عنوان مقاله [English]
Snow shower is the phenomena that caused problems in mountainous roads of North Khorasan province. The goal of this paper is the analysis of three case studies of snow shower in Bojnurd during 2011-2013. We used Bojnurd synoptic data to determine three snow shower case studies during 2011-2013 and then analyzed using synoptic maps of surface, 850, 500, and 300 hpa. Dynamical analysis is done by using sodar output data, helicity, the cross section of relative humidity and vertical wind speed. The maximum value of helicity was 348 (m2/s2) on 17.3.2012 which show sever instability during this phenomena. The vertical wind speed from sodar data in the third case study is very similar to meso scale shear instability. The cross section of vertical wind speed and humidity shows an increasing in relative humidity that accompanying with improvement in upward and downward motions which increase instabilities in the clouds and increase snow shower. Finally we concluded that atmospheric instabilities such as temperature reduction, increase in pressure, humidity and wind speed, increasing in helicity, suddenly changes in wind speed and direction which improve instabilities are the main factors of the formation and intensification of snow showers in the region.