The role of orographic effects on heavy rainfall event over central vietnam in october 2021

THE ROLE OF OROGRAPHIC EFFECTS ON HEAVY RAINFALL
EVENT OVER CENTRAL VIETNAM IN OCTOBER 2021
Phong Le Van1, Phong Nguyen Binh2, Thuong Le Thi2*
Vietnam Institute of Meteorology, Hydrology and Climate Change
Hanoi University of Natural Resources and Environment, Vietnam
*Corresponding author. Email:

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ABSTRACT
In this paper, we use observation data and high-resolution (3 km) simulations from the WRF
model, driven by FNL data, to investigate the causes of heavy rain over Central Vietnam from
22nd to 29th October 2021. Our findings confirm the significant role of topography in causing
heavy rain. In terms of the atmosphere’s vertical structure, the surface’s northeast wind, almost
perpendicular to the Central region’s topography, combined with the east wind and trade winds
above, caused strong convection. Additionally, medium-scale formations such as tropical cyclones
also contributed to heavy rainfall over a short period in this area.
1.INTRODUCTION
The heavy rainfall in recent years has shown significant changes in both its distribution and
quantity. These transformations are increasingly challenging for weather forecasting and early
warning efforts. Numerous forecasting technologies have been implemented; however, heavy rain
events occurring over short periods of time continue to result in substantial consequences.
Meanwhile, the influence of low-level cold air combined with various atmospheric patterns
and disturbances, such as storms, tropical depressions, the Inter-Tropical Convergence Zone
(ICTZ) and eastward wind disturbances, has led to substantial rainfall in the Central region of
Vietnam (Anh V. P., 2002 and Quang L. D., 2005). However, to assess the causes of heavy rainfall
and the vertical structure of the atmosphere during these events, relying solely on synoptic charts
is insufficient. Hence, numerical models have emerged as a solution. In 2011, Toan N. T., (2011)
utilized the WRF model as a powerful tool for weather prediction. The author ran the WRF model
for the Central region days in advance of heavy rain events, comparing cases with and without local

data updates for 14 heavy rainfall episodes. The results indicated that better local data improved
the model’s accuracy.
Additionally, in a recent study, Tuan B. M., (2019) reanalyzed ECMWF data for 30 years
and employed dynamic statistical methods to demonstrate that wave-like atmospheric disturbances
within the East Asian monsoon zone are a major factor contributing to heavy rainfall in Vietnam.
The development of these disturbances can sometimes intensify into storms or tropical depressions
and, at other times, only persist in regions of weak convergence and divergence. However, when
these wind convergence zones combine with tropical extratropical factors or high terrain in
Vietnam, rapid enhancement occurs, creating a favorable environment for heavy rainfall events.
On the other hand, during the heavy rainfall event in October 2020 in the Central region
of Vietnam, Mau N. D et al., (2022) pointed out that the activity of the monsoon trough, coupled
with the presence of the Madden-Julian Oscillation (MJO), created favorable conditions for the
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development of tropical cyclones. Furthermore, during the La Niña period, there was an increase
in the activity of the eastward winds (trade winds) in the Northwest Pacific, playing a leading role
in directing the monsoon trough directly towards the Central region, resulting in historic heavy
rainfall in this area.
2.

DATA AND METHODS

2.1Data
This research utilizes two datasets to assess and understand the factors leading to heavy
rainfall. Firstly, daily rainfall observations from 8 weather stations in the Central region are
employed. Additionally, the “Final” dataset (FNL) from the U.S. National Centers for Environmental

