Water cloud optical and microphysical properties are required for a better understanding of the impact of aerosols on the solar radiation feedback. The introduced retrieval technique provides droplet concentration, effective radius and optical thickness on a basis of ground-based remote sensing observations and a vertical cloud model. Mainly cloud radar, microwave radiometer and ceilometer observations are used in this approach. The model assumptions are related to a sub-adiabatic approach, in which cloud mixing processes are predefined to be homogeneous. A gamma droplet size distribution with a fixed shape parameter is considered to relate the observations with the retrieval products. The technique is applied on a water cloud case and the uncertainties in relation to the model assumptions and errors in the measurements are determined. The greatest uncertainty in the retrieval of droplet concentration is related to the fixed shape parameter, which requires in-situ data for validation processes to quantify the results. The optical parameters are less sensitive to the model assumptions and they are evaluated with a radiative closure experiment. They are used as input for radiative transfer calculations in order to compare the simulations with radiation measurements at the ground. There is a relatively good agreement between the simulated and measured radiation considering the horizontal cloud inhomogeneity, although a bias of around 5% still exists. Therefore the technique might be a suitable approach of retrieving quantitative cloud optical properties independently from radiation observations.@en

For a better understanding of the complex processes from increased anthropogenic aerosol production to changes in cloud albedo the relevant aerosol, cloud and radiation features are required. The detection of these properties can be achieved through the synergy of ground-based remote sensing measurements with high spatial and temporal resolution. This work introduces a retrieval technique of microphysical and optical cloud properties on the basis of radar, microwave radiometer and ceilometer observations in combination with a vertical model. The greatest difficulty within this strategy is the quantification of the impact of cloud properties on the surface radiation budget, which is dominated by uncertainties in the model assumptions and the errors in the surface observations. In previous studies the optical retrievals are proved by a radiative closure experiment, which compares radiation observations with simulations based on the retrieved optical properties. A further step towards the quantification of the cloud albedo effect requires an improvement of various cloud property retrieval techniques in order to elucidate the physical processes behind. This paper presents the application of the introduced retrieval technique on water cloud cases, whereas the results and the sensitivity will be analyzed in comparison to results of similar ground-based retrieval techniques. The intercomparison of different cloud retrievals will demonstrate the accuracy of various retrieval techniques, which could lead to an improvement of the quantification of the complex processes involved in the first indirect aerosol effect.@en