The Influence of Zagros Mountains on Iran's Rainfall Cyclones

Document Type : Full length article

Authors

1 PhD Student of Synoptic Climatology, Faculty of Geographical Sciences, Kharazmi University, Iran

2 Professor of Climatology, Faculty of Geographical Sciences, Kharazmi University, Iran

3 Associate Professor of Climatology, Faculty of Geographical Sciences, Kharazmi University, Iran

4 Assistant Professor of Climatology, Faculty of Geographical Sciences, Kharazmi University, Iran

Abstract

Introduction
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.
Conclusion
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.

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Main Subjects


احمدی گیوی، ف. و نجیبی‏فر، ی. (1383). مطالعة چرخندزایی در پشت به باد کوه‏های آلپ و اثر آن بر آب و هوای خاورمیانه برای دورة یک‏ساله، مجلة فیزیک زمین و فضا، 30(2): 1ـ19.
خلج، ع. (1381). تحلیلی بر تأثیر رشته‏کوه زاگرس روی سیستم‏های سینوپتیکی مؤثر بر اقلیم ایران مرکزی، رسالة دکتری، دانشگاه تربیت مدرس.
سلطان‏زاده، ا.؛ احمدی گیوی، ف. و ایران‏نژاد، پ. (1386). بررسی سه‏ماهة تأثیر رشته‏کوهای زاگرس بر جریان‏های میان‏مقیاس منطقة شرق زاگرس با استفاده از مدل منطقه‏ایRegCM ، مجلة فیزیک زمین و فضا، 33(1): 31ـ50.
علیزاده، ا.؛ آزادی، م. و علی‏اکبری بیدختی، ع. (1387). بررسی رشته‏کوه البرز در تقویت سامانه‏های همدیدی، مجلة فیزیک زمین و فضا، 34(1): 9ـ24.
مرادی، م.؛ مشکواتی، ا.ح.؛ آزادی، م. و علی‏اکبری بیدختی، ع. (1387). شبیه‏سازی عددی نقش کوهستان در یک سامانة بارش‏زا روی ایران، مجلة فیزیک زمین و فضا، 34(1): 25ـ44.
Ahmadi Givi, F. and Najibifar, Y. (2004). The study of Cyclones on lee of the Alps mountains and its effect on the Middle East climate for the one-year period, The Journal of the Earth and Space Physics, 30(2):1- 19.
Alizadeh, A.; Azady, M. and Aliakbary Bidokhrt, A. (2008). Survey Alborz mountain range in strengthening synoptic systems, The Journal of the Earth and Space Physics, 34(1): 9-24.
Alizadeh, T.; Azizi, Gh.; Moheflhojah, A. and  Khoshakhlagh, F. (2017). Identification of the spatial- temporal variations of the intense cyclonic in the Mediterranean, with a numerical algorithm, The Journal of the Earth and Space Physics, 42(2): 405- 417.
Barry, R.G. (1992). Mountain Weather and Climate, Third Edition, Cambridge University Press.
Dacare, F. and Gray, S.L. (2009). The Spatial Distribution and Evolution Characteristics of North Atlantic Cyclones, Monthly Weather Review, 137:99-115.
Doyle, J.D. and Durran, R.D. (2002). The dynamics of mountain-wave induced rotors, J. Atmos. Sci., 59: 186-201.
Ferrero, E.; Longhetto, A.; Briatore, L.; Chabert d’Hieres, G.; Didelle, H.; Giraud, C. and Gleizon, P. (2002). A laboratory simulation of mesoscale flow interaction with the Alps, Dynamics of Atmosphere and Oceans, 35: 1-25.
Hayes, J.L.; Williams, R.T. and Rennick, M.A. (1987). Lee cyclogenesis. Part I, Analytic studies, J. Atmos. Sci., 44, 432-442.
Horvath, K.; Fita, L.; Romero, R.; Ivancan-Picek, B. and Stiperski, I. (2006). Cyclogenesis in the lee of the Atlas Mountains: a factor separation numerical study, Advances in Geosciences, 7: 327-331.
Iordanidou, V.; Koutroulis, A.G. and Tsanis, I.K. (2015). Mediterranean cyclone characteristics related to precipitation occurrence in Crete, Greece, Nat. Hazards Earth Syst. Sci., 15: 1807-1819.
