عنوان مقاله [English]
The activity of synoptic cyclones plays an important role in determining the local climate and forming large-scale atmospheric circulation through the vertical and horizontal exchange of heat, humidity and momentum, coupled with interaction with large scale circulation centers. The cyclones are generally transmitter of the bad weather conditions and also represent the initial mechanism of transmitter moisture and heat to the pole. Systematic changes in geographical location or in the intensity / frequency of cyclone activities will make significant disparities among other regional climate impacts. The effects of mountainous obstacles on synoptic systems, especially the cyclone systems, are recognized. The mountain range is one of the factors that in addition to disrupting the uniformity of the earth face, it also disrupts the climatic uniformity. The purpose of this study is to determine the influence of thermodynamics of Zagros Mountain on the changes in cyclones entering the country from the west.
Materials and methods
For this purpose, the daily precipitation data were obtained from 13 stations of the Meteorological Organization in west Iran. Also geopotential data were extracted from the NCEP / NCAR databases with spatial resolution of 2.5 × 2.5 degrees and ERA-Interim data from ECMWF databases with spatial resolution of 0.125 × 0.125 degrees, their framework is 0 to 80 degrees east and 0 to 60 degrees north. Using the Factor Analysis method, April 14th-18th, 2003 was selected as the best pattern. After selecting the sample day, sea level pressure maps and geopotential heights of different levels were prepared and analyzed.
Results and discussion
The results of the analysis of these maps showed that the cyclone reaching the Zagros Mountains are dynamically strengthened from the day it formed until it arrived in Iraq. When they approach the Zagros, the vorticity and its omega are reduced, but crossing Zagros, a positive vorticity increase happens. These types of cyclones call Zagros cyclones. The relationship between the amplified cyclone with the divergence region of the middle and middle levels were observed at all stages. The Zagros roughness, like a wall, initially weakens the cyclone reached Iran and makes them bipolar. However, the passage of the cyclones from the mountain make the thermodynamic conditions of the descending air in the lee mountain range. The condition makes them revival. As the air reaches the roughness, a weak core remains in the Zagros range, and another nucleus is formed by passing through the mountains in the central regions of the country, and is reinforced in the next hours. Finally, the cyclone is amplified and leaves its moisture completely on Iran. These cyclones can be called Zagros second cyclones.
Mountain barriers are considered as the factors destroying the homogeneity of the local climate. Sometimes they act in the planet scale like the Rocky Mountains. Iran has a heterogeneous environment in term of geomorphology and climatology. One of the most outstanding effects of roughness on the climate is the change in the structure of systems passing through these barriers. Zagros Mountains is one of the main mountain ranges of Iran, with an almost northwest-southeast direction and with a maximum height of about 4,400 meters at Zardkuh Peak. It has a significant impact on immigrant systems to the country.
A study on the cyclone on April 14th, 2003 showed that this cyclone was formed on the April 12th on the northwest Europe, moving towards the Mediterranean Sea. Its trough arrives in the country on the April 14th and it reaches the slopes of Zagros on the 16th. As it is approaching Zagros, changes in pressure in the back and the lee of Zagros are increasing. Vorticity and divergences are completely different in two parts. In the Zagros, during a few days when the cyclone pass across the range, there is a negative vorticity. The vertical velocity also demonstrates subsidence in Zagros altitudinal areas. The results vividly prove that the cyclone gets weakened in collision with the mountain, and its movement gets slow, but it does not disappear. It is re-reinforced on the Zagros lee in the central part of the regions, and continued its route to outside the borders of the country.