جستجوی مقالات مرتبط با کلیدواژه "چرخندگی" 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.

Abstract In this research, the seasonal behavior of polar jet streem has been identified based on the cold season (autumn and winter) on Iran. General analysis of jet streem process based on what is mentioned in the methods section was done in order to find the behavior patterns of jet streem in GRADS software environment . The results of the analysis indicate the existence of two main medium nuclei, one above Europe and a wider nucleus over North Africa and Saudi Arabia. The prevalence of nuclei in Europe has been higher than in North Africa. In autumn, the incidence of North African nuclei was about 30% lower than in Europe, and in winter the frequency of both nuclei was similar. The core of the polar jet streem in the two seasons, is located approximately 35 degrees north latitude. According to the information obtained from the average speed maps in autumn and winter, it seems that the speed change has been more than before.

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.

In order to investigate the spatial and temporal variations of relative vorticity in Iran, we have extracted the mean monthly relative vorticity data from the NCEP Reanalysis data site for the window of 45 to 62.5 degrees east and 25 to 40 degrees north for the 1971-2005 period with 2.5 degrees resolution. The mean monthly maps of the extracted data were drawn and described. The results showed that in winter season most of the country is under the positive relative vorticity indicating the existence of instability over the country and invasion of westerlies resulting in rain. The spring season was the same as winter, but toward the end of the season the negative vorticity was invading the country from the south. So that, in summer most of the country was occupied by the negative voriticity and resulted stability. During this season only the southern coasts of the Caspian was out of the negative vorticity occupation. During fall season the winter conditions were dominating the country from the north.

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.