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Simulation of Aerosol Impacts on Weather over Vietnam Region by WRF/CHEM Model

1Dao Thi Hong Van, 1Nguyen Quang Trung, 2Hiep Van Nguyen, and 1Phan Van Tan

1VNU University of Science, 334 Nguyen Trai St., Thanh Xuan Dist., Hanoi, Vietnam

2Vietnam Institute of Meteorology Hydrology and Climate Change Abstract

The couple Weather Research and Forecast/Chemistry model (WRF/CHEM) version 3.4 has been used to investigate the effects of aerosols on temperature and rainfall over Vietnam region under the northeast monsoon conditions. Three experimental runs with turning on chemistry module for three different chemical options have been performed and compared with the no aerosol run for analysis. The results show that most of the aerosols over Vietnam region are come from China continent. Under winter monsoon conditions, the aerosols have significant effects on both temperature and rainfall over Vietnam. In addition to cooling air temperature at lower levels, the aerosols also cause secondary effects in which an anomaly high pressure is formed to enhance the northeasterly winds and to bring relatively cooler air from high latitudes to the north of Vietnam to further reduce air temperature over Vietnam region. The stronger winter monsoon flows due to effects of aerosols result in increase in orographic rainfall over the Central Vietnam region.

1. Introduction

In recent decades, researchers had paid much attention on the important role of aerosols in climate system. Aerosols affect the climate system in many ways, such as influencing the radiative transfer and atmospheric chemistry processes. The impacts of aerosols might result in the redistribution of energy budgets in the climate system, leading the change in the atmospheric and oceanic circulations.

Aerosols are emitted in both natural and anthropogenic processes. The main natural sources of aerosols are desert dust, sea spray, volcanic explosions or biogenic organic emissions. Anthropogenic aerosols may come from fossil fuel combustion, biomass burning and industrial production. Due to their relatively short lifetimes in atmosphere, some aerosols generally show highly spatial variability. The spatial distribution of aerosols is mainly controlled by local sources, chemical transformations, transport and

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removal processes. Therefore, the effects of anthropogenic aerosols on climate system are expected to be at the regional scale. Giorgi et al. (2002, 2003) and Qian et al. (2003) assessed the regional climatic impacts deriving from the direct and indirect effects of anthropogenic aerosols over East Asia and found that these effects can contribute to a cooling trend observed over various regions of China during the last decades of the 20th century. Menon et al. (2002) found substantial regional climatic effects induced by large black carbon aerosol forcing over south Asia.

During the past three decades, the continuous rapid economic and population growth in East Asia have resulted in substantial increases in anthropogenic emissions of aerosols (Xueyuan Wang et al., 2010). In 1981, with the represented of MAPS (Measurement of Air Pollution from Satellite), high concentrations of CO2 and air pollution over large areas of Asia, Africa and South America were observed for the first time (H. G. Reichle Jr. et al., 1986). Although Asian NO2 emissions only contributed a minor fraction of global emissions during the 1970s, from 1990s, the Asian NO2 emissions have increased rapidly and surpassed emissions from North America and Europe in the mid-1990s (Hajime Akimoto, 2003). The NO2 emissions from Asia are predicted to continue to increase in the next two decades (Z. Janusz Cofala et al., 2007).

Although numerical approaches using couple chemical/atmospheric models have been used to study the emission sources, transportation and effect of aerosols on weather and climate for more than three decades in Europe and U.S and other regions of the world (Langman B., 2000; Chung C. E. et al., 2005; Xiaoming Hu, 2008;Y. Zhang et al., 2009; Jung Yoon Kang et al., 2011; Paolo Tuccella et al., 2012), few of those are conducted or related to Vietnam region. In this study, the main objective is to investigate the arerosol sources, transportation, as well as the impacts of aerosols on temperature and rainfall over Vietnam using the WRF/CHEM (Weather Research and Forecasting/Chemistry model). The rest of this paper presents experiment design and data on Section 2, some primarily results on Section 3, and summary and discussion on Section 4.

2. Experiment design and data In this study, a fully couple “online” Weather Research and Forecasting/Chemistry

model (WRF/CHEM) version 3.4 (Grell et al., 2005) was used to investigate the effect of aerosols on weather over Vietnam region. WRF/CHEM is integrated continuously from 01 January 2006 to 05 January 2006 over a domain extending from 1.5oS to 32oN and from 88oE to 124oE (Fig. 1). The model horizontal resolution is 27km. There are four experiments including WRF without chemistry module (WRF_NOCHEM), and three run cases with turning on chemistry module for three different chemical options (Table 1). The emission sources, transportation and effects of aerosols on the air temperature, rainfall are analysed.

The meteorological initial and 6-hourly boundary conditions are from National Centers for Environmental Prediction (NCEP) Operational Model Global Tropospheric Final Reanalysis (FNL) at 1.0-degree resolution.

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The emission data such as NH4, NO3, SO2, elemental carbon, organic matter, aerosol water, sea salt and mineral dust are from RETRO (http://retro.enes.org) and EDGAR (http://www.mnp.nl).

