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In this paper, we use observation data and highresolution (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, mediumscale formations such as tropical cyclones also contributed to heavy rainfall over a short period in this area
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: ltthuong.kttv@hunre.edu.vn 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 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 322 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 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 DATA AND METHODS 2.1 Data This research utilizes two datasets to assess and understand the factors leading to heavy rainfall Firstly, daily rainfall observations from 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 2.2 Methods Synop Method: The Synop method involves the analysis of Synop charts at various pressure levels, combined with observational data from 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, km and km Region 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 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) International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 323 Figure The three nest domains employed in the model 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 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 324 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 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 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 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 325 326 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 Figure 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 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 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 327 328 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 Figure 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 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 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 329 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 330 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 Figure 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 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 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023 331 (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 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 to 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 332 International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Allied Sciences 2023