تحلیل سینوپتیک یخبندان شدید، دیرهنگام و فراگیر فروردین ماه 1388

  Synoptic analysis of sever, late and widespread frost in April 2009         Gh. Azizi ( * ) ,  Associate Professor of Climatology, University of Tehran, Tehran, Iran.   e-mail: ghazizi@ut.ac.ir     A. Hanafi, M. A. of Climatology, University of Tehran, Tehran, Iran.     M. Soltani, M. A. of Climatology, University of Tehran, Tehran, Iran.     M. Aghajani, Graduated in Physical Geography, University of Tehran, Tehran, Iran.   Received: 10 May 2010 / Accepted: 14 Desember 2011, 1-4 P       Extended abstract   1- Intro duction   Frost identified as one of the most catastrophic natural disaster accompanied by usually plentiful losses and damages. Freezing and frost play a significant role on plants phenology for agriculture crops, as if occur, resulting in production limit (Alijani and Hajbarpour 2007). Freezing refers to conditions that the air temperature at a height of about two meters from ground reaches to zero or below zero degrees Celsius (Azizi, 2004). From the viewpoint of agricultural meteorology, frost occurred in extreme low temperatures that resulting to plant tissue damage. This type of freezing is different for each product according to the critical temperatures (Mohammadi, 2004).     Generally we can say that freezing caused to restriction in growth, activity or decrease in function.     2- Methodology   In order to synoptic analyses the frost of 12th to 15th April 2009 periods and its effects on vast country of Iran, the relevant meteorological data including mean surface pressure hPa and temperature (degC) mean geopotential height hPa streamlines fields – u & v wind components (m/s-1) and 850 hPa and 500 hPa were selected from NCAR/NCEP data and produced by GrADS and then were analyzed. In this study, 500 hPa and temperature have been used as a representative of middle atmosphere, especially moisture and wind in 850 hPa were used for analysis moisture of system and SLP used for studying the position of low pressure and high pressure systems. For this purpose, synoptic maps for SLP, 850 and 500 hPa producted by GrADS software and analyzed.     3- Discussion   The mean sea level pressure indicates a ridge of large scale anticyclone centered over Black Sea extends southwards and prevails over northern Iran on 12 April 2009 resulted in cold advection from high latitudes over northern half of Iran. Simultaneously, a low center (1014 hPa) located above latitude 50o N over Siberia area influenced the study area to some extent. In the northern half of Iran the circulation is characterized by a remarkable intensification of the pressure gradient, due to the presence of a deep trough positioned over Caspian Sea with its axis oriented along the northwest-southeast direction field . As a result of being meridian the circulation patterns over the northwest parts of the country resulted in the cold air influx from Polar Regions into Iran. At the level of 500 hPa, a low center established on latitude 55o N and longitude 70o E over the Russia resulted in a deep trough across the southwest – northeast and Iran located in front of the trough. The wind direction is eastwards and the temperature varies from -12 oC to -27oC in Iran. In majority of the country the relative humidity values are between 30% - 50% apart from a small section over the northwest, in which is more than 80% caused snowfall in some stations. A cut-off low generated in the cold system center as a result of being existing the meridian circulation, which it has a huge difference temperature as compared to the adjacent air. The average temperature decreased to less than -5 oC in the low center. The low system cells displaced towards southeast and it was provided a very suitable pressure gradient over the northern border of Iran (on 13 April). The cold system, on the next day, left the study area towards east and its great influence weakened and as a result, the temperature began to increase (on April 14). Besides, at the isobaric level of 850 hPa, most parts of the country surrounded by northerly flows in consequence of the anticyclone activity over the latitude 50o N and longitude 24o E caused cold air advection from high latitudes into Iran, as most stations under study experienced under zero temperature (–0oC). Moreover, at the level of 500 hPa a huge trough as a result of a large scale cyclone centered over Russia extended southwards and entirely predominated whole Iran on April 14, resulted in cold air advection from high latitudes over more than 70% of Iran in particular north and northwest regions of the country highly were affected by the cold system as compared to other sections. On the contrary, northeast divisions of Iran recorded a more chilly temperature because of the cold system eastward displacement on the next day in the meantime the western half of Iran was experienced a rise in temperature gradually (on 15 April).     4- Conclusion   As mentioned above, during 12th – 15th April 2009 episode a chilly system had been affected by approximately whole Iran resulted in a sensible and considerable drop in temperature and some snowfall over most regions of the country, which identified as an exceptional late frost phenomenon within a couple of recent years. Synoptic pattern of meteorological charts of 12 April 2009 indicated that a high system accompanied by two isobaric centers nearly 1029 hPa and 1014 hPa over Black Sea and Russia (Siberia zone) were located, which its vortex was extended by approximately southern half of Iran. In this research have been distinguished the western high – over Black Sea – played a significant role on advective, or air mass frosts over Iran resulting from displacing eastwards and cold advections from high latitudes. On next day, the mentioned anticyclone had not a considerable displaced. A cut-off low was formed in center of the high system in consequence of a great pressure gradient because of being meridian the contours, resulted in a dramatically fall in temperature as compared to its own adjacent surroundings. Consequently, the mean temperature value was dropped in less than -5oC in the low center. On 14th April, the high cell was created a very suitable pressure gradient in north half of the country as it was displacing southeastwards, which resulted in to intensify the instabilities over north half of Iran, as a result on this day, most stations were recorded minus temperatures zero 0oC nationwide except for the southern border of country. Eventually, the temperature slightly increased in most stations over western half of the country, meanwhile the temperature values kept going to drop in to some extent in eastern stations, followed by eastwards displacement of the high system. As well as, in order to indicate the cold system track over the country and the occurred frosts during the examined period, the min temperature values interpolated by using GIS. The results indicated well the cold system’s entrance, activity and going out of the country.   Key words : Synoptic, sever Frost, Regionalization, April, Iran.     References   Alijani., B. & Hajbarpour, GH. (2007). 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