Table 1. Experiments and descriptions

Experiment Chemical mechanism Aerosol schemes

WRF_C300 No ozone chemistry The Goddard Chemistry Aerosol Radiation and Transport (GOCART) model

WRF_C301

Regional Atmospheric Chemistry Mechanism (RACM) - Kinetic PreProcessor (KPP)

The Goddard Chemistry Aerosol Radiation and Transport (GOCART) model

WRF_C11 Regional Acid Deposition Model, 2nd generation (RADM2)

Modal Aerosol Dynamics Model for Europe (MADE)/Secondary Organic Aerosol Model (SORGAM) including aqueous reactions

Figure 1. Model domain and terrain height (shading, m)

3. Preliminary Results

3.1. Simulation of aerosol concentrations Figure 2 shows the vertical profiles averaging over Vietnam region of particle

pollution (or particulate matter) for particles less than 2.5 micrometers in diameter (PM2.5), particles between 2.5 and 10 micrometers in diameter (PM10) and SO2 (left to right) from WRF_C300 (red), WRF_C301 (green), WRF_C011 (blue) on 04th January 2006. It can be seen on Figure 2 that among the three experiments there are no significant differences in concentration of PM2.5, PM10, and SO2 at levels higher than 750 hPa. However, at levels below 750 hPa, the concentration of PM2.5, PM10 in

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the WRF_C11 run is much larger than in the other two runs. In contrast, the concentration of SO2 at lower levels is the highest in the WRF_C300 run.

In terms of horizontal distributions, Figure 3 shows that the region of local maximum concentrations of PM2.5, PM10 and SO2 locates over China continent. The particulate matters and SO2 are migrated following dominated northeast monsoon (winter monsoon) flows toward Vietnam region. The effects of particulate matters and other aerosol on Vietnam weather for the days from 01 to 04 January 2006 will be investigated in the next section.

Figure 2. Profiles of PM2.5, PM10 and SO2 (left to right) concentration (*1000 µg/kg) from WRF_C300 (red), WRF_C301 (green), WRF_C011 (blue) for 04 January 2006

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Figure 3. Spatial distribution of PM2.5 (top), PM10 (middle) and SO2 (bottom) concentration (*1000 µg/kg) and wind vectors from WRF_C300, WRF_C301,

WRF_C011 (left to right) at 850-hPa level for 04 January, 2014. 3.2. Effects of aerosols on simulated temperature and precipitation

In this section, the differences in temperature and rainfall (WRF/CHEM minus WRF_NOCHEM) are computed to evaluate the effects of aerosols on weather over Vietnam region.

Figure 4 shows that aerosols result in cooling air temperatures at lower level in WRF/CHEM. The temperature in the WRF/CHEM runs is about 0.5oC to 3.0oC lower than in the WRF_NOCHEM run. The most significant decrease in air temperature occurs near 900-hPa level. At higher levels, from 500 hPa to 200 hPa, air temperature in the WRF/CHEM runs is warmer than in WRF_NOCHEM run (Fig. 4). The warmer air temperature may relate to the absorption of upward radiative emission from aerosol at lower levels.

WRF/CHEM runs simulated 2m air temperature (T2m) lower than in WRF_NOCHEM over the North of Vietnam. The largest cooling in temperature can reach up to 3.0°C. The cooling in Central Vietnam is lower with the values of about 1.0oC (Fig. 5, top). The decrease in temperature in WRF/CHEM runs over the Northern part of Vietnam and Southern China may relate to the cooling effects of aerosol and a secondary effect. The secondary effect associates with an anomaly high pressure originally caused by aerosol cooling effect. The anomaly high (Fig. 5, top)

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enhances the Northeast winds to bring relatively cooler air from high latitudes to the North of Vietnam.

There is a region of relatively warmer temperature located to the East of Vietnam (Fig. 5, top). The region of warmer temperature may relate to anti-cyclonic anomaly circulation from the anomaly high pressure formed over southeast China due to cooling effects of aerosols. The anti-cyclonic winds help to bring relatively warmer air from lower latitudes toward the region East of 104oE (Fig. 5, top).

In terms of rainfall over Vietnam, the aerosols in WRF/CHEM run may result in increase rainfall over the Central Vietnam (WRF_C11, Fig. 5). Specifically, the increase of rainfall in the region cause Northeasterly winds stronger. As a consequence, orographic lifting is enhanced over the East side of the Truong Son mountain range to increase rainfall in the Central Vietnam region.

Figure 4. Verital profile of differences (WRF/CHEM - WRF_NOCHEM) in regional mean

air temperature for WRF_C300, WRF_C301, WRF_C011 from 01 to 04 January 2006

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Figure 5. Differences (WRF/CHEM - WRF_NOCHEM) in mean T2m (oC), wind

vectors (m s-1), and surface pressure (hPa) (upper), and total daily precipitation (mm) (bottom) for WRF_C300, WRF_C301 and WRF_C11 for 04 January, 2006

4. Summary and discussions In this paper, the fully couple “online” Weather Research and Forecas

/Chemistry model (WRF/CHEM) version 3.4 has been used to investigate the effects of aerosols on temperature and rainfall over Vietnam region under the Northeast monsoon conditions. Four experiments, including WRF without chemistry module (WRF_NOCHEM), and three run cases with turning on chemistry module for three different chemical options have been performed. The model simulations showed that most of the PM2.5, PM10 and SO2 aerosols over Vietnam regions are come from China continent. Under winter monsoon conditions, the aerosols have significant effects on both temperature and rainfall over Vietnam and Southeast Asia. In addition to cooling air temperature at lower levels, the aerosols also cause secondary effects in which an anomaly high pressure is formed to enhance the northeasterly winds and to bring relatively cooler air from high latitudes to the north of Vietnam to further reduce air temperature over Vietnam region. The anomaly high pressure also creates anomaly anti-cyclonic winds to bring relatively warmer air from lower latitudes to form a relative warm region east of 104oE. The stronger winter monsoon due to effects of aerosols from China may increase orographic winter rainfall and in some cases can possibly cause more severe heavy rainfall and floods over the Central Vietnam region.

Acknowledgements

This study was partially supported by funding from the Consortium of Asian Universities in Fukuoka (CAUFUK), Fukuoka Prefecture, Japan.

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