سازوكار شكل‌گيري باد گرمش در‌ ‌‌البرز ‌

Mechanism of Garmesh wind formation on western half of Alborz Mountains

براي بررسي شرايط جوي در زمان وقوع باد گرمش، روزهاي شاخص اين پديده از بانك اطلاعاتي 29 ساله ي باد گرمش گيلان (2010-1982) استخراج شد. ميدان هاي فشار، دما، نم ويژه، ارتفاع ژئوپتانسيلي، سرعت قائم، مؤلفه هاي مداري و نصف النهاري باد، فرارفت رطوبت و دما، جريان، تاوايي نسبي و برش قائم كميت هاي ديناميكي در سامانه هاي منجر به اين پديده در همه ترازهاي جوي مطالعه شد. از تصاوير سنجنده ي موديس، ماهواره هاي ترا و آكوا براي تأييد وجود ابرناكي و بارش (برف) در دو سوي رشته كوه البرز استفاده شد. به سبب ابرناكي و وقوع بارش در هنگام باد گرمش سه دسته الگو شناسايي شد: دسته ي اول موارد رخداد باد گرمش همراه با آسمان صاف و بدون پديده در دو سوي رشته كوه البرز، دسته دوم فقط وجود ابرناكي در هنگام باد گرمش و دسته سوم موارد همراه با وقوع بارش را در دامنه ي جنوبي رشته كوه البرز در زمان باد گرمش شامل مي شود. نتايج نشان مي دهد كه تفاوت اين سه دسته، علاوه بر الگوي سطوح مياني و فوقاني وردسپهر، در الگوي همديدي توده هواي مستقر در سطح زمين است. در دسته اول و دوم، استقرار توده هواي پرفشار در نواحي مركزي فلات ايران و نفوذ زبانه ي كم فشار در شمال رشته كوه البرز موجب شكل گيري جريان هاي جنوبي به سمت سواحل جنوبي درياي كاسپين و افزايش سرعت باد در لايه هاي زيرين وردسپهر مي گردد. اما در دسته ي سوم گسترش كم فشار به سمت دامنه ي جنوبي البرز و نواحي شمال غربي ايران، شكل گيري مولفه ي مثبت باد نصف النهاري را به سمت دامنه هاي شمالي و پشت به باد البرز در پي دارد.

Mountain systems have an important role on meteorological variations. Different components of the mountain affect the atmospheric parameters and have essential role in atmosphereic circulation. Garmesh wind is one of the most wellknown phenomena that are related to mountain systems. In this research, mechanism of garmesh wind are identified using database of garmesh wind in the last 29 years and using remote sensing technology from 2005 to 2010. To survey the Synoptic and dynamic conditions of atmospheric patterns in the Garmesh windrsquo s events in the region, SCDATA of several synoptic stations in Gilan province, including Rasht, Bandar Anzali, Astara and Jirandeh are used which had continuous longterm data in 19822010period After Identification of days with Garmesh wind, daily images of Modis sensor of terra and aqua satellites in visible band and 721 band are monitored for checking the cloudiness on the both sides (southern and northern slops) of Alborz mountains and data of Jirande station in southern slop of Alborz mountains are used for detecting atmospheric phenomena like precipitation and snowfall. Also for studying the synoptic and dynamic pattern of this phenomena, reanalysis data from NCEP/NCAR were used. In this research, Based on the presence or absence of the atmospheric phenomenon (like rainfall and snowfall), three categories were identified. In the first category, Garmesh winds were happened in clear sky conditions and without any atmospheric phenomena on both side of mountainrsquo s slope. In the second category, only cloudiness was seen at the time of the Garmesh wind. In the third category, precipitations (in this research, snowfall) were seen in southern slope of Alborz Mountains. Statistical analysis of Garmesh wind in central plains of Gilan Totally, Occurrence of Garmesh wind was 479 days in Rasht, during 19822010. The frequency of occurrence of this phenomena was in January, February, November and December and rarely, in September and June. Clouds that observed in the time of Garmesh wind were: Altocumulus (type 4), Cirrus, CirroCumulus. Patterns of Garmesh wind mechanisms on western half of Alborz Mountain: B1. Garmesh wind without any phenomena This category includes11 cases of total 47 studied cases. 29 January 2008 is an exle of clear sky condition in the time of Garmesh wind. In this pattern, in the surface zonal extension of Mediterranean dynamical low pressurersquo s contours from west of Caspian to Gilan plain and at the same time formation of cold high pressure cell on Zagros mountains caused strong pressure gradient on southern coastal zone of Caspian Sea, As it led to the the increase of wind velocity in Rasht airport synoptic station from 11 kilometer per hour in 00 UTC to 36 kilometer per hour in 12 UTC. Dominance of warm core on southern Caspian versus dominance of cold surface air on Iran Plateau indicates adiabatic warming in northern slope of Alborz Mountains. B2. Garmesh wind with cloudiness This category includes 34 cases of total 47 studied cases. Free of air massrsquo s patterns in the surface and conditions of atmospheric flows in lowtroposphere that are similar to previous category, transition of height trough in midtroposphere and hightroposphere can be name variant component verses previous category. B3. Garmesh wind and precipitation (snowfall) This category includes 2 cases of total 47 studied cases. At the same time, surface high pressure was on Iran Plateau and low pressure system was on Caspian Sea and also Gilan providence that caused the formation of Northerly stream and westeast stream to southern coastal zone of Caspian Sea and backward of Alborz Mountains like other patterns, snowfall occurred on southern slope of Alborz Mountains. Strong southern and southwestern stream and strong positive vorticity on southern slope of Alborz Mountains by deep height trough in lowtroposphere has an important role on intensification of vertical motions on lee ward of Alborz Mountains. Garmesh wind is an atmospheric phenomenon that occurs as a result of interaction between atmospheric systems in synoptic scale and topography on back ward of mountain. In the other words, existence of Alborz Mountainrsquo s as a great wall has an important role in the interaction between synoptic systems and formation of Garmesh wind. Formation of Garmesh wind phenomena in Gilan province, is affected by extension of Siberian high pressurersquo s counters and subtropical high pressure on central of Iran Plateau and also existence of advection of pressurersquo s counter like subpolar low pressure and or the Mediterranean Sea on north of Alborz mountains are required. Without any notification to origin of air masses, three categories has been observed based on existence or absence of Phenomena (in this research, sowfall) In 700 and 500 hPa, Geopotential height patterns and relative vorticity field indicate that in the first category, wide parts of Iran is affected by high height and negative vortisity like low troposphere, during peak hours the wind. But in the second and third category (specially in third category ) existence of upper trough and easterly extension of trough caused to reduction of height and formation of strong positive vorticity in upper level and all over of air column in both south and north slopes of Alborz mountains.