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Investigating the variability of the Siberian high pressure intensity in relation to the anomalous winter circulation of the atmosphere
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.

Study of seasonal distribution of dust in the Middle East region using CALIOP and MODIS data
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.

Analysis of merging polar-front and subtropical jet streams and its effect on rainfall in western Iran from December 12 to 15, 2010
Ms. Aida Faroghi, Professor Manuchehr Farajzadeh, ,
Volume 25, Issue 78 (9-2025)
Abstract
In this study, the frequency of merging events between the polar-front jet stream and the subtropical jet stream, along with their impact on precipitation patterns in western Iran, was analyzed over a ten-year statistical period (2010–2019). Utilizing coding in GrADS, 300 hPa jet stream maps were produced at six-hour intervals. Throughout the study period, the axes of these two jet streams merged on several occasions.
An examination of the frequency of merging indicated that, prior to 2015, the frequency of merging in December exhibited an increasing trend. However, this trend diminished in 2016 and 2017, only to experience a resurgence in 2018 and 2019. It is noteworthy that not all instances of jet stream merging resulted in significant precipitation events (e.g., December 2011, 2014, and 2017). For instance, in light of the substantial rainfall of 110 mm recorded at the Dehloran station, the period from December 12 to 15, 2010, was selected for detailed analysis to elucidate the atmospheric mechanisms responsible for the rainfall. From December 12 to 15, 2010, a decline in air temperature over Europe and Southwest Asia prompted a considerable meridional displacement of the polar-front jet stream, resulting in its merger with the subtropical jet stream. On December 12, 2010, as the polar-front jet stream underwent meridional movement and extended into tropical regions, its velocity core merged with that of the subtropical jet stream over the northern Arabian Peninsula, the Red Sea, and northeastern Africa. The convergence of these two jet streams led to a vertical expansion of the jet stream into lower atmospheric levels. At the mid-levels of the atmosphere, minimal meridional movement was observed. As a result, the Sudan low-pressure system migrated to higher latitudes, merging with the Mediterranean low-pressure system.

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