Abstract
Abstract. Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-Solar Radiation (HALO-SR) instrument onboard the High Altitude and Long Range Research Aircraft (HALO) in November 2010. The data are used to quantify the influence of surface albedo variability on the retrieval of cirrus optical thickness and crystal effective radius. The applied retrieval of cirrus optical properties is based on a standard two-wavelength approach utilizing measured and simulated reflected radiance in the visible and near-infrared spectral region. Frequency distributions of the surface albedos from Moderate resolution Imaging Spectroradiometer (MODIS) satellite observations are used to compile surface-albedo-dependent lookup tables of reflected radiance. For each assumed surface albedo the cirrus optical thickness and effective crystal radius are retrieved as a function of the assumed surface albedo. The results for the cirrus optical thickness are compared to measurements from the High Spectral Resolution Lidar (HSRL). The uncertainty in cirrus optical thickness due to local variability of surface albedo in the specific case study investigated here is below 0.1 and thus less than that caused by the measurement uncertainty of both instruments. It is concluded that for the retrieval of cirrus optical thickness the surface albedo variability is negligible. However, for the retrieval of crystal effective radius, the surface albedo variability is of major importance, introducing uncertainties up to 50%. Furthermore, the influence of the bidirectional reflectance distribution function (BRDF) on the retrieval of crystal effective radius was investigated and quantified with uncertainties below 10%, which ranges below the uncertainty caused by the surface albedo variability. The comparison with the independent lidar data allowed for investigation of the role of the crystal shape in the retrieval. It is found that if assuming aggregate ice crystals, the HSRL observations fit best with the retrieved optical thickness from HALO-SR.
Highlights
Passive remote sensing techniques, either satellite-/aircraftbased or from the ground, for retrieving cirrus optical and microphysical parameters are well established (Hong et al, 2007; Eichler et al, 2009; Yang et al, 2005)
While High Altitude and Long Range Research Aircraft (HALO)-Solar Radiation (HALO-SR) performs passive measurements in the solar spectral range, the High Spectral Resolution Lidar (HSRL) is an active instrument providing vertically resolved and surface-albedo-independent data that can be analyzed without radiative transfer simulations
The slightly larger standard deviations in some cases in the τ retrieval are not reproduced for the reff retrieval of this time series. This states that the bidirectional reflectance distribution function (BRDF) influence is minimal for the reff as the directional scattering is most significant at a lower wavelength with regard to the cirrus optical thickness
Summary
Either satellite-/aircraftbased or from the ground, for retrieving cirrus optical and microphysical parameters are well established (Hong et al, 2007; Eichler et al, 2009; Yang et al, 2005) These retrievals are complicated and uncertain, (a) partly due to the complex nonspherical shape of ice crystals and the spatial heterogeneity of cirrus and (b) the impact of surface albedo on the measured signal. While HALO-SR performs passive measurements in the solar spectral range, the HSRL is an active instrument providing vertically resolved and surface-albedo-independent data that can be analyzed without radiative transfer simulations. This allows deriving cirrus optical thicknesses not affected by the surface albedo. HALO-SR, like most passive satellite-based sensors, measures vertically unresolved and relies on accurate assumptions of crystal shape and surface albedo in the radiative transfer
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