Investigation of the Role of Polar Vortex in Iranian First and Last Snowfalls

From a climatological point of view, a winter day usually is symbolized with cold weather or a snowfall day. The first and last days of snowfall may be considered as winters’ real starting and finishing points. Starting and finishing points of seasons are affected by sun location, which is fixed yearly whereas snowfalls usually begin with cooling and they may happen in any time. Cooling has several reasons, the most important of which may be approaching and locating of polar vortex trough or isolated cut-off low systems from polar vortex on a region. The current research aims at identifying the role of polar vortex in creating cooling necessary for Iranian first and last snowfalls and finding a way for estimating its occurrence time. Firstly, snowfall data of all synoptic stations is collected from Islamic Republic of Iran Meteorological Organization (IRIMO) in 3-hourly basis. After statistical review, snowfall data related to 38 stations which have at least 33 years continuous statistics from 1976 to 2008 is considered proper in order to be used in the research. Then, after determining cold and warm peak times in summers and winters and basing them, the first and last days occur snowfall after warm and cold peak in each year have been respectively identified first and last snowfalls. The first and last snowfall days (calendar series) changed into Julian days. Using Smada 6.0 software, resulted series have been fitted by different statistical distributions (for example normal, 2 parameter log normal, 3 parameter log normal, pearson type 3, log pearson type 3 and gumball distributions) and most proper distributions have been recognized. Ultimately, using those distributions, occurrence probability of snowfall days has been calculated. Geopotential height data for those selected days is collected from National Center for Environmental Program and National Center for Atmospheric Research (NCEP/NCAR). Using Grads software, related maps have been drawn. Resulted maps have been analyzed through the using of Skew-T Log P diagrams obtained from University of Wyoming. In most of the cases, Iranian first and last snowfall days happen because of cooling resulted from locating of polar vortex troughs or their isolated cut-off low systems on Northwestern of Iran or its neighborhood. Using astrological calendar and getting assistance from probability rules, can be estimate Iranian first and last snowfall occurrence times.