عنوان مقاله [English]
Cut-off low pressure systems are defined as closed lows in the upper troposphere that have become completely detached from the main westerly current.These systems are generally identifiable at mid and upper levels, by closed geopotential meters around the low center. Cut-off low systems are closed cyclonic eddies isolated from the main western stream. These lows are upper and midtropospheric features and consequently they do not need to have a corresponding low at the lower levels of the troposphere (Palmen and Newton, 1969). However, sometimes a Cut-off low may start as an upper-level trough extending to the surface after its development. Its intensity is higher in the upper troposphere, decreasing downward and being even possible to find anticyclonic circulation at the surface. They are defined as mid and upper tropospheric cold lows, which generate and develop in the westerlies. A split of the westerlies and a breaking of mid and upper level jet stream appear simultaneously with the generation of the closed low (e.g.Ndarana and Waugh 2010). Cut-off lows are upper-level low-pressure areas formed on the equatorward side of the maximum westerly winds in the polar or the subtropical jet stream. Traditionally, Cut-off lows have been recognized as depressions mostly located in mid latitudes, which are characterized by closed geopotential contours in isobaric maps (with a cold core) that have more or less concentric isotherms around the central core Because the jet stream corresponds to the boundary between two very different air masses, the air mass trapped within a COL maintains its polar characteristics.
Materials and methods
The characteristics of Cut-off Lows in Iran are studied for the period 1976–2015. To identify these systems, the Raul Nieto algorithm was used with automatic detection capabilities. The systematic identification of Cut-off Lows is realized by applying an original automated scheme using mean daily geopotential height, U wind and air temperature at 500 hPa NCEP Reanalysis data.
Results and discussion
According to the results the main regions of Cut-off lows formation affecting Iran in the east, west and south of Turkey, the eastern Mediterranean, and the Syrian and Jordanian countries.
The origin of these systems was also reviewed seasonally. In this study, Iran was divided into four regions and the frequency of Cut-off low in these areas was investigated. The results showed that the most frequent occurrence of these systems occurred in A region (northwest quarter of Iran) and then in region C (northeastern quarter). Out of a total of 628 cut-off low that affected Iran over the course of 40 years, 552 cut-off low have affected region of A (northwest quarter of the country). After that, the area C (quarter northeast) is passing through 283 cut-off lows.
The country's B region (quarter of southwest), with 162 cut-off lows, is ranked the next. Eventually, the D region (southeast quarter of the country) has the lowest abundance with 87 cut-off low during the 40-year period. In order to study more precisely the spatial distribution of cut-off low in the quadric areas, we analyze the spatial distribution of these systems Seasonally. In all four seasons, the region with the highest frequency of cut-off low is A (northwest quarter). This is due to the northwest quadrant on the main cut-off low including Turkey, the Black Sea and the Eastern Mediterranean. In areas B and D, the highest incidence of cut-off lows belongs to the winter season. Due to the fact that the western winds in the southern regions are too late in the middle of fall season and It also leaves the area very early in the spring, in the middle of the spring season, Therefore, the only season in which western winds are fully present is the winter. Therefore, the highest abundance of cut-off lows in these two regions is in winter. In areas A and C, most of the cut-off lows is in the spring and then there is a relatively small winter season. In total, in the A region, 552 cut-off low was identified during the 40 years period. Which is an average of 13.8 events per year. 1982, with 19 cut-off low incidents, and in 1992, 1996, and 2012, with 18 incidents, are most frequent in area A. The least frequent cases belonged to 2001 with the occurrence of 8 and 1978 and 1985 with event 9 in area A.
According to the results the main regions of Cut-off lows formation affecting Iran in the east, west and south of Turkey, the eastern Mediterranean, and the Syrian and Jordanian countries. In this study, Iran was divided into four regions and the frequency of Cut-off low in these areas was investigated. The results showed that the most frequent occurrence of these systems occurred in A region (northwest quarter of Iran) and then in region C (northeastern quarter). It was also found that in areas A and C, the largest Cut-off lows occurred in the spring and in the B areas (southwest quarter) and D (southeast quarter) in winter. The annual distribution of snow in the four regions of Iran was also examined. So, during the statistical period of 40 years, in 1982 in all four regions of Iran, the highest frequency of Cut-off low occurred.