Abstract
Abstract. Spectral airborne upward and downward irradiance measurements are used to derive the area-averaged surface albedo. Real surfaces are not homogeneous in their reflectivity. Therefore, this work studies the effects of the heterogeneity of surface reflectivity on the area-averaged surface albedo to quantify how well aircraft measurements can resolve the small-scale variability of the local surface albedo. For that purpose spatially heterogeneous surface albedo maps were input into a 3-dimensional (3-D) Monte Carlo radiative transfer model to simulate 3-D irradiance fields. The calculated up- and downward irradiances in altitudes between 0.1 and 5 km are used to derive the area-averaged surface albedo using an iterative retrieval method that removes the effects due to atmospheric scattering and absorption within the layer beneath the considered level. For the case of adjacent land and sea surfaces, parametrizations are presented which quantify the horizontal distance from the coastline that is required to reduce surface heterogeneity effects on the area-averaged surface albedo to a given limit. The parametrization which is a function of altitude, aerosol optical depth, single scattering albedo, and the ratio of local land and sea albedo was applied for airborne spectral measurements. In addition, the deviation between area-averaged and local surface albedo is determined for more complex surface albedo maps. For moderate aerosol conditions (optical depth less than 0.4) and a wavelength range between 400 and 1000 nm, the altitude and the heterogeneity of the surface albedo are the dominant factors determining the mean deviation between local and area-averaged surface albedo. A parametrization of the mean deviation is applied to an albedo map that was derived from a Landsat image of an area in East Anglia (UK). Parametrization and direct comparison of local and area-averaged surface albedo show similar mean deviations (20% vs. 25%) over land.
Highlights
This study investigates the effect of local surface albedo heterogeneity and aerosol parameters on the retrieved areaaveraged surface albedo from airborne upward and downward irradiance measurements. 3-D radiative transfer simulations were performed for different local surface albedo and aerosol situations
Calculated upward and downward irradiances in altitudes zflight between 0.1 and 5 km were used to retrieve the surface albedo using an iterative method that is based on 1-D simulations
For surface albedo ratios δ ≥ 2, the parametrization is a function of flight altitude zflight, aerosol optical depth (AOD), ωand δ
Summary
With respect to the retrieval of the aerosol optical depth (AOD), surface albedo heterogeneity can lead to an overestimation of AOD due to the so-called adjacency effect (Lyapustin and Kaufman, 2001) This effect influences satellite or airborne radiance measurements. The agreement between the area-averaged surface albedo ρret retrieved by the method of Wendisch et al (2004), based on airborne measurements and the heterogenous local surface albedo ρloc, depends on the flight altitude and aerosol properties. Since many remote sensing applications need knowledge about ρloc instead of ρret, this paper investigates how well the surface albedo can be spatially-resolved by airborne measurements depending on the heterogeneity of the local surface albedo and flight altitude.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
