فهرست مطالب abass ranjbar saadat abadi

The Great Khorasan in northeast Iran has a variety of surface structures and plains and high peaks, but due to its vicinity to the deserts of Afghanistan and Turkmenistan, it is affected by dust all the time, especially in summer. The purpose of this study was to simulate summer dust in this region by RegCM model. For this purpose, during the period 2000 to 2017, three extreme dust events were selected. The satellite image used to confirmed dust mass presence and then the synoptic structure was analyzed. Finally, the simulation results of RegCM 4.6 model were compared with the observational data including the horizontal visibility and aerosol optical depth (AOD) of Aqua satellite. The synoptic analysis showed that during the summer, low thermal pressures form in the southern Afghanistan and high pressure in the north. This structure lead to the development of north and northeast winds with speeds of 12 to 21 m / s and dust emission on the eastern border of Iran and western Afghanistan. Investigation of RegCM accuracy done by visibility, Aquas’s AOD showed that model performance in South Khorasan is better as Razavi Khorasan. The highest correlation coefficients of AOD of model and horizontal visibility were obtained at Khorasan central stations including Gonabad, Ferdows, Nahaband and Ghaen at -0.82, -0.77 and -0.44 respectively. RegCM model performed a better dust simulation in severe dust with a horizontal visibility down to less than 1000 m, high continuity and horizontal extension. Overall, the RegCM model underestimates the AOD value for the Aqua satellite algorithm.

A synoptic perspective was adopted in this study to investigate two winter precipitation systems that caused a flood in Sistan and Baluchestan and a heavy snowfall in Gilan in January 9-12 and February 9-12, 2020, respectively. The results demonstrated that although different in shape, both systems were cold-core and similar in their formation and strengthening. Amplification of the mid-level trough, formation of cyclonic rotation, positive relative vorticity advection to the east of the trough, convergence in lower levels, and formation or strengthening of low-pressure at the Earth’s surface to the east of the trough, played the key role in developing and strengthening these two systems. These mechanisms are consistent with the theory of development and strengthening of mid-latitude low-pressure systems. A noticeable difference between these two systems is the continuity and increasing speed of the north wind in the first system, caused by the meridional shift in the polar jet stream. These conditions have caused the cyclonic curvature formed in the subtropical jet stream axis in the first system to be increase and extended to the south. These curvatures are located to the east of the Red Sea in the first system and northeast of the Mediterranean Sea in the second system. Also, in the first system, the advection of warm moist air from the Arabian Sea and the Gulf of Oman, and in the second system, the advection of cold air from the Caspian Sea have provided suitable conditions for convectional rainfall and heavy snowfall, respectively.