Synoptic systems and the widespread - heavy rains in Middle area of Vietnam
Dr. Nguyen Thi Tan Thanh*1, Ms. Dao Thi Loan*1*1: Aero-Meteorological Observatory, of NHMS of MONRE, SR.Vietnam
Abstract
Vietnam is a country in the monsoon activity area. In winter the cold air can give their affectup to southern part of Vietnam. In summer the south – west monsoon impact will be observed
over southern and middle area. When monsoon system will interact with other synoptic systemfor example typhoon, tropical depression, ITCZ est.… the weather in this area in most of thecases will be especially affected. Heavy rainfall or very heavy rainfall have been recorded,sometimes it reached to the historical disaster. Base on the analyzing of synoptic situations, theiraffected area and the resulted quantitative daily rainfall during period of 1993 - 2002, theprecipitation variation could be noticed and must be considered during forecasting service and as
some special characteristic of monsoon activity in Vietnam.

1. Introduction
The network of rain-gauge stations in Vietnam is not dense enough, therefore usingrainfall data measured by weather radar is necessary for a very-short range prediction ofwidespread heavy rain and for studying the flood phenomena in Vietnam, especially in theMiddle part of the country where the flood event is offen very serious due to the small, short,sloped rivers with narrow flat area along the coast line. Most of the Middle part of Vietnam is on
the eastern side of Truong Son high mountain ridge. To get quantitative data for studying theprocess of a widespread-heavy rain in time and space, weather radar is required to operate in aspecial schedule. The variation of quantity of rainfall depends on the synoptic systems, whichcaused the widespread-heavy rains. Base on the data, gathered during last 10 years, we can findthe most affected synoptic situations on precipitation phenomena in this area.
2. Distribution widespread heavy rains in Middle (1993-2002)
1640

148

27.8307(% )

(% )

127

30.6

20

16.7
12.5

4

6.9

10
2.8 1.4
0.0

0

1.40
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

VYear

VI

VII VIII IXXMonthProbabilities of widespread heavy rains in Middle areasFig. 1: Probabilities of widespread heavy rains in middle areas.

1XIXIIITable 1: Number of widespread-heavy rainfall events in Middle part of Vietnam (1993-2002)
Month1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
Sum

V

VI

VII VIII

1
1

1
1
2

10

1
2
1
5IX
1
1
3
12

1
9

X
3
2
3
2
2
1
1
2
1
3
20

XI
2
3
3
1
4
2
3
1
3
22XII
2
1
1
1
1
2
2
2
12

I

IIIII

IV

0

00

11Sum
8
598
665
10
7872

During the period of 1993-2002, there are strong variations in distribution of widespreadheavy rains. Annual distribution widespread-heavy rains are not equally. Highest probabilities ofwidespread-heavy rains fall in 1994, 1999 and lowest is in 2000. Mostly they concentrate fromAugust to December every year (maximum probability is in December). Rarely there arewidespread-heavy rains in the period from January to July, even there is not in February, March,
April, July.
3. Synoptic systems cause widespread heavy rains in Middle part of Vietnam
There are 72 large-heavy rains happened in Middle areas in 10 years (1993-2002). Nine
primary synoptic systems (independent type or combine type) cause that rains are found as
follows:
1/ Typhoon + Cold Air (14%)
2/ Tropical Low Pressure + ITCZ (13%)
3/ Tropical Low Pressure + Cold Air (8%)
4/ Cold Air + Eastern wind streams (8%)
5/ Tropical Low Pressure (7%)
6/ Cold Air (5.5%)
7/ Tropical Low Pressure + ITCZ + Cold Air (4%)
8/ ITCZ + Cold Air (4%)
9/ ITCZ + South Western Monsoon (4%)
In addition, there are also 19 other synoptic systems can bring large-heavy rains to study
areas, however, occurred probabilities are very low. Although appearance is only 2 times in 10years for the system of Cold Air + North Eastern Monsoon, Tropical Low Pressure+ South
Western Wind Convergence, Typhoon and Typhoon +ITCZ, is only 1 time in 10years for remain
systems, their influences are very serious. Typical example as the famous rain occurred in 16/XI/1999 in Trung Bo with rainfall of 1000mm, reached to more than 2000mm at somewhere,
its damage was strongest in many last decades. This rain was caused by synoptic system of ITCZ+ Cold Air + Eastern wind streams.
Cold Air have very important role in combining with other synoptic systems cause largeheavy rains in Middle areas, its number of appearance times in 72 rains in 10 years is 40.

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Table 2: Nine primary synoptic systems cause widespread heavy rains in Middle areas in 10years (1993-2002).
YearRainfall (24h)from-to (mm)occurred days
1/ Typhoon + Cold Air (14%)
1993
211300-500
1993
21250-1001995
310200-450
1995
211100-350
1995
511100-150
199739150-300
1997
311100-250
1998
311100-200
1998
312250-400
2001
411150-2502/ Tropical Low Pressure + ITCZ (13%)
1993
29200-400
1993
310150-250
1995
59100-200
1995
59200-350
1996
108200-350
1996
79100-300
1999
612200-250
2000
1550-100
2000
31270-90
3/ Tropical Low Pressure + Cold Air (8%)
1993
31250-100
1996
51050-100
1997
510100-300
1998
310250-350
1998
2001

Number of

63Month1112300-400
80-100Year

Number ofMonthoccurred days

Rainfall (24h)from-to (mm)4/ Cold Air + Eastern wind streams (8%)
1994
2000
2000
2000
2000
2001

310100-300
311100-150
311200-300
211150-250
21280-100
31260-100
5/ Tropical Low Pressure (7%)
1994
2650-100
1994
31050-100
1995
29100-100
1996
510100-100
199721030-50
6/ Cold Air (5.5%)
1995
211100-150
1996
411100-250
1997412100-100
2000
51100-200
7/ Tropical Low Pressure + ITCZ + Cold Air (4%)
1993
510300-500
1993
610200-300
1997
39150-300
8/ ITCZ + Cold Air (4%)
1993
511200-400
1995
410300-600
1999
612300-800

9/ ITCZ + South Western Monsoon (4%)
1996
2001
2001

4441288100-150
50-100150-200

References
1. Pham Ngoc Toan & Phan Tat Dac. Climate of Viet Nam. 1993
2. National Center of Hydro-Meteorological Forcasting. Charateristics of Hydro-Meteorology inVietnam. 1993-2002
3. Mai Trong Thong et all. Estimation the role of synoptic systems cause the heavy rains, specialfor forming the flood in 1996 in valleys in South Middle area. National Center of Science andTechnology. 12/1997

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4. Hydro-Meteorological Service of Vietnam. Regulations of Warning for Typhoons and Floods.
10/1998
5. Hydro- Meteorology in Leningrad. Measurement the rainfall by the weather Radar.
6. Louis J. Battan. University of Chicago 1973. Radar Observation of the Atmosphere.
7. Alanseed. Bureau of Meteorology, Australia. Quantitative Precipitation Estimation.
8. Alanseed. Bureau of Meteorology, Australia. Lectures of Training Course for forcasting rainsin tropical areas.

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