Aerosol- Climate Interactions: Examples from Hong Kong and the Pearl River Delta Region (pp. 125-138)
Authors: (Long S. Chiu)
Abstract: While the effect of increases in atmospheric carbon dioxide on climate has been relatively well documented, there is still large uncertainty associated with aerosol effects on climate. Aerosols affect both the hydrological and energy cycles. Their presence is crucial for the formation of clouds and hence they will affect the radiative budget and precipitation type, amount and processes. Aerosols associated with burning of biomass have been shown to inhibit rain formation. Aerosols also reflect and absorb shortwave radiations, causing surface cooling and upper level heating, thus contributing to stabilize the atmosphere. Because of their shortwave absorption, they act as elevated atmospheric heat sources and induce low level convergence, and hence possibly early onset of monsoon systems.
We analyzed the aerosol and cloud parameters derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA‘s EOS satellite and TRMM rainfall over the Pearl River Delta (PRD) region. While the seasonal variations of AOD over southern China show a peak in the summer, there are two AOD peaks, respectively in the spring and fall, in the PRD. This bimodal behavior is due to the influence of the modulating effect of wind changes and wet deposition by monsoon rain. Increases in the aerosol optical depth (AOD) is associated with increases in the Fine Mode Fraction (FMF) and the Angstrom Exponent, suggesting that the increases in AOD are associated with small size (<2.5μm) aerosols. Significant correlations exist between AOD and columnar water vapor (WV), and marginally significant correlation with cloud fraction (CF), cloud top pressure (CTP) and rain rates (RR). The rain rate tends to be associated with large (large cloud coverage) and high (low cloud top temperature and pressure) clouds. Linear trend analyses show significant increases in stratiform rain in June and
decrease in January. This is associated with significant increases in AOD for both months. Correlation analyses also show significant negative correlation between AOD and rain rate in December and January. It is suggested that in the presence of moisture abundance, aerosols act as cloud condensation nuclei and increases the cloud fraction. The compensating effect of aerosol removal by wet deposition reduces the positive correlation between AOD and rain rate, and hence no significant rainfall trends are observed. This contrasts with the conditions observed in central southeast China which show a general drying trend due to increases in aerosol concentration.
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