Radiation Measurements

Abstract

Surface albedo, the ratio of the upwelling solar irradiance to the downwelling irradiance at the surface of the Earth, is a significant indicator of environmental conditions and a key parameter for calculation of the radiation budgets of the atmosphere and surface. By knowing the surface albedo (α), one can estimate the amount of heat produced at the surface by the absorption of incident solar radiation. The albedo has strong temporal and spatial variations that influence the regional and global surface energy budget. For a vegetated surface, α can change with plant phenology, soil moisture content, and fractional canopy cover, as well as solar zenith angle. Uncertainties in α translate almost directly into errors in the calculation of net radiation and energy fluxes and have been investigated in many studies. In a review of studies on the sensitivity of climate models to surface albedo changes, (1983) concluded that an accuracy of ±0.05 in α was needed for climate modeling purposes. To evaluate in detail the variations in α on diurnal and seasonal scales, ground-based measurements are usually essential. In principle, factors internal to the vegetative canopy should be examined separately from external factors, but albedo measurements made above vegetation usually do not distinguish between canopy and noncanopy effects. Hence, the role of noncanopy surfaces often has to be inferred, which requires careful analysis. In this chapter, the diurnal asymmetry in α is investigated using field observations above the canopy. The contributions of the phenological effects on α excluding the influence of solar zenith angle are examined. In addition, phenological effects on the conversion of narrowband satellite data to the broadband albedo α are explored.