PIs: Manfred Wendisch, Georg Heygster

In the Arctic, a discontinuous albedo contrast between highly reflecting snow/ice and mostly absorbing sea surfaces is typical. So far it has not been quantified how these surface reflection heterogeneities affect the (i) the radiative forcing of clouds (warming/cooling), and (ii) the remote sensing products of clouds and aerosol particles. To quantify these effects, airborne measurements and Radiative Transfer (RT) modelling studies are proposed. An imaging spectrometer for measuring reflected radiances with high temporal and spatial resolution, combined with a digital camera, will be installed on the Polar 5 & 6 aircraft to measure the reflectance during ACLOUD (Arctic Clouds – Characterization of Ice, aerosol Particles and Energy fluxes) in 2017. Simultaneously, spectral irradiances reflected by the surface will be measured to deduce the surface albedo. Furthermore, a three–dimensional (3D) radiative transfer (RT) model will be applied to reproduce the airborne measurements in different spatial resolutions. The results of the measurements will be compared to satellite–derived surface reflection properties, in particular the Bidirectional Reflectance Distribution Function (BRDF) and albedo. The measured spatial and spectral distribution of BRDF and surface albedo will serve as input for 3D (three–dimensional) RT simulationsto quantify 3D radiative effects and their sensitivity on the retrieval of cloud and aerosol properties as well as the sensitivity on the radiative cooling or warming by Arctic clouds, which influence feedback mechanisms within the Arctic Amplification. We will investigate, if 1D assumptions are sufficient to quantify the radiative cooling or warming due to clouds for a representative area for heterogeneous surfaces. Measurements of terrestrial broadband radiation will be used to determine the net radiative effects.

• Deliver measures of heterogeniety, surface albedo
• Estimate of CRF over highly variable Arctic surface (contrast between ice/snow/open water/melt periods)
• Sensitivity of the existing aerosol/cloud retrievals on surface heterogeniety
• Recommondations on how to compare satellite, airborne and in-situ measurements made over challenging surfaces in the Arctic