In this thesis upward spectral radiances are analyzed, which were sampled above subtropical trade wind cumuli near Barbados. Data were collected by a new measurement setup consisting of two payloads attached to a helicopter. This unique approach allows for collocated measurements of high–resolution microphysical cloud data, aerosol particle number size dis- tributions and the radiation reflected from cloud top, yielding a quantification of the first indirect aerosol effect of shallow water clouds. With the help of the spectral radiation data methods are introduced which help to identify cloud data that are influenced by the albedo of the water surface, as well as three–dimensional radiative effects. A statistical measure of cloud inhomogeneity is introduced, characterizing the observed trade wind cumuli to exhibit medium to high inhomogeneity with respect to upward radiances in the visible wavelength range. The influence of the substantial cloud inhomogeneity on the remote sensing of the cloud optical thickness and the effective droplet radius is quantified. It is shown, that misrep- resentation of the often observed overlying thin cirrus layers in the usual retrieval approaches yields an overestimation in retrieved effective droplet radius of up to 50% and and underestimation in retrieved cloud optical thickness of up to 6%. A new retrieval algorithm is introduced which minimizes the effects of the overlying cirrus without a priori knowledge of its properties. The results from the new algorithm are compared to in situ observations of the effective droplet radius, yielding a high agreement of ±1 μm. Relative susceptibilities of the retrieved microphysical and optical cloud parameters, as well as cloud top reflectivities, are derived to quantify the first indirect aerosol effect for subtropical trade wind cumuli. These relative susceptibilities are in the range of the maximum possible susceptibility following the theory for homogeneous clouds. Parameterizations of the cloud optical thickness and the effective droplet radius as a function of the derived relative susceptibilities and the liquid water path are introduced. The parameterized and measured parameters agreee with a correlation coefficient of 0.97, showing that the parameterization can reliably represent the measurements.

In this thesis upward spectral radiances are analyzed, which were sampled above subtropical trade wind cumuli near Barbados. Data were collected by a new measurement setup consisting of two payloads attached to a helicopter. This unique approach allows for collocated measurements of high–resolution microphysical cloud data, aerosol particle number size dis- tributions and the radiation reflected from cloud top, yielding a quantification of the first indirect aerosol effect of shallow water clouds. With the help of the spectral radiation data methods are introduced which help to identify cloud data that are influenced by the albedo of the water surface, as well as three–dimensional radiative effects. A statistical measure of cloud inhomogeneity is introduced, characterizing the observed trade wind cumuli to exhibit medium to high inhomogeneity with respect to upward radiances in the visible wavelength range. The influence of the substantial cloud inhomogeneity on the remote sensing of the cloud optical thickness and the effective droplet radius is quantified. It is shown, that misrep- resentation of the often observed overlying thin cirrus layers in the usual retrieval approaches yields an overestimation in retrieved effective droplet radius of up to 50% and and underestimation in retrieved cloud optical thickness of up to 6%. A new retrieval algorithm is introduced which minimizes the effects of the overlying cirrus without a priori knowledge of its properties. The results from the new algorithm are compared to in situ observations of the effective droplet radius, yielding a high agreement of ±1 μm. Relative susceptibilities of the retrieved microphysical and optical cloud parameters, as well as cloud top reflectivities, are derived to quantify the first indirect aerosol effect for subtropical trade wind cumuli. These relative susceptibilities are in the range of the maximum possible susceptibility following the theory for homogeneous clouds. Parameterizations of the cloud optical thickness and the effective droplet radius as a function of the derived relative susceptibilities and the liquid water path are introduced. The parameterized and measured parameters agreee with a correlation coefficient of 0.97, showing that the parameterization can reliably represent the measurements.