Study on heavy rain and "unusual heavy rain" in the area from Hai Van Pass to Ca Pass, South - Central Region of Vietnam (period 1986-2010) shows that: - Statistically, in the studying area there are ~ 3 heavy and very heavy rain spells annually (of which, there are ~ 1 very heavy rain spell). Rain spells often occur from August to November; but tend to concentrate in the period from October to December. - Heavy rains are caused by 7 types of synoptic weather situations (SWS). However, 4 following SWS have high frequency: Typhoon or Tropical Depression - 22.0 percent; Cold North-East monsoon - 20.5 percent; Typhoon (or Tropical Depression) combined with Cold North-East monsoon - 19.2 percent; and Inter-tropical convergent combined with Cold NorthEast monsoon -19.2 percent. - The combination of the above SWS at the same time or join together (one after another) can potentially cause heavy rain in large area and for a long time. Time of heavy rain spell can be varied from 2-6 days, but mainly: 2-4 days, the 5 days spell is limited, normally combined of main Cold North-East monsoon and additional one, or Inter-tropical convergent rain. - The SWSs are occuring in the midle of rain season (the months September-December, especially in October and December) often causing heavy and very heavy rains. These spells are often causing by Cold North-East monsoon combined strong East Wind Zone; Cold North-East monsoon combined Typhoon (Tropical Depression) or Inter-tropical convergent rain. - The SWS Typhoon (or Tropical Depression) made landfall in the studying area can resulting to heavy rain but not more then 3 days. - Besides heavy rains in the season, over 25 years (1986-2010) "unusual heavy rain" can occur 5 spells 125 years in period from January to July (~ 0,2 unusual heavy rain spell/year), of which, there is only one .unusual very heavy rain (~ 0,04 unusual heavy rain spell/year).
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