Prediction (NCEP), a global reanalysis data set with a spatial resolution of 0.25×0.25 degrees
longitude and latitude, is used as input for the WRF model in this study.
2.2Methods
Synop Method: The Synop method involves the analysis of Synop charts at various pressure
levels, combined with observational data from 8 weather stations in the Central region. This
approach is employed to investigate and comprehend large-scale circulation patterns and weather
configurations contributing to heavy rainfall.
Modeling Method: The modeling method utilizes the high-resolution numerical model WRF
(Weather Research and Forecasting) with input data derived from the global reanalysis dataset
known as FNL (Final) provided by the U.S. National Centers for Environmental Prediction
(NCEP). This modeling approach aims to simulate the occurrence of heavy rainfall during the
period from October 22 to October 29, 2021, in order to elucidate the dynamic and thermodynamic
mechanisms responsible for rainfall, both in cases with and without geographical influences.
2.3 Experimental design
The WRF model is a numerical weather prediction and research model developed through
the collaboration of the United States National Center for Atmospheric Research (NCAR), the
National Centers for Environmental Prediction (NCEP), the Forecast Systems Laboratory of the
National Oceanic and Atmospheric Administration (NOAA) and a group of scientists at various
universities. The clWRF version is a development from the WRF model for the purpose of climate
research and prediction. In this study, the WRF model is utilized with three nested grids, each
having different resolutions: 27 km, 9 km and 3 km. Region 1 extends to approximately 26oN
and off the coast of the Philippines to “capture” large-scale processes, specifically the intrusion
of cold air and tropical cyclones. Region 2, with a resolution of 9 km, covers the entire territory
of Vietnam and the East Sea. Region 3, with a high resolution, covers the entire Central region to
simulate heavy rain events (Figure 1)
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Figure 1. The three nest domains employed in the model.
3.RESULTS
During the heavy rainfall event from October 22 to October 29, 2021, in the Central region
of Vietnam, various types of rainfall occurred, including light rain, moderate rain and heavy rain.
In some areas from Quang Tri to Quang Ngai, extremely heavy rainfall was observed, with total
precipitation ranging widely from 300 to 500 mm and, in many places, exceeding 500 mm.
Furthermore, Vietnam stretches across multiple latitudes and features complex terrain, making
the interaction between topography and weather patterns a significant and frequently discussed issue
in studies. One of the notable geographical features that influence Vietnam’s climate is the Truong
Son Mountain Range, which runs along the country’s length and acts as a barrier that obstructs lowerlevel airflows. During the summer, the Truong Son Range blocks the southwest monsoon winds
from the west, resulting in the Foehn effect and intensifying heat in the Central region. Therefore,
summer is not the main rainy season in this area. However, during the autumn, when the monsoon
disturbances within the East Asian monsoon combine with the northeast monsoon winds at lower
levels, the Truong Son Range once again plays a critical role in blocking low-level airflows and
causing heavy rainfall in the Central region. Consequently, the rains in the Central region are often
of high intensity and frequently lead to flash floods and landslides.
In this study, the interaction between topography and weather patterns is also considered
and calculated using numerical modeling experiments. In these experiments, two scenarios are
run: one with the presence of topography and one without topography. This approach allows for
an assessment of how terrain influences weather conditions in the region, helping to elucidate the
role of geography in heavy rainfall events.
The simulation ran from October 22 to October 29, 2021, during a period of intense rainfall
in the Central region of Vietnam. This was due to a combination of factors: the intrusion of cold
air, disturbances in the eastward wind and the emergence of tropical cyclones. The outcomes of the
simulation are presented in Figure 3.
From October 22 onwards, potent easterly and northeasterly winds from the East Sea had
a direct impact on the Central region of Vietnam. The downward movement of the cold air mass
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caused what is known as front rainfall over the East Sea. Despite the circulation patterns being
similar, there was a significant difference in rainfall between the two simulation scenarios. In the
scenario without topography, the majority of the rainfall occurred over the sea and was relatively
light. However, in the scenario with topography, the simulated rainfall was considerably heavier,
with a tendency for the precipitation to accumulate on the eastern slopes of the Truong Son
Mountain Range, extending from Ha Tinh to Quang Ngai.

Figure 2. Shows the total daily rainfall at several weather stations in the Central region from
October 22 to October 29, 2021.
In the days that followed, the scenario without topography did see some rainfall over land,
but it was significantly less than in the scenario with topography. These results clearly highlight the
role of topography in obstructing low-level airflows, which results in heavy rainfall.

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Figure 3. Illustrates the interaction between topography and the weather patterns that

contributed to heavy rainfall from October 22 to October 23, 2021 in two cases with terrain
(left) and without terrain (right).
To better understand the relationship between topography and weather patterns that result in
heavy rainfall, an analysis of the atmospheric vertical structure at 14.8 degrees North Latitude and
108.0 degrees East Longitude was carried out in two scenarios: one incorporating topography and
one without. Figure 4 displays the findings.
The scenario that includes topography, cold air and disturbances in the East Asian monsoon
converged at a height of 1,500 meters on the eastern side of the Truong Son Mountain Range. The
Convective Available Potential Energy (CAPE) index was extremely high over the eastern Truong
Son area and the coastal regions of the Central region. This suggests that the convergence of cold
air and eastward wind disturbances, which blow almost perpendicular to the Truong Son Range’s
topography, played a significant role in the very heavy rainfall experienced in this region.

