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Abstract:
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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 |