عنوان مقاله [English]
The atmosphere is characterized by the ubiquitous presence of a variety of swirling fluid eddies. Indeed, this observational fact prompted us in the previous chapter to consider the effects of flow curvature on some simple force balances. In the mid-latitude atmosphere the most important of these many eddies is the large-scale cyclone, also known as the synoptic-scale cyclone.
The surface analysis illustrated clearly demonstrates that in the northern hemisphere the winds circulate around the center of lowest pressure in a counterclockwise fashion over an enormous geographical area. Conversely, the surface winds circulate clockwise over an equally large area around the center of the northern hemisphere surface high-pressure system depicted. The ubiquity of large-scale, rotating disturbances in the mid-latitude atmosphere compels us to understand better the nature of fluid rotation and the resulting circulation of these eddies. Therefore, formation positive and negative eddies in each level shows stability and instability sequence. Very studies has been done on eddies, map patterns at varies regions and relationship with indexes or climatic elements that following mentioned.
Alijani (2001) studied the variations 500 hPa flow patterns over Iran and surrounding areas and their relationship with the climate of Iran. Time confine of this study was October to March during 1961-1990 years. The results this study showed that the characteristics of the 500 hPa flow patterns varied over monthly and annual time scales. Also in other part of results have been clears that troughs and ridges located close to Iran had more influence on the climate of Iran. Two troughs were identified and named the Caspian and Syrian troughs.
Tymvios et al (2010) studied the geopotential height patterns with heavy rainfall events in Cyprus. They believed that dynamically induced rainfall is strongly connected with synoptic atmospheric circulation patterns at the upper levels. The results showed that negative eddies at 500 hPa geopotential height had a important role in sever cyclonic conditions and creative heavy rainfall.
Gong et al (2007) have developed a statistical method and have reconstructed Northern Hemisphere 500 hPa heights back to the late 19th century using one temperature and three sea level pressure (SLP) data sets. First, the relationship between ERA40 500 hPa heights and surface temperature and SLP was screened using stepwise multiple regressions based on the calibration period of 1958â2002 (1998=2000 according to the availability of SLP data). Second, the regression equations were applied to compute the 500 hPa height through to the late 19th century for the whole Northern Hemisphere.
Hafez (2011) studied the relationship between 500 hPa geopotential height anomalies over North America and Europe and the USA landfall Atlantic Hurricanes activity. The results revealed that there are significant positive correlations between the anomalies in geopotential height over North America and East Europe simultaneously and existence of Atlantic hurricanes of category 3 that strike USA. Also to that, significant positive correlations between the anomalies in geopotential height over North America and existence of major Hurricanes (category 3, 4 and 5) that landfall USA is found too. However, significant negative correlations between the anomalies in geopotential height over North Atlantic and existence of all USA landfall Atlantic Hurricane category are existed else category 1. In general one can conclude that anomalies in geopotential height at 500 hPa level over North America and Europe are control the USA landfall Atlantic hurricanes activity.
Eddies, there are anomalies that have been created in different geopotentiol height atmospheric layers. But eddies intensity and frequency in 500 hPa geopotentiol height more than other levels geopotentiol height. Study on 500 hPa eddies, by reason of Effective role negative eddies (positive eddies) to creative positive vorticity (negative vorticity) that on climateâs world, have an especially important. In this research, for studying of eddies intensity and frequency effected on Iranâs climate during warm period (Spring and Summer seasons), have been used form 500 hPa geopotentiol height six hourly data, in geographical confine from to and to during 1960-2010. Therefore databases in MatLab software have been created from 500 hPa geopotentiol height data during 1960-2010. Use to this database intensity and frequency of eddies derivation and calculated.
Study on 500 hPa eddies, by reason of them role in formation of atmospheric stability and instability condition have a particular important. In this research frequency and intensity of eddies had been studied at 500 hPa geopotentiol height. Then in surfer software deriving maps from the results of calculations related to negative and positive eddy frequency and intensity
The results this research shows that in the start of spring season, Tender band from north of Persian Gulf to North West of Iran have been salient negative anomaly and gently to last of spring season whole of Iran affected negative anomalies on the 500 hPa geopotentiol height. Also in summer season, negative anomalies 500 hPa geopotentiol height have been seen in the south east of Iran and to the north west of Iran (in the central, north east and south west of Iran) had a trend decreasing and in the north west negative anomalies 500 hPa geopotentiol height had a trend increasing.