Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology Zurich (IAC-ETHZ), Winterthurerstrasse 190, 8057 Zurich, +41 1 635 5208 Switzerland
This work presents a field Gonio-Spectrometer developed at the Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich (IAC-ETH).The main motivation to built this Gonio-Spectrometer was the study of the Hemispherical Distribution Reflectance Factor (HDRF) of dry snow on the Ice Sheet of Greenland and to examine the influence of the HDRF on the surface energy balance.
The surface Albedo is of great importance for both, large scale and small scale climate modeling and energy balance studies. Especially for remote regions, satellites provide an extraordinary means to measure reflected sunlight. However, raw satellite data have to undergo several corrections depending on the viewing angles of the sensors relative to the targets and the irradiance source (sun). The function that describes the distribution of reflected radiance with angle is called Bidirectional Reflectance Distribution Function (BRDF). The HDRF is the commonly used dimensionless form of the angular distribution of reflectance. BRDF and HDRF are functions of four angles: incoming (solar) zenith angle and azimuth, and outgoing (reflected) zenith angle and azimuth. In situ measurements of HDRF data, a combination of multidirectional and hyperspectral data, require complex and demanding experiments. Therefore, existing data sets a rare. However, the advent of new satellite systems that offer hyperspectral resolution and off-nadir tilting capability ask for ground truth data sets.
The IAC-Goniometer is operated with a standard field spectroradiometer, 'FieldSpec® Pro Dual VNIR' from ASD. It measures the HDRF with a distance of 1 meter between sensor and target. The sensor, an optic cable, can be placed on an arbitrary place on the hemisphere and always points towards the same surface area. Depending on the viewing geometry, the diameter of the footprint area varies from 5 cm (at nadir) to 20 cm (at 75 degree zenith angle). The pointing accuracy, analyzed in a laboratory experiment with a laser beam, was measured at ± 1.4 cm.
In the summer field season of 2004, ninety complete samplings of snow HDRF were accomplished on the Ice Sheet of Greenland. The measurements were made for various solar illumination geometries and various snow surface properties. The snow surface changed from very smooth, either after a snowfall or during wind drift, to very rough, after rime incidents. New snow showed the highest anisotropy in the HDRF distribution while a rough surface exhibited a more isotropic distribution. Moreover, the HDRF became more anisotropic with increasing solar zenith angle with values over 200% for wavelength around 1000 nm. Video clip
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