dc.description.abstract | The aerosols from biomass burning have a non-negligible contribution to atmospheric composition, as well as to climate change. Indochina is one of the prevailing source regions of biomass burning in the world. Despite the large–scale climate has been known a possible factor to biomass-burning actives in this region, the long-thern variability and transport mechanisms in different climate patterns still remain unstudied. Therefore, this study focuses on analyzing long-term (2003-2014) distribution and transport characteristics of biomass-burning aerosols and meteorology in March using NASA/MERRAero data set. Combining with satellite data (e.g., MODIS fire counts, aerosol optical depth, and cloud effective radius) to investigate the correlation between biomass-burning aerosols and regional climate. In addition, the potential impacts of biomass-burning aerosols on regional radiation budget, cloud microphysics, and rainfall distribution are also discussed in this study.
The study domain covers 85°-143°E and 8°-44°N. The northern Indochina is the main biomass-burning region in the dowmain according to satellite data. Analysis of the Niño3.4 sea surface temperature (SST) anomaly and the fire counts in north Indochina, we proposed three climate scenarios associated with fire activites: (A) years with extreme biomass-burning activity associated to El/La Niño climate; (B) years with strong biomass-burning activity under normal climate, and (C) years with weak biomass-burning activity under normal climate. For the scenario A, the years of 2008 and 2011 are typical examples showing weak fire activities corresponding to low Niño3.4 SST. An overwhelming northeast monsoon extends its influence to the South China Sea and Indochina. Meanwhile, the tropical easterlies at 850hPa moves toward northwest. As a result, moist air converges over Indochina, setting an unfavorable condition for biomass burning. In contrast, in 2010, the northeast monsoon shows weaker than climate average due to El Niño climate. The overall climate pattern of 2010 represents a favorable environment for biomass burning. For the scenario B, the year of 2007 shows no correlation to Niño3.4 SST but strong biomass-burning activity were observed. The location of the Pacific High plays an important role in this scenario. When the Pacific High extends westerly to Asian continent, it obstructs continental cold High from moving southward. The wind pattern over Indochina is dominated by southwester. As a result, stronger biomass-burning activity was observed. For the scenario C, the years of 2003 and 2005 showed weak biomass-burning activities under normal climate. A common climate pattern for these two years is that more moist air transported from the Indian Ocean. .
Regarding to the transport of biomass-burning aerosols, we found that the years (i.e. 2008 and 2011 as examples) with weak fire activity are also showing an adverse long-range transport mechanism. In contrast, for strong biomass-burning activity year (i.e. 2007), the large-scale circulation accompanied with orographic lefting can help polluted airmass aloft to higher level and long-range tranport within westerlies.
The radiative and water-cycle effects of biomass-burning aerosols are also investigated in this study. The estimates of aerosol direct radiative forcing at the top of atmosphere for strong and weak biomass years are -13.36 ~ -16.24 Wm-2 and -11.78 Wm-2 ~ -9.64 Wm-2, respectively, showing two order greater than global average of -0.27 Wm-2. In addition to aerosol direct effect, we also assessed the indirect effect. We found that the increase of biomass-burning aerosols can alter the cloud microphysics, in terms of cloud droplet size and cloud fraction. In a normal climate condition, the biomass-burning aerosols tend to decrese cloud droplet size and clould fraction, which in turns reducing precipitation amount or delaing the precipitation time.
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