Marine stratocumulus cloud regimes exert a strong climatic influence through their high solar reflectivity. Human-induced changes in stratocumulus clouds, attributed to an increase of the aerosol burden (indirect effects), can be significant given the cloud decks proximity to the continents; nevertheless, the magnitude and the final climatic consequences of these changes are uncertain. This thesis investigates further the interactions between aerosols, cloud microphysics, regional circulation, and radiative response in the Southeast Pacific stratocumulus cloud deck, one of the largest and most persistent cloud regimes in the planet. Specifically, three different aspects are addressed by this thesis: The importance of the synoptic atmospheric variability in controlling cloud microphysical and radiative changes, a validation analysis of satellite retrievals of cloud microphysics from MOderate Resolution Imaging Spectroradiometer (MODIS), and the quantitative assessments of cloud aerosol interactions along with their associated radiative forcing using primarily aircraft remote sensing data. Synoptic and satellite-derived cloud property variations for the Southeast Pacific region associated with changes in coastal satellite-derived cloud droplet number concentration (Nd) are analyzed through a composite technique. MAX and MIN Nd composites are defined by the top and bottom terciles of daily area-mean Nd values over the Arica Bight, the region with the largest mean oceanic Nd, for the five October months of 2001, 2005, 2006, 2007, and 2008. The MAX-Nd composite is characterized by a weaker subtropical anticyclone and weaker winds than the MIN-Nd composite. Additionally, the MAX-Nd composite clouds over the Arica Bight are thinner than the MIN-Nd composite clouds, have lower cloud tops, lower near-coastal cloud albedos, and occur below warmer and drier free tropospheres. At 85˚W, the top-of-atmosphere shortwave fluxes are significantly higher (50%) for the MAX-Nd, with thicker, lower clouds and higher cloud fractions than for the MIN-Nd. The change in Nd at this location is small, suggesting that the MAX-MIN Nd composite differences in radiative properties primarily reflects synoptic changes. The ability of MODIS level 2 retrievals to represent the cloud microphysics is assessed with in-situ measurements of droplet size distributions, collected during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The MODIS cloud optical thickness (t) correlates well with the in-situ values with a positive bias (1.42). In contrast, the standard 2.1 micron-derived MODIS cloud effective radius (r_e) is found to systematically exceed the in-situ cloud-top r_e, with a mean bias of 2.08 um. Three sources of errors that could contribute to the MODIS r_e positive bias are investigated further: the spread of the cloud droplet size distribution, the presence of a separate drizzle mode, and the sensor viewing angles. The sensor zenith viewing angles were found to have little impact, while the algorithm assumption about the cloud droplet spectra and presence of a precipitation mode could affect the retrievals but not by enough to fully explain the positive MODIS r_e bias. The droplet spectra effects account for r_e offsets smaller than 0.6 um, 0.9 um, and 1.6 um for non-drizzling, light-drizzling, and heavy-drizzling clouds respectively. An explanation for the observed MODIS bias is lacking although three-dimensional radiative effects were not considered. This investigation supports earlier studies documenting a similar bias, this time using data from newer probes. MODIS r_e and t were also combined to estimate a liquid water path (LWP) and Nd. A positive bias was also apparent in LWP, and attributed to r_e. However, when selected appropriate parameters a priori, the MODIS Nd estimate was found to agree the best with the insitu aircraft observations of the four MODIS variables. Lastly, the first aerosol indirect effect (Twomey effect) is explicitly investigated with VOCALS-REx observations, collected during three daytime research flights (Nov 9, 11, and 13), utilizing an aerosol-cloud interactions metric, and defined as ACI=dln(t)/dln(Na), with Na corresponding to the accumulation mode aerosol concentration, t derived from a broadband pyranometer, and ACI binned by cloud LWP derived from a millimeter-wavelength radiometer. Aircraft remote sensing estimates of the ACI, during sub-cloud transects, show that the cloud aerosol-interactions are strong and close to the maximum theoretical value for thin clouds, with a decrease of ACI with LWP. Although an explanation for the dependence of ACI on LWP is lacking, we found that a decrease in ACI with LWP is associated with decreases in both surface meridional winds and Nd. Similar to ACI, albedo fractional changes due to Nd fractional changes also tended to be smaller for higher LWPs, but with an overall radiative forcing larger than conservative global estimates obtained in global circulation models. The findings of this thesis emphasize the strong stratocumulus albedo response to an aerosol perturbation and its dependence on the regional scale atmospheric configuration. The results presented here can be used as a benchmark for testing regional and climate models, as well as helping to improve the current parameterizations of the first aerosol indirect effect.

In this thesis a novel approach to compare airborne observations of spectral upward and downward irradiances with along-track radiative transfer simulations (RTS) are presented. The RTS are performed with the ecRad radiation scheme of the Integrated Forecast System (IFS) operated by the European Centre for Medium Range Weather Forecast (ECMWF) and the library for Radiative transfer (libRadtran) on basis of hourly 0.1° IFS analysis data (IFS AD). The comparison aims to investigate the general capability of the utilized models to reproduce the observed radiation field. Simultaneous utilization of ecRad and libRadtran, driven by the same IFS AD, and comparison with observations enables to separate for potential errors in the applied IFS AD and ecRad.

In this work spectral airborne measurements of upward irradiance and a novel remote sensing technique for the cloud droplet number concentration are used to evaluate the representation of clouds in current operational weather prediction models.