EXPERIMENTAL WEATHER FORECAST IN VIETNAM
Collaborators: Prof. Nguyen Thong and Dr. Ho Tuan Duc (Faculty of Civil Engineering, HCMC University of Technology and CARE research center in Vietnam)
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The Weather Research and Forecast (WRF) modeling system is run on a domain spanning the Vietnam East Sea (aka South China Sea) at 20km resolution (soon will also be proposed a refinement of the horizontal resolution over HCM City through 2-way nesting). Terrestrial data are provided by the US Geological Survey (30'). The forecast uses initial and boundary data from the 0.5-degree, 3-hourly product from the Global Forecast System of the National Center for Environmental Prediction (NCEP, NOAA). The simulation starts one day before present using initial and boundary data interpolated from GFS data. Next, a 3 day forecast is conducted using boundary data from GFS.
GFS is run four times per day (00 UTC, 06 UTC, 12 UTC, and 18 UTC) out to 16 days. For the first week, computation is done at a resolution of about 28-km. The GFS data is then gridded for users on a global 0.5-degree or even 0.25 degree resolution grid.
Vietnam is located between 9 and 23 degrees north. The country has a long coastline on the Gulf of Tonkin and the South China Sea. It is mountainous in the northwest and in the central highlands facing the South China Sea, with peaks reaching up to 2450m. In the north around Hanoi and in the south around Ho Chi Minh City, there are extensive low-lying regions in the Red River delta and the Mekong delta respectively.
Vietnam's climate can be divided into three different zones: North, Central and South Vietnam. The climate in North Vietnam is humid and subtropical, while South Vietnam has a tropical climate all year round, with the central region in-between. As Vietnam is a long, narrow country, climatic conditions vary considerably with temperatures ranging between 20°C and 35°C. Due to higher altitudes, it is coolest in the mountains.
Overall, Vietnam has a tropical monsoon type of climate. From May to September, the south monsoon sets in, and the country is dominated by south to southeasterly winds. From October to April, the north monsoon is dominant with northerly to northeasterly winds affecting the country. During the north monsoon, northern Vietnam has cloudy days with occasional light rain, while southern Vietnam tends to be dry and sunny, sheltered by the central mountain range. Between each monsoon season lies a transition month when winds are light and variable.
Vietnam has an actual rainy season during the south monsoon (May to September). Rainfall is light and infrequent for the remainder of the year. Even so, annual rainfall can be considered abundant and exceed 1000mm almost everywhere. This figure is even higher in the hills, especially those facing the sea, in the range of 2000-2500mm. For coastal areas and the parts of the central highlands facing northeast, the season of maximum rainfall is between September to January. During this period, these regions also receive torrential rain from typhoons which move in from the South China Sea.
Temperatures are high all year round for southern and central Vietnam. Northern Vietnam, however, has a definite cooler season as the north monsoon occasionally advects cold air in from China. Frost and some snow may occur on the highest mountains in the north for a few days a year. The lowlands in southern Vietnam are sheltered from outbreaks of colder northerly air. These areas have a dry season that is warm to hot, with much sunshine.
In more detail:
- South Vietnam: there are two main seasons: the wet and the dry. The wet season lasts from May to November (June to August are the wettest months). During this time, there are heavy but short-lived downpours almost daily, usually in the afternoon. The dry season usually runs from December to April. Late February to May is hot and very humid, but it cools down slightly when the rainy season begins. In Ho Chi Minh city, the average annual temperature is 27°C. In April, daily highs are usually in the low 30s. In January, the daily lows average 21°C. Average humidity is 80% and annual rainfall average 1979mm. The coldest temperature that has ever been recorded in HCM city is 14°C.
- Central Vietnam: the coastal lowlands are denied significant rainfall from the southwestern monsoon (April to October) by the Truong Son Mountain Range, which is very wet during this period. Much of the coastal strip's precipitation is brought by the northeastern monsoon between December and February. Nha Trang's long dry season lasts from late January to October, while Da Lat's dry season is from December to March. Da Lat, like the rest of the central highlands, is much cooler than the Mekong Delta and the coastal strip. From November to March, Da Lat's daily highs are usually in the low to mid-20s. The cold and wet winter weather of the north-central coastal lowlands is accompanied by fog and fine drizzle.
- North Vietnam: areas north of the 18th Parallel have two seasons: winter and summer. Winter is quite cool and wet, and usually lasts from around November to April. February and March are marked by a persistent drizzling rain that the Vietnamese call crachin. The hot summers run from May to October. The north is subject to occasional typhoons during the summer months.
