Observational and numerical studies of the boundary layer, cloud, and aerosol variability in the southeast Pacific coastal marine stratocumulus

This dissertation investigates the impacts of meteorological factors and aerosol indirect effects on the costal marine stratocumulus (Sc) variations in the southeast Pacific, a region that has been largely unexplored and is a major challenge of the modeling community, through both observational and numerical studies.;This study provides a unique dataset for documenting the characteristics of the marine Sc-topped BL off the coast of Northern Chile. The observational study shows that the boundary layer (BL) over this region was well mixed and topped by a thin and non-drizzling Sc layer on days synoptically-quiescent with little variability between this region and the coast. The surface wind, the surface fluxes and the BL turbulence appeared to be weaker than those over other ocean regions where stratocumulus clouds exist. The weaker turbulence in the BL may contribute to a relatively low entrainment rate calculated from the near cloud top fluxes. This in-situ data set can help us better understand cloud processes within this coastal regime, and also be valuable for the calibration of the satellite retrievals and the evaluation of numerical models operating at a variety of scales.;A strong positive correlation between the liquid water path (LWP) and the cloud condensation nuclei (CCN) was observed under similar boundary layer conditions. This correlation cannot be explained by some of the hypotheses based on previous modeling studies. The satellite retrievals obtained upstream one day prior to the flight observations reveal some sign that the clouds under the high CCN concentrations have minimal LWP loss due to precipitation suppression effects.;The results from large eddy simulations with a two-momentum bulk microphysics scheme under different idealized environment scenarios based on aircraft observations indicate that (1) the simulated Sc responds more quickly to changes in large-scale subsidence than to those changes in surface fluxes, free-tropospheric humidity, and the BL-top stability; (2) large-scale vertical wind shear clearly induces cloud-top mixing and enhances entrainment rate; (3) the solar radiation could weaken the BL turbulence, reduce the entrainment rate and decouple the BL; and (4) the impact of the reduced cloud sedimentation due to increasing aerosol on the cloud is small.