عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In order to Statistically and Synoptically analyze and predict the dry and wet periods in
Tehran during the cold period, the days with 0/1 milimeter rain or more , mehrabad
station was daily chosen and surveyed as wet days during 1985-2003. The second
grade markov chain model was used For determinig dry and wet periods. At first
frequancy wet and dry days according to their continuation were classified and
frequancy eash one was studied individually.Tthen the possibility of every sequence
was calculated monthly and a six month of cold ness period.
The most frequancy of rainy days 51 days and its least is 25 days a year. March
with 188 and October with 81 days rain is the Maximum and the Minimum frequency.
1995 and 1996 years have been the driest and wettest years.After determining the
sequences , the survey of effective pressure patterns in creating rain during the days 27
to 30 of November ,1 to 7 of December 2003 as a longhest period and 10 to 13 of
December 1995 as a period of 4 days of wetness during the driest year , the survey
period was synoptically analyzed for this reason. It has been necessary to use sea level
pressure maps and 500 hp and also maps of direction and speed of the wind and
spessific humidity 700 hp.
The comparison of the frequency of predicted sequences with the frequancy of
observed sequences , shows markov chain model exactness in predicting the dry and
wet sequences of Tehran region which have sharp mismathch of rain.
In synoptically maps it was specified that the most important source of humidity
in Iran have been the Red sea, Arab sea, Adan gulf, which concidence of these wetness
sources with the Persian gulf and Saudi Arabia high pressure has caused more
transition of wetness into Iran and Tehran.