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.
Atefeh Rezaei Talei, Zahra , Buhlol Alijani, Hematolah Roradeh, Taher Safarrad,
Volume 0, Issue 0 (3-1921)
Abstract
| The role of regional and extra-regional forcings causes changes in the Siberian high-pressure cyclonic circulation every year. In this regard, an attempt was made to investigate the variability of the intensity of the Siberian high pressure in relation to the abnormal winter circulation of the atmosphere. For this purpose, gridded data of sea level pressure, geopotential height, orbital and meridional wind components of Center for Environmental Prediction/Atmospheric Sciences (NCEP/NCAR) between the years until 2020 (December to February) were used. After applying the Siberian high pressure intensity index, the extreme periods of this high pressure in winter were extracted. Next, the development of this system and the systems affecting the Siberian high pressure were investigated using the quantity of relative Vorticity advection. The results showed that during the winter period, the high pressure center of Siberia has undergone changes and has taken on a growing trend between 4 and 6 hectopascals. On the other hand, it was observed that the role of atmospheric systems such as dynamic ridges on the Siberian region along with advection The negative relative humidity, the location of the Siberian region in the east of the ridge, and the formation of omega bands play an important role in strengthening this system, which has caused this high pressure to develop from east to west or north to south. On the other hand, the role of the polar vortex in the cold advections of the region and the displacement towards the equator has caused the Siberian high pressure to sometimes increase by more than 11 hectopascals compared to its long-term average. Finally, it was observed that the atmospheric circulation in mid-latitudes plays a transitional role in the high pressure changes in Siberia and the polar and subpolar currents. |
Dr. Ruhallah Moradhaseli, Dr. Ali Bayat, Mrs. Fateme Radmehri,
Volume 23, Issue 70 (9-2023)
Abstract
Aerosol optical depth in 550 nm and angstrom exponent measurements with MODIS have been studied with 1-degree resolution for the period 2006-2017 in the middle east. Moreover, tropospheric aerosol optical depth and depolarization ratios measured at 532 nm with CALIOP have been studied for same area and same period of time too. These parameters have been classified seasonally. Optical depth results show high values for the region especially in spring and summer seasons. During the cold seasons, optical depth values are much less compared with their values at warm seasons. At spring, dust sources located in northern Iraq and those located in central and northern parts of Arabian Peninsula are much more active. Sources located in southern parts of Arabian Peninsula get more active by summer. Angstrom exponent results show that in arid and semi-arid parts of middle east, aerosol sizes are mainly in coarse mode. In arid parts of Iraq and Arabian Peninsula coarse mode particles are dominant during 4 seasons, but for arid parts inside Iran coarse mode is dominant during warm seasons and a modification in suspended particle sizes can be seen during cold seasons. Depolarization measurements of CALIOP show that almost in all seasons, non-spherical particles are ready in middle east atmosphere which is usual for an area inside the dust belt.
