‘Sudden Stratospheric Warming and its Influence on the Tropical Atmosphere'

The stratospheric warming events are categorized into major and minor warming depending on the temperature increase in the polar stratosphere. The warming is called a ‘major’, when the polar temperature increases poleward from 60 degree latitude and followed by a reversal in the zonal wind at 10 hPa (~32 Km). Usually major warming events are associated with the displacement of polar vortex from high to mid latitudes or the splitting of vortices in to two. The warming is called a "Minor", when the polar temperature increases more than 25 degree in a period of a week or less, at any stratospheric level with less intensified easterly wind anomalies. The stratospheric warmings generated during the transition period of winter to spring are called final warmings. The warming events observed in the early winter period (November to early December) over Canadian region are called Canadian warmings. There is strong interaction between stratosphere and troposphere during SSW period over high and low latitudes regions. The thesis consists of 7 chapters

Meteorological aspects of an abnormal cooling event over Iran in April 2009

During the period from 12 to 15 April, 2009 nearly the entire Iran, apart from the southern border, experienced an advective cooling event. While winter freezing concerns are typical, the nature of this freezing event was unusual with respect to its date of occurrence and accompanying synoptic meteorological situation. To analyze the freezing event, the relevant meteorological data at multiple levels of the atmosphere were examined from the NCEP/ NCAR reanalysis dataset. The results showed that a polar vortex was responsible for the freezing event over the country extending southward extraordinarily in such a way that its ridge influenced most parts of Iran. This was recognized as an abnormal extension of a polar vortex in the recent years. The sea-level pressure fields indicated that a ridge of large-scale anticyclone centered over Black Sea extended southward and prevailed over most parts of Iran. This resulted in the formation of a severe cold air advection from high latitudes (Polar region) over Iran. During the study period, moisture pumping was observed from the Arabian Sea and Persian Gulf. The winds at 1000 hPa level blew with a magnitude of 10 m s-1 toward south in the region of convergence (between -2 9 10-6 s-1 and -12 9 10-6 s-1). The vertical profilesof temperature and humidity also indicated that the ICE structural icing occurred at multiple levels of the atmosphere, i.e, from 800 hPa through 400 hPa levels. In addition to the carburetor (or induction), icing occurred between 900 and 700 hPa levels in the selected radiosonde stations during the study period. In addition, the HYSPLIT backward trajectory model outputs were in quite good agreement with the observed synoptic features