Airborne hyperspectral observations of surface and cloud directional reflectivity using a commercial digital camera

Abstract

Abstract. Spectral radiance measurements by a digital single-lens reflex camera were used to derive the directional reflectivity of clouds and different surfaces in the Arctic. The camera has been calibrated radiometrically and spectrally to provide accurate radiance measurements with high angular resolution. A comparison with spectral radiance measurements with the Spectral Modular Airborne Radiation measurement sysTem (SMART-Albedometer) showed an agreement within the uncertainties of both instruments (6 % for both). The directional reflectivity in terms of the hemispherical directional reflectance factor (HDRF) was obtained for sea ice, ice-free ocean and clouds. The sea ice, with an albedo of ρ = 0.96 (at 530 nm wavelength), showed an almost isotropic HDRF, while sun glint was observed for the ocean HDRF (ρ = 0.12). For the cloud observations with ρ = 0.62, the cloudbow -a backscatter feature typically for scattering by liquid water droplets -was covered by the camera. For measurements above heterogeneous stratocumulus clouds, the required number of images to obtain a mean HDRF that clearly exhibits the cloudbow has been estimated at about 50 images (10 min flight time). A representation of the HDRF as a function of the scattering angle only reduces the image number to about 10 (2 min flight time).The measured cloud and ocean HDRF have been compared to radiative transfer simulations. The ocean HDRF simulated with the observed surface wind speed of 9 m s −1 agreed best with the measurements. For the cloud HDRF, the best agreement was obtained by a broad and weak cloudbow simulated with a cloud droplet effective radius of R eff = 4 µm. This value agrees with the particle sizes derived from in situ measurements and retrieved from the spectral radiance of the SMART-Albedometer.

Optical thickness and effective radius of Arctic boundary-layer clouds retrieved from airborne nadir and imaging spectrometry

“…The final radiometric uncertainty is given as 8 % for the AisaEAGLE and 9 % for the SMARTAlbedometer radiance, which is mostly due to the uncertainty of the reference standards. The radiometric calibration of AisaEAGLE has been verified by comparing the upwelling radiances with that of the well-established SMARTAlbedometer (compare Ehrlich et al, 2012). Using the INS attitude records, the AisaEAGLE pixels that are located in the field-of-view of the SMART-Albedometer radiance sensor are identified for each time step.…”

Validation of the sea ice surface albedo scheme of the regional climate model HIRHAM–NAOSIM using aircraft measurements during the ACLOUD/PASCAL campaigns

“…They concluded that the parameterization of snow and ice surface albedo in several models contributes significantly to the bias in the magnitude of the feedback, as a result of the strong sensitivity of simulated snow-covered surface albedo to surface warming. Flanner et al (2011) and Crook and Forster (2014) have identified an underestimation of the snow-ice albedo feedback in models compared to observations.…”

High Angular Resolution Measurements of the Anisotropy of Reflectance of Sea Ice and Snow

“…More recently, directional reflectance has also been measured with commercial digital cameras. For instance, Ehrlich et al () collected directional reflectivity over clouds, sea ice, and water using an airborne wide‐angle lens reflex digital camera. Their measurements highlight the capability of digital cameras to instantly measure directional reflectance over an entire hemisphere without the need for high‐precision movable components.…”