اثرات باد گرمش بر وضعيت جوي ايستگاه همديدي رشت

The Effects of Garmesh Wind on Weather Conditions of Rasht Synoptic Weather Station

گرمش بادی گرم و خشك است كه بیشتر در فصل سرد سال، در شمال رشته كوه البرز می وزد. علت اصلی ایجاد باد گرمش استقرار سامانه پرفشار و یا زبانه آن بر روی فلات ایران و سامانه كم¬فشار و یا زبانه آن بر روی جنوب دریای كاسپین است. باد گرمش باعث افزایش پتانسیل آتش¬سوزی جنگل¬ها، آلودگی هوا، ذوب برف، ایجاد حساسیت و بیماری، تنش گرمایی بر روی محصولات كشاورزی و باغی، افزایش تبخیر و تعرق، از جا كندن درختان و در برخی موارد نیز سبب تخریب سازه‌ها و غیره می‌شود. در این مطالعه به بررسی دامنه تغییرات عناصر جوی در زمان رخداد باد گرمش در رشت در دوره آماری 1982 لغایت 2010 پرداخته شده است. نتایج بدست¬آمده نشان داد كه، با شروع باد گرمش میانگین دمای هوا در ایستگاه رشت 9 درجه سلسیوس افزایش و میانگین رطوبت نسبی 47% كاهش پیداكرده است. باد غالب در گلباد گرمش، دارای سمت جنوبی بوده و به طور میانگین سرعت متوسط آن در دوره آماری پیش گفته از 2 متر بر ثانیه به 5 متر بر ثانیه افزایش یافته است. بیشترین فراوانی رخداد باد گرمش در رشت برای ماه های دسامبر و ژانویه و در ساعات 09 و 12 گرینویچ ثبت شده است. در طی رخداد این پدیده دید افقی افزایش قابل ملاحظه می‌یابد و در بیش از 40% موارد آسمان صاف تا كمی ابری است. ابرهای ظاهر شده در زمان رخداد باد گرمش بیشتر از نوع ابرهای سطوح بالا و متوسط هستند، ابر صدفی شكل نیز از ابر‌های شاخص در زمان باد گرمش در رشت مشاهده شده است.

Abstract In general, Garmesh wind in Rasht province increases the temperature by 9°C and reduces relative humidity by 47%. The direction of prevailing wind during the Garmesh wind is Southern and its average speed increases by 2 m/s to 5 m/s. The highest frequency of Garmesh wind occurs at 09 and 12 UTC in December and January. The occurrence of this phenomenon substantially increases the horizontal visibility, so the sky would be clear in more than 40% of the cases. 1. Introduction Wind is one of the atmospheric elements which has a great impact on the region and local climate. Wind transfers air from one area to another. Thus, wind is very important in the regional climate study. Garmesh is a hot and dry wind that often blows from the north of Alborz Mountains in the cold season. The main reason for the creation of Garmesh wind is the simultaneous presence of high pressure system or its ridge on Iranian plateau and its association with low pressure system on the south of Caspian SeaGarmesh wind that is warm and dry is a well-known wind for the people in Gilan provience. This wind blows from the northern slope of Alborz Mountains and flows from Iranian plateau to the northern slopes of Alborz Mountains. The wind can last from several hours to several days. Relative humidity suddenly reduces and air temperature increases during the Garmesh wind. In many cases, the wind mostly covers the southern part of the Caspian Sea. According to Gilan Governor Documents, financial losses due to the Garmesh wind and fire forest was about 8.71478 million Rials in the period of 1381 to 1388. 2. Material and Methods Synoptic weather station of Rasht is located in the south of the Caspian Sea and north of Alborz Mountains in Gilan province. Its height is about 8 meters above the sea level. Atmospheric data including wind, pressure, cloudiness, air temperature and relative humidity were extracted in the period of 1982-2010. Then Visual Basic program was used to decode the data. Finally, these data were analyzed using statistical software such as Excel. 3. Results and Discussion The frequency of days with Garmesh wind during the statistical period (1982-2010) was 479 days. Mean kjnhgftr6yikmn bvds3 annual days with Garmesh wind is 16.5 days per year. In some cases, Garmesh wind blows for just one hour but in other cases it last for several days. This phenomenon is more frequent in the evenings times. The highest frequency of the Garmesh wind occurs in January, February and December. In more than 75% of cases the direction of this wind is southern. Under normal conditions 60% of times in the year, wind is calm in the Rasht station and more than 91% of cases, the wind speed is less than 3.6 m/s, but in more than 69% of the cases, the Garmesh wind speed is between 2.1 and 8.8 m/s. Normally, the relative humidity in Rasht station is 82% but during Garmesh wind it decreases to 34%. With the onset of Garmesh wind, temperature increases and this increment continues until the end of Garmesh wind period. In some cases in the other regions in the world, during Garmesh wind, temperature increases or decreases by 20°C in less than three hours. But in Rasht station it increases more than 9°C. In most cases of Garmesh wind occurrences, temperature varies between 15 to 25°C, and in 92% of Garmesh wind occurrences, it varies between 10 and 30°C. The most important cause of Garmesh wind in Rasht is the presence of high pressure system or high pressure system tongue in Iranian plateau and low pressure system on the south of Caspian Sea. In more than 94% of cases mean monthly pressure (QFF) increases and varies between 1000 to 1020 hPa during Garmesh winds. Usually, there are low values for horizontal visibility along with low pressure system, but at the beginning of Garmesh wind, horizontal visibility increases and then at the end of this phenomenon horizontal visibility decreases. In 23% of cases during the Garmesh wind, the sky is clear and in 43% of cases the sky is slightly cloudy (2.8 or less). In 13% of cases, Altocumulus cloud type 4 has been seen with Garmesh wind phenomenon. 4. Conclusion The results showed a jump in the number of hydro-climatic variables such as temperature, relative humidity, horizontal visibility, wind direction and wind velocity when Garmesh wind occurs in Rasht. The mean No. of days with Garmesh wind is 16.5 days during the year. In more than 91% of cases the wind speed is less than 3.6 m/s, but during Garmesh wind, the wind speed in 69% of cases is between 2.1 and 8.8 m/s. Normally the average relative humidity is 82% in Rasht, but during Garmesh wind, it decreases to 34%. In some cases, in less than 3 hours during Garmesh wind, temperature increases by 20 degrees Celsius or decreases on the contrary. Normally low horizontal visibility is along with low pressure system. At the start of Garmesh wind, horizontal visibility decreases and at the end of it, horizontal visibility increases.