BRDF Characteristics of Westsiberian and Alaskan Tundra Vegetation Communities
Buchhorn, Marcel¹; Heim, Birgit¹; Walker, Donald²; Epstein, Howard³; Leibman, Marina^4
1Alfred Wegener Institute for Polar and Marine Research, GERMANY; ²Alaska Geobotany Center, University of Alaska Fairbanks, UNITED STATES; ³Dept. of Environmental Sciences, University of Virginia, USA, UNITED STATES; ⁴Earth Cryosphere Institute Siberian Branch of Russian Academy of Sciences, RUSSIAN FEDERATION

The bidirectional reflectance distribution function (BRDF) effects satellite data from platforms with pointing capabilities and sensors with wide swath causing significant changes in the measured spectral surface reflectance depending on the solar illumination geometry and sensor viewing conditions.
The Environmental Mapping and Analysis Program (EnMAP), a German hyperspectral mission with expected launch in 2017, will provide high spectral resolution observations with a ground sampling distance of 30 meters. Since the EnMAP sensor has pointing capabilities, both spectral and directional reflection characteristics need to be taken into account for the algorithms development for vegetation parameters. The â€~hyperspectral method development for Arctic VEGetation biomes’ (hy-Arc-VEG) project is part of the national preparation program for the EnMAP mission. We developed a portable field spectro-goniometer, named Manual Transportable Instrument for Spherical BRDF observations, ManTIS (International patent AWI2011/07/01) for the in-situ measurements of anisotropic effects of tundra surfaces. The goniometer was designed for field use in difficult and challenging terrain and weather and is therefore of low weight, without electrical devices and weatherproof and can be disassembled into its components for transport. The current off-nadir viewing capacity is matched to the EnMAP sensor configuration (up to 30A°).
We carried out spectral field and goniometer measurements on the joint YAMAL 2011 expedition (RU-US-DE) organized by the Earth-Cryosphere Institute (ECI) in August on the Yamal Peninsula, northwestern Siberia (RU) and on the joint NAAT (North American Arctic Transect) expedition together with the University of Alaska, Fairbanks, in 2012 along the North Slope, Alaska (US). The field goniometer measurements (conducted under varying sun zenith angles), as well as field spectro-radiometrical measurements, were carried out at the NASA Yamal Land Cover/Land Use Change (NASA Yamal-LCLUC) transects and rA©leves at Laboravaya (southern Yamal) and Vaskiny Dachi (central Yamal) and at the long term monitoring grids of the University of Fairbanks, Alaska along the North Slope. All investigated grids, transects and rA©leves are part of the Greening of the Arctic programme (http://www.geobotany.uaf.edu/goa/index).
BRDF processing for the tundra test sites demonstrate the mirror asymmetry in relative azimuth with respect to the principal plane. It also showed that the maximum scattering appears in the backward direction, but that there is no minimal forward scattering. Moreover, the analysis of the anisotropic behaviour of moss-dominated tundra types with 10 to 15% vascular plant cover show that the BRDF influence on vegetation indices (VI) of low-growing arctic vegetation communities can be up to 15% of the nadir value. The low sun elevation at the arctic latitudes prevents hotspot-effects, but a BRDF normalization still should be taken into account for the development of tundra-adapted vegetation indices.