Assessment of ground-based remotely sensed liquid water cloud properties based on radiation observations

The microphysical and optical properties of liquid water clouds are essential parameters for the quantification of the surface radiation budget since they have a strong impact on the absorption and scattering processes within the clouds. Ground-based remote sensing enables continuous and long-term observations of the cloud-radiation interaction on a regional scale. Often, shortwave radiometry is used to retrieve the cloud optical depth and effective radius in combination with radiation transfer calculations. This work introduces a retrieval technique of the microphysical and optical properties of liquid water clouds independently from shortwave surface radiation measurements. The retrieval method combines cloud radar, ceilometer and microwave radiometer observations with a sub-adiabatic cloud model in order to retrieve the droplet number concentration, the profiles of effective radius and the cloud optical depth. This approach has the advantage that the retrieved cloud physical properties can be evaluated by means of a radiative closure experiment. The retrieved cloud properties are used as input parameters for the radiative transfer calculations to simulate the irradiances and to compare them with the observations of the radiation on the ground with the aim to draw conclusions on the quality of the retrieved cloud parameters. The procedure to evaluate the retrievals has been performed for various water cloud cases from several Atmospheric Radiation Measurement (ARM) observational sites. The comparison of the simulated irradiances, based on the retrieved products, with the radiation measurements show a good agreement considering the uncertainties in the measurements and in the cloud model. Therefore the presented cloud radar-based technique could be a good opportunity for retrieving reliable optical thickness and effective radius values on a long-term regional scale. Furthermore, this technique could provide the ability to enhance the studies of the effect of aerosol on the cloud microphysical properties. Additionally it can be used for the validation of the cloud properties retrieved from passive satellite observations. In fact, to conclude this work, the ground-based retrieved cloud properties are compared with the same properties retrieved using satellite measurements at non-absorbing and absorbing channels for several cases.