ENSO IMPACT ON VIETNAM CLIMATE
There is high interannual variability observed in winter over central Vietnam, which is forced in part by coherent large-scale phenomena, such as ENSO (El Nino Southern Oscillation). Anomalous anticyclonic winds over the SCS are largely modulated by ENSO (Juneng and Tangang, 2005). There is weaker winter monsoon winds over the SCS during El Nino (and stronger during La Nina). Weaker winds also correspond to weaker waves and lower significant wave heights. However, in summertime (JJA), positive correlation values are observed in southern SCS due to strengthening of summer monsoonal winds over the southern SCS during El Nino years.
For the deltas and central climatic zones, seasonal rainfall and number of heavy rainfall months are significantly higher during La Niña than during El Niño episodes. It help explain flooding anomalies: flooding occurs is at least twice as frequent during La Niña as compared to El Niño conditions, particularly in Central Vietnam. Because ENSO cycles have an impact on flooding, it provides prospects for early warning, differentiated for different zones and rainfall regimes.
The Northwest Pacific Ocean is the most active tropical cyclone basin on the planet. Annually, an average of about 26 tropical cyclones in the basin acquire tropical storm strength or greater and 16 of them become typhoons (Joint Typhoon Warning Center database). The risk of landfall of a typhoon or tropical storm depends on its trajectory, varying strongly with the season, as well as on interannual (Chan 1985) and interdecadal time scales (Ho et al. 2004). Cyclogenesis over the tropical northwest Pacific takes place in a broad region west of the date line. Most of these tropical cyclones (TCs) follow rather straight west-northwestward tracks. The large-scale circulation of the atmosphere has a predominant role in determining a TC’s motion through the steering by the surrounding large-scale flow and other factors such as the beta-drift, which promotes northwestward propagation. About one-third of these Northwest Pacific TCs continue in a westward direction and make landfall in Southeast Asia (Camargo et al., 2007). There are about 5 landfalls in Vietnam on average, mostly over the northern provinces. These numbers have been shown to vary significantly from year to year, partly due to the influence of El Niño–Southern Oscillation (ENSO; e.g., Camargo and Sobel, 2007). There is a tendency in El Niño years toward tropical cyclones that turn northward while avoiding the South China Sea, as opposed to La Niña years, when Vietnam is statistically more affected than in normal years.
The year 2013 was qualified as a normal year but followed long-lasting La Niña events from 2010 to 2012. As a result, large positive anomalies of subsurface waters were still observed in 2013, which is particularly favorable to TC intensification. Whether this can explain the exceptional 2013 Pacific typhoon season is an open question. In any case, with 51 named systems, it was the most active typhoon season since 2004, as well as the deadliest since 1975. The first two-thirds of the season were weak but the season became dramatically active starting from mid-September. 10 named systems crossed the South China Sea and landed in Vietnam, most noticeably Typhoon Nari in October and in early November, Typhoon Haiyan, which became one of the most intense tropical cyclones on record.
The summer monsoon drives relative weak, short-period southwesterly waves, which contrasts with the severity of tropical storms. However, the onset of northeast winter monsoon (October-April) resulting from the formation of winter anticyclones over the Siberian region leads to high wind conditions, particularly off northern and central Vietnam. As new Siberian anticyclones form every 3 to 10 days, surges of northeast winds set in then wane with this cycle. The winter mean significant wave height is in excess of 2.5 m off the Nha Trang area (Chu et al., 2004) and the 90th percentiles can even reach 4 m (Mirzaei et al., 2013). Heavy swells during strong surges in the northeast monsoon can cause significant wave heights to rise to more than 6 m for one to a few days. Winter monsoon surges can also be reinforced by the occurrence of low-pressure typhoons by increasing the pressure gradients between the southeast asian continent and the ocean. However, coastal waves are generally lower than offshore waves due to sheltering and dissipation effects over the continental shelf (Mirzaei et al., 2013).
Strong wave height inter-annual variability is also observed during the winter months in the SCS. It correlates negatively with El Nino during the winter season but becomes positive in the summer monsoon, consistent with surface wind anomalies over the SCS (Juneng and Tangang, 2005). Therefore, winter wave height increases during La Nina off Nha Trang with a standard deviation of 0.6 m in December (Mirzaei et al., 2013).
Northeast Monsoon conditions prevailed over the South China Sea in late November and December 2013. A monsoon surge affected the region on 1-3 December and again on the 19-21 December 2013. These events thus closely followed the end of typhoon season and probably contributed to the continuing Nha Trang coastal erosion in December.
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