*7187 dimensions and every pixel on each day map has its own mean of temperature value. According to the definition by «The Join World Meteorological Organization Commission for Climatology”، a hot day is a day when the temperature of each pixel in the country and each day is more than 90th percentile of a given pixel and a given day. Therefore، 366 maps of hot day threshold have been created. These maps were hot days criterions. Country experienced the most pervasive hot day in 2004/3/7. In this day، virtually 96. 7% of the country experienced temperature which was above the defined threshold temperature. 2- The atmospheric database for the day with the most pervasive hot and also for the period of 1963-2009 which was obtained from NCEP/NCAR contains sea level pressure (SLP)، meridian and zonal components of wind، temperature and 500 hp geopotantal heights. Synoptic and dynamic analyses were carried out in order to investigate atmospheric situation synchronized with hot day: - Synoptic analyses of SLP and 500 hp were analyzed to reveal the pressure and the height of atmosphere and their anomalies in 2004/3/7 and also they were compared with those in 1963-2009 mean. - Dynamic analyses were considered jet stream، advection and front genesis function. The jet stream was investigated in four levels (300،400،500 and 600 hp). Finally، in order to analyze the dynamic – thermodynamic relations of atmosphere، the relationship of vorticity and temperature in aforementioned level has been estimated based on Pearson correlation coefficient. The maximum temperature in (2004/3/7) occurred in southeast of the country specified from 39 to 40 degree centigrade، while the minimum temperature specified from 0 to 3 degree centigrade has happened in a small area at north east of Eastern Azerbaijan. There are only small distinct areas that cover 3. 3% of the country (Northwest، Caspian coast and in Northeast of the country) where the positive anomalies have not occurred. Generally، most parts of the country experienced positive anomalies and there are also areas in Kerman،Yazd، Semnan And Khorasan Razavi characterized by up to 16 degree centigrade positive anomalies. Sea level pressure showed two high pressure systems were located in east and west of Iran، meanwhile the polar low came from southward to north of Iran. Therefore، a dramatic pressure gradient in between caused a massive hot advection toward Iran. The pressures anomalies in all over Iran were positive and up to 5 hp were negative through northern part and in small area of Oman sea coast. In the 500hp level، a two center cut off system was observed on Russia and Iran is located in front of Mediterranean-Red sea trough. This pattern moved air masses from lowest latitude، northeast Africa، toward Iran. Also a ridge on Iran caused a dramatic height gradient due to its adjacency with northern cut off. This condition caused a negative anomaly pattern all over Iran. The anomalies are -550 to -300 meters. Investigating westerly''s jet stream، it revealed that jet stream came to 600 hp in southwest-northeast orientation which was covered northwest of Iran to northeast of Kazakhstan. A vast area of Iran was located in southern mouth quarter in which cyclonic convergent air and dropping pressure level were naturally expectable. Calculating front genesis function، it is clear that there was front genesis potential all over Iran. All situations mentioned tend to have warm advection in all atmospheric levels toward Iran although there are some reigns characterized by cold advection in which the temperature anomalies are still positive. The correlation between lowest levels of atmosphere and uppers indicated a strong positive correlation. This indicates a thick atmospheric layer warming limiting heat transfer. The pervasive hot day was due to difference in pressure pattern causing a deep atmospheric front and a high and deep jet stream. There are other patterns such as negative anomalies in atmospheric height، warm advection. Warm convergence in all atmospheric levels tends to decrees spatial temperature difference. Consequently، it caused difficult situation for heat vacation. It is obvious that surveying hot days necessitate thermodynamic as well as dynamic consideration.

Climate in the south of the Caspian Sea is very different from other parts of Iran. The existence of the Caspian Sea، complex terrain، and the fact that large-scale circulation patterns، as well as local، regional، and extra-regional circulations affect this region، has caused the climatic characteristics of the region to become very unique. The south part of the Caspian Sea receives a significant rainfall in autumn and winter، but in contrast، the least amounts in spring. The incidence of extreme rainfall and flooding in this season could impose damages to the region. Therefore، it is reasonable to identify mechanisms of development، strength، and laws of movement and extension of the synoptic and dynamic patterns to amid positive effects and avoid the harmful results or decrease them. Dynamical studies have a very special place in the study of climate of a region، so that by using the dynamic indices، we can analyze more accurate atmospheric phenomena. This paper attempts to apply a dynamical and synoptic approach to investigate extreme spring precipitation، zoning amounts of total rainfall of every pattern، and provide a model for dynamic and synoptic patterns in the study area. Study Area: The study area is includes the northern provinces of Iran، Gilan، Mazandaran and Golestan in the southern coast of the Caspian Sea and north of the Alborz mountain. This area is between latitudes 36 degree and 34 minutes to 38 degree and 28 minutes of north and longitude 48 degree and 50 minute to 54 degree and 2 minute of east (figure1). Also the rainiest stations and consequently perennial rivers and dense forest cover has been in this region of Iran

