Albedometer
Astrophysics and Astronomy
Final Report Abstract
The Albedometer was primarily planned for measurements on the HALO (High Altitude and LOng Range Research) aircraft. A first version including spectrometers for visible and near infrared radiation but without horizontal stabilization platform was used on a helicopter-based platform during the international Intensive Measurement Period at the Cabauw Tower (IMPACT) campaign within the EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality Interactions) project in May 2008. This campaign had the primary objective to study interactions between low-level clouds and aerosol processes. In situ data of the microphysical cloud properties were collected by the Airborne Cloud Turbulence Observation System (ACTOS) attached to the helicopter by a 145 m long rope, while the downward-looking optical inlets of the Albedometer were installed underneath the helicopter which was flying above the cloud. It allows collocated measurements of cloud microphysical properties within the cloud and radiative quantities of the cloud top without temporal or spatial displacement. That is exemplarily needed for the analysis of the link between cloud parameter and reflected radiation (keyword: indirect aerosol effect). From the measurements during the campaign two cloud cases were studied in detail. The optically thicker cloud in our limited data set exhibited an anticorrelation between cloud top albedo and the effective droplet radius. Spectral radiance data of the Albedometer were used for the remote sensing of the effective radius and the optical thickness based on common retrieval techniques. The retrieved cloud properties of the Albedometer were compared to the collocated in situ cloud measurements and the satellite retrieval results from the Moderate-Resolution Imaging Spectroradiometer (MODIS). Despite low optical thickness and effective radius values for both cases the mean retrieved effective radius deviated only up to 0.2 µm from the in situ measurements, while the MODIS retrievals of the effective radius deviated by a factor of 2-3. During the first HALO flight with operational scientific instruments (Techno-Mission in October 2010) the Albedometer was operated together with an instrument for measurements of actinic radiation in cooperation with the Forschungszentrum Jülich. The date are currently used in combination with radiative transfer calculations to derive information on the shape and spatial distribution of cirrus cloud ice crystals, supported by Lidar data gathered by the DLR during the Techno-Mission. In addition we try to reach closure of the actinic radiation measured at the location of the aircraft and model simulations based on atmospheric properties retrieved from independent radiance observations. Based on the atmospheric aerosol profile, gathered out of the Lidar data, the ground surface albedo is iterated out of the radiance and actinic flux density measurements. It is varying due to the heterogeneity of the ground below the flight routes and delivers values between 0.1 and 0.4. The resulting surface albedo allows the retrieval of cloud properties, in particular the effective radius and the optical depth, based on the algorithm given by Nakajima & King (1990). By varying the modeled ice crystal shape, information on the ice crystal shape and the spatial inhomogeneity during the measurements are derived afterwards.
