Document Type : Research Paper

Authors

• saeed jahanbakhsh ¹
• Ali Mohammad Khorshiddoust ²
• Hamid Mirhashemi ³

¹ proffesor Professor of Climatology, University of Tabriz

² Professor of Climatology, University of Tabriz

³ PhD Candidate in Ecology and the University of Tabriz

Abstract

Introduction
Due to their height, geometry and orientation, the mountains affect the atmospheric currents. Therefore, mountains cause the formation, intensification or weakening of many atmospheric phenomena. One of the most obvious of these phenomena is lee cyclogenesis whose formation and evolution is subject to interaction between mountains and atmospheric currents.
The lee sides of mesoscale or large-scale mountains, such as the Alps, Rocky Mountains, the East Asian mountains and the Andes, are favorable regions of cyclogenesis. This type of cyclogenesis is known as lee cyclogenesis, and can be defined as the formation of a cyclone with strong positive vertical vorticity or an appreciable fall in pressure with a closed circulation formed in the lee of a mountain that then drifts away (Chung. et al, 1976). In this regard, most studies have been made about the lee cyclogenesis of the Alps and the Rocky Mountains. As a result, the general understanding of mechanism of lee cyclogenesis is derived from studies conducted on these two mountains and partially due to the Alpine Experiment (ALPEX) field project held in 1982. In other words, the general framework of the mechanisms that exists today about lee cyclogenesis is based on the results obtained from the study of the Alps and the Rocky Mountains.
 
Methodology
The variables used in this study are consists of horizontal wind speed (u and v), vertical wind speed vertical pressure coordinates (w), temperature (T) geopotential pressure levels (z). The data variables used 1 and 0.5 degrees in the horizontal and vertical distance of 50 hPa from 1000 to 200 hPa. The data were extracting from the archival database ECMWF ERA Interim version with six-hour monitoring. Preliminary investigations showed that unstable systems under certain elevation models are located in the middle of the atmosphere in the region of the west, northwest of the west and west to the upstream slopes of the Zagros Mountains. so that the study of middle - level maps showed how the trough and the ridge of such systems has played an important role in determining the direction and movement of these systems towards zagros because it is different in terms of the confluence or diffluence system, curvature and sheer vorticity advection and ageostrophic wind of the trough, and finally the direction in the trough. In general, the conditions prevailing on these systems and the it direction they pass through to the Zagros have an important role in how they interact with the zagros. Therefore, it is necessary to examine how these systems approach to Zagros. For this purpose, this important issue was further investigated using the ageostrophic wind equation. Because the ageostrophic wind indicates the imbalance of geostrophic, and in fact the change of wind both in space and time causes this imbalance of geostrophic and the creation of the ageostrophic wind (Lynch and Cassano 2006, 123). Also used curvature vorticity in the natural coordinates, quasi-geostrophic height tendency equation and divergence of the isallobaric wind.
 
Results and discussion
An examination of the maps of the lower atmospheric levels shows that in some cases the cyclones approaching the windward slopes of the Zagros, under the baroclinic wave’s atmosphere, lead to the formation of a secondary rotation in the lee ward Zagros. Therefore, it is necessary to have a primary cyclone on the windward slope of the Zagros Mountains to form the lee cyclone. So the detection of cyclone approaching windward of the Zagros Mountains shows that cyclones which move and even higher than the Mediterranean latitude to the east, would have approached the Zagros in a way that was accompanied by a pattern of confluence ridge - diffluence trough in the middle of the atmosphere. While the cyclones, accompanied by a trough, were moving northward along the path of their eastward before reaching the Zagros in the north east. On the other hand, the cyclons, formed in the Red Sea region or Sudan, were required to reach the Zagros Mountains (the maximum velocity at the base of the trough) or the diffluence trough to approach the zagros. At the same time, the cyclones that were located in the area were under a confluence trough, never moved to the Zagros Mountains. So, according to the latitude of the primary cyclone and how the stack - trough is the level of 500 hpa, the path of the approaching Zagros is different. However, in many cases the nature of the trough and the level of the level of 500 hpa on the path of the eastward movement changes from difflunce to confluence and vice versa, which occurs due to the changing location in the baroclinic wave
 
Conclusion
Because The Zagros Mountains create changes in approaching baroclinicity waves causes advent thermal anomaly and consequently cause the occurrence of cyclone, so, such cyclogenesis  is interpreted adjust according to the theory modify baroclinicity waves. Because this theory clearly states that mountain lead to lee cyclogenesis by modify baroclinicity. It is worth noting that many of the much complexities of the relationship between the Zagros Mountains and baroclinicity waves that may cause the occurrence of cyclones are a variety of other theories proposed in the literature to be more consistent Atmospheric Sciences. In any case, this study is based on small portions and a special kind of cyclogenesis can be in the Zagros leeward.

Keywords

• Lee Cyclogenesis
• Occluded Front
• potential vorticity
• confluence
• diffluence