In this study، a statistical - synoptic method is used to investigate the dynamical patterns of the extreme spring precipitation in the southern coast of the Caspian Sea. In this regard، the daily rainfall data from 11 weather stations located on the southern shores of the Caspian Sea over a period of 50 years (1961-2010) were obtained from Iran Meteorological Organization. The selection criterion for extreme precipitation is the 24-hour rainfall in which 70% of stations would be equal or more than 20% of the long-term average of the seasonal rainfall. Using data from National Centers for Environmental Prediction / National Center for Atmospheric Research with a horizontal resolution of 2. 5°×2. 5°، the prevailing daily average atmospheric conditions were analyzed. The data include geopotential height، the zonal (u) and meridional (v) wind components، specific humidity and vertical velocity for 1000 hPa and 500 hPa levels. By using abovementioned data، the dynamical parameters of relative vorticity، vertical velocity، divergence and convergence of wind fields and vertical profiles of relative vorticity، divergence and convergence of wind fields separately for each model studied and produced by GrADS software. The maps were analyzed and finally dynamical patterns were identified.

The main objective of this study is to determine the extreme spring precipitation patterns associated with the dynamical quantities in the north coastal strip of Iran، so the weather map of severe-pervasive rainfall are extracted and dynamical analysis is done. The 500 hPa level maps show that the extreme spring rainfall in the southern coast of the Caspian Sea mainly follows four patterns. The main feature of the first pattern is the blocking system (Omega)، which is primarily located in the east of Europe. The system is created from the northward extension of high pressure of North Africa and southward development from the low level polar front in high latitude and also strengthening of a blocking system by the Mediterranean cyclone. In this pattern، Mediterranean cyclones are a major factor of spring rainfall in the southern coast of the Caspian Sea. Also، the European cyclones with amplification by low heights of the high latitude will cause a large amount of rainfall in this season in the north of the country. In addition، the extension of the Saudi high pressure to the East، and expansion of the subtropical high pressure outbreaks to above 80 degrees north latitude، makes a good condition to merge the European cyclones and Mediterranean cyclones in the study area، which eventually lead to create the heavy rainfall in the northern region of Iran. The third spring rainfall pattern is influenced by blocking systems in the Central Asia and Siberian regions. However، the expansion of the subtropical high pressure in the north-east and existence of the Saudi Arabian high pressure in the south of country، have a negative effect on the trough of low heights so that it prevents the system to create heavy and flash flood rainfall and it decreases the rainfall. In the fourth pattern، the blocking system is the dominant form of the spring precipitation in the southern coast of the Caspian Sea. This pattern is somewhat similar to the first pattern، which the different subtropical high pressure systems abnormally extend to the north in the spring. It is observed that even outbreaks reach to the 50°-60° N. In addition، low heights of the high latitude and Mediterranean cyclones also play a major role in the precipitation pattern. To provide a comprehensive view of the nature of atmospheric circulation in the southern coast of the Caspian Sea in spring، vertical profiles of divergence and convergence in patterns of along the 37 N were prepared. The results showed that the southern coast of the Caspian Sea and East Mediterranean during the active patterns of precipitation in the spring have a prevailing western streams (Eastward) at levels of 500 hPa to 150 hPa. Eastward flows through the orbital inclination have the maximum intensity in 300 hPa to 100 hPa levels and areas of divergence maxima and minima convergence and also the fields of horizontal divergence and convergence match at 1000 hPa.

The results showed that extreme spring precipitation in the southern coast of the Caspian Sea follows four major patterns. The main feature of the first pattern is the blocking- omega system that is located in Eastern Europe and Ural Mountains that lead precipitation induced systems to the southern shores of the Caspian Sea. In the second pattern، the Mediterranean cyclones are considered as a main factor of the spring rainfall in the study area. The third pattern of spring rainfall is formed when a blocking system exist over Siberia and Central Asia. Finally، in the fourth pattern، the blocking system is also the main cause of spring precipitation in the southern shores of the Caspian Sea. This pattern is somewhat similar to the first pattern، which is different from the subtropical high pressure systems abnormally extend to the north in spring. It is observed that even outbreaks of it reaches to the 50°-60° N. Investigation of the dynamical parameters showed that during the activities of the system، in all cases، the vertical velocity of the atmosphere is negative and the positive vorticity is high and the convergence zone is located on the shores of the Caspian Sea. All these factors imply that the dynamical weather condition in the region is appropriate for dynamically fast rising of particles to the middle and upper atmosphere، which eventually create heavier spring rainfall.

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.