Ms. Aida Faroghi, Professor Manuchehr Farajzadeh, ,
Volume 0, Issue 0 (3-1921)
Abstract
In this research, the frequency of merging of the polar-front and subtropical jet streams and its effect on the amount of precipitation received from the atmospheric pattern in western Iran during the 10-year statistical period of 2010-2019 was investigated. Then, by coding in Grads, the maps of jet stream level 300 hPa were drawn with a time interval of 6 hours. During the statistical period, these two jet streams merged along their axis. By examining the frequency of the merging of two jet streams, it was found that until 2015, the frequency of the merging of two jet streams in December was an increasing trend and in 2016 and 2017, there was a decreasing trend. It has increased again in 2018 and 2019. In all cases, the merging of two jet streams has not resulted in heavy precipitation events (December 2011, 2014 and 2017). So, considering the heavy rainfall of 110 mm from Dehloran station, the date of 12th to 15th of December was chosen to analyze and understand the system that led to the occurrence of rainfall. From December 12 to 15, 2010, the decrease in air temperature over Europe and Southwest Asia led to a large meridional movement of the polar-front jet stream, which resulted in merging with the subtropical monsoon. On the 12th of December 2010, along with the medicalization of the polar-front jet stream and its penetration into the tropical regions, its velocity core merged with the core of the subtropical jet stream over the northern Arabian Peninsula, the Red Sea and northeastern Africa. The merging of these two jet streams has led to the vertical expansion of the jet to lower levels, and in the middle level of the atmosphere, very low meridional movement is observed. As a result, the Sudan low moves to higher latitudes and merges with the Mediterranean low pressure.
Mahdi Khazaei,
Volume 21, Issue 63 (12-2021)
Abstract
This study investigates the subtropical anticyclones and summer monsoons of the north hemisphere in the 500 hPa level, effective on Iran in a 38-year period (1975-2012). For this purpose, the data of geopotential heights of 500 hPa at 12 UTC with a spatial resolution of 2 by 2 degrees of the (NCEP/NCAR) were used. Results show that three independent patterns (Azores anticyclone, Saudi anticyclone and North West Africa anticyclone) and four combinatory patterns derived from the independent patterns have controlled the atmosphere of 500 hPa level of Iran. In most cases, the maximum in geopotential heights in above-mentioned anticyclone centers reach to more than 5920 geopotential meters, but often the tongue of these anticyclones control the atmosphere of 500 hPa levels of Iran with the geopotential height of 5880 to 5900 meters. In contrast, at this pressure level, the large-scale monsoon system is very weak and the maximum geopotential height of this system is about 5840 meters. The extent and intensity of the monsoon system has been reduced so that its tongues do not affect the 500 hPa level of Iran at any time. Given that the main factor in the development of large-scale monsoon system is the surface heating (bottom-up formation) and the main factor in the development of subtropical anticyclone is descending of atmospheric dynamics, the main reason for the development of summertime subtropical dynamic anticyclone should be a very strong and unit agent, something other than the summer monsoons.
