Radiative and microphysical properties of Arctic stratus clouds from multiangle downwelling infrared radiances

The information content of multiangle downwelling infrared radiance spectra of stratus clouds is investigated. As an example, 76 sets of spectra were measured at angles of 0, 15, 30 and 45° from zenith, using an interferometer based at the Surface Heat Budget of the Arctic Ocean (SHEBA) drifting ice camp. Exploiting the angular variation of radiance in infrared microwindows, a “geometric” algorithm is used to determine cloud temperature and optical depth without auxiliary information. For comparison, a spectral method allows us to infer cloud microphysical properties for each angle; each multiangle set therefore constitutes a microphysical characterization of horizontal inhomogeneity of the cloudy scene. We show that cloud temperatures determined with both approaches agree with temperatures obtained from lidar/radiosonde data. The multiangle radiance observations can also be used to calculate the longwave flux reaching the surface. We find that up to 14 W m−2 of the overcast fluxes can be attributed to horizontal variations in cloud microphysical properties.