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Figure 4. Depicts the vertical atmospheric cross-section at 14.8N in two scenarios: One with
topography (left) and one without topography (right), simulated during the heavy rainfall
event from October 22 to October 29, 2021 in two cases with terrain and without terrain.
To delve deeper into the interplay between topography and weather patterns that result in
heavy rainfall, an analysis was performed on the vertical atmospheric structure at 14.8 degrees

North Latitude and 108.0 degrees East Longitude under two conditions: one incorporating
topography and one without. The findings are shown in Figure 4.
The scenario that includes topography, cold air and disturbances within the East Asian
monsoon converged at a height of 1,500 meters on the eastern side of the Truong Son Mountain
Range. The Convective Available Potential Energy (CAPE) index was extremely high over the
eastern Truong Son area and the coastal regions of the Central region. This suggests that the
convergence of cold air and eastward wind disturbances, which blow almost perpendicular to the
Truong Son Range’s topography, played a significant role in the very heavy rainfall experienced
in this region. Moreover, on October 26, due to the influence of the tropical cyclone circulation
combined with disturbances within the East Asian monsoon, rainfall continued in this region. The
convergence process was strong and prominently focused around the center of the tropical cyclone.
In contrast, in the scenario without topography, at 14.8 degrees North Latitude, cold air with
a stable northeasterly wind pattern affected the Central region of Vietnam. The entire region was
directly impacted by a relatively strong cold wave. The CAPE index also showed clear signs but
did not exhibit the same convection around the center of the tropical cyclone as in the topography
scenario. This suggests that the energy provided by the environment for convergence into the
center of the tropical cyclone was relatively weak.
When analyzing the atmospheric structure along the longitudinal axis at 108.0 degrees East
Longitude, similar observations were made. In the topography scenario, there was clear convergence
between the northeasterly winds and the terrain of the Central region, extending to approximately
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16 degrees North Latitude (Bach Ma Range). Furthermore, a relatively well-defined low-level
trough (convergence between the Northeast monsoon and the Southwest monsoon) was observed
from October 26 in the region from 16 to 18 degrees North Latitude. In the following days, this
low-level trough gradually moved Northward following the motion of the tropical cyclone. At this

time, the convective potential index primarily existed over the Southern East Sea and the Gulf of
Thailand.
Conversely, in the scenario without topography, even though there was still a Northward
shift of the low-level trough, the influence of the northeasterly wind was considerably strong,
dominating most of the Central region. In this case, the low-level trough was positioned much
lower, at around 12 to 14 degrees North Latitude. The CAPE index in this scenario also differed
significantly from the topography scenario. On October 22, this index was very strong in the 14
to 16 degrees North Latitude region, indicating strong convergence in this area, but it rapidly
weakened in the following days.

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Figure 5. Displays the vertical atmospheric cross-section at 108.0E in two scenarios: one with
topography (left) and one without topography (right), simulated during the heavy rainfall event
from October 22 to October 29, 2021, in two cases with terrain and without terrain.
4.CONCLUSION
The results from the analysis of observational data and high-resolution WRF model
simulations regarding the interaction between heavy rainfall and topography in the Central region
during the period from October 22 to October 29, 2021, indicate the following:
(1) Heavy rainfall in the Central region is attributed to the influence of cold air in combination
with the topography, as the Truong Son Mountain Range acts as a barrier, enhancing convective
activity and rainfall in this area.
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(2) The impact of tropical cyclone circulation is one of the factors leading to an increase in
both the extent and amount of rainfall over a short period.
(3) In the case of topography, the simulated rainfall field using the high-resolution WRF
model is significantly more intense. Additionally, the rainfall tends to concentrate on the eastern
slope of the Truong Son Range, spanning from Ha Tinh to Quang Ngai. Conversely, in the scenario
without topography, rainfall is primarily distributed over the sea and the amount of rainfall is
relatively weak over the Central region.
(4) When analyzing the vertical atmospheric structure at 14.8N and 108.0E, the study
observed that during this rainfall event, the Northeasterly winds blew almost perpendicular to the
topography. This, combined with upper-level eastward wind disturbances and the influence of the
tropical cyclone carrying abundant moisture, created a strong upward converging flow, resulting
in widespread heavy rainfall.
5.REFERENCES
Anh V. P., 2002. Trade wind and the research situation of heavy rainfall influenced by trade wind
in the Central Region. Meteorological and Hydrological Journal, Vol. 499, 22-28.
Mau N. D., Thang, N. V., Duong T. H., Tuan B.M., Tuan V. Q., Hien D. T., 2022. Large-scale
circulation associated with heavy rainfall on October 2020 in Central Viet Nam. Climate
Change Science Journal, Vol. 21, March 2022.
Quang L. D., 2005. Characteristics of Heavy Rainfall in Central Vietnam. Meteorological and
Hydrological Journal, Vol.536, 1-6.
Toan N. T., 2011. Prediction of Heavy Rainfall due to KKL combined with ICTZ for 1 to 3 days
in the Central Central Region using the WRF model. Master’s thesis, University of Natural
Sciences, Hanoi National University.
Tuan B. M., 2019. Extratropical Forcing of Submonthly Variations of Rainfall in Vietnam.
J. Climate, 32, 2329-2348.

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