| For a better understanding and projection of future climate changes, it is important to quantify and reduce the uncertainties of aerosol direct and indirect effects. This dissertation focuses on these issues.; The aerosol direct effect (i.e., scattering the incoming solar radiation) ranges from -0.1 to -1.0 W/m2. This high uncertainty may arise from aerosol loading, aerosol optical properties, and radiative transfer models (RTMs). By comparing aerosol loading from different chemical transport models, comparing theoretical and measured aerosol optical properties, and comparing aerosol forcings simulated by different RTMs, we show that the largest uncertainty is associated with aerosol loading.; The first aerosol indirect effect (AIE) (i.e., modifying the initial cloud drop size distribution) is generally supported by observations and model results. Our numerical results show that the cloud droplet number concentration increases and droplet size decreases with increasing aerosol loading.; The second AlE (i.e., modifying cloud lifetime and morphology) is not easily observed. For a spring continental stratus observed at the Southern Great Plains, our numerical results show that the cloud liquid water path (MP) could either increase, decrease, or remain unchanged with increasing aerosol loading. For summer maritime stratocumulus observed at the sub-tropical northeast Atlantic, our numerical results show that the LWP and cloud fraction (CF) could decrease or remain nearly unchanged with increasing aerosol loading. Further investigation indicates that thermodynamic feedbacks (more vapor condensation near cloud base caused by drizzle evaporative cooling) and the large-scale meteorological conditions (large-scale subsidence) are important for the response of the LWP and CF to changes in aerosols when precipitation is so small that it is not a dominant sink of cloud water. |