Khalaj, A. (2002). An Analysis on the Impact of Zagros Mountain Range on Synoptic Systems Affecting Central Iran Climate, P.H.D thesis, Tarbiyat Modares University.
Lionello, P.;  Malanotte-Rizzoli, P.; Boscolo, R.; Alpert, P.;  Artale, V.; Li, L.; Luterbacher, J.; May, W.; Trigo, R.;  Tsimplis, M.; Ulbrich, U.; and Xoplaki, E. (2006). The Mediterranean climate: An overviewofthe main characteristics and issues, Developments in Earth and Environmental Sciences, 4, 1-26.
Maheras, P.; Flocas, H.A.; Patrikas, I. and Anagnostopoulou, CHR (2001). A 40 Year Objective Climatology of Surface Cyclones in the Mediterranean Region: Spatial and Temporal Distribution, Int. J. Climatol., 21: 109-130.
Morady, M.; Meshkati, A.H.; Azadi, M.; Aliakbary Bidokhti, A. (2008). Numerical simulation of the impact of orography on active synoptic weather systems over Iran, Journal of Earth and Space Physics, 34(1): 25-44.
Newton, C.W. (1956). Mechanisms of Circulation Change During a Lee Cyclogenesis, Journal of Meteorology, 13: 528- 539.
O’Handley, C. and Bosart, L. (1996). The Impact of the Appalachian Mountains on Cyclonic Weather System. Pat I: A Climatology, Monthly Weather Review, 124: 1353-1373.
Petterssen, S. (1956). Weather Analysis and Forecastining, Vol. 1, Motion and motion systems, Mc Graw-Hill, New York, 428 pp.
Reitan, C. (1974). Frequencies of cyclones and cyclogenesis for North America, 1951-1970: Mon. Wea. Rev., 102: 861-868.
Simpson. R, Isla, Seager, Richard, Shaw. A,  Thffany and Ting, Mingfang (2015), Mediterranean Summer Climate and the Importance of Middle East Topography, Journal of Climate, 28, 1977- 1996.
Soltanzadeh, A.; AhmadyGivy, F.; Irannezhad, P. (2007). Three Months Investigation of the Effect of the Zagros Range on Mesoscale Streams of the Eastern Zagros Region Using the RegCM Regional Model, Journal of Earth and Space Physics, 33(1): 31-50.
Tibaldi, S. and Buzzi, A. (1983). Effect of orography on Mediterranean lee cyclogenesis and its relationship to European blocking, Tellus, 35A: 269-286.
Wang, Xiaolan L.; Feng, Y.; Compo, G.P.; Swail, V.R.; Zwiers, F.W.; Allan R.J. and Sardeshmukh, P.D. (2013). Trends and low frequency variability of extra-tropical cyclone activity in the ensemble of twentieth century reanalysis, Clim Dyn, 40: 2775-2800.
Wang, L.X.; Swail, V. R. and Zwiers, F.W. (2006). Climatology and Changes of Extratropical Cyclone Activity: Comparison of ERA-40 with NCEP–NCAR Reanalysis for 1958–2001, Journal of Climate, 19: 3145-3166.
Zaitchik, B.F.; Evans, J.P. and Smith, R.B. (2007). Regional impact of an elevated heat source: The Zagros Plateau of Iran, J. Climate, 20: 4133-4146, doi:10.1175/JCLI4248.1.
Zarrin, A.; Ghaemi, H.; Azadi, M.; Mofidi, A. and Mirzaei, E. (2011). The effect of the Zagros Mountains on the formation and maintenance of the Iran anticyclone using RegCM4, Meteor. Atmos. Phys., 112: 91-100.
Zhu, X.; Sun, J.;  Liu, Z.;  Liu, Q.; Martin, J. (2007). A Synoptic Analysis of the Interannual Variability of Winter Cyclone Activity in the Aleutian Low Region, Journal of Climate, 20: 1523-1538.
Ziska, K.M. and Smith, P.J. (1980). The climatology of cyclones and anticyclones over North America and surrounding ocean environs for January and July, 1950-1977, Mon. Wea. Rev., 108: 387-401.