Dependence Of Albedo On Sun Direction And Cloudiness

This section presents some observations to emphasise that albedo is not only controlled by the surface but also by the illumination, in particular the solar zenith angle and cloudiness. The measurements were collected at a grazed grassland site at Uardry in southeastern Australia and have been processed into the 30-minute mean albedos presented here. Prata et al. (1998) and Grant et al. (2000) give details on the site, and the measurements and their analysis.

Figure 9 shows the diurnal variation of the albedo on three clear days in three different months. The albedo has a minimum near noon on each day, as has previously been well reported from observations, and as is expected from theoretical considerations. However, the amplitude of the diurnal variations reduces markedly over several months, there is sometimes a difference between the morning and afternoon albedo at corresponding solar zenith angles, and departures on the time scale of a few hours from a smooth variation are sometimes apparent. These three features have been seldom, if ever, noted, but if neglected will introduce errors in any estimation of the diurnal cycle of albedo, or daily or monthly mean albedo, from an estimate of the instantaneous albedo at one local time such as would be made with a polar orbiting satellite. Grant et al. (2000) show that for the Uardry site an estimate of the daily mean albedo from the 1030 local time albedo could be in error by 0.01-0.03 for an albedo of 0.20. That is, a relative error of 5 to 15%, depending upon the available level of detail of knowledge of the diurnal variation of albedo.

Figure 9. The diurnal cycle of albedo at Uardry on three clear days over several months. The dots are the observed 30-minute mean albedos. The smooth curves are a fitted model and are included as an aid to judging the symmetry and smoothness of the observed variations

Figure 10 shows the diurnal cycle of downwelling solar irradiance and albedo for three days of an eight-day period. The first and third days are clear throughout and the albedos are similar. Presumably the surface changed only slightly over the period, perhaps in greenness or soil wetness. The second day was cloudy throughout, reducing the downwelling irradiance to a roughly constant fraction of its clear-sky value. The albedo on the cloudy day was significantly below that on the two clear days, and had a flatter diurnal variation. Satellite-based estimates of land surface albedo rely on measurements in clear-sky conditions. A likely approach to the estimation of cloudy-sky albedo from satellite observations is to measure the surface BRDF under clear conditions and integrate it with a cloudy-sky irradiance field. For sufficiently stringent demands on the accuracy of the albedos input to climate models, the dependence of the albedo on the sky condition will have to be taken into account.

Local time (hours)

Figure 10. The diurnal cycle of downwelling solar irradiance Si (lower three curves) and albedo (upper three pairs of curves) at Uardry on three days, two of which are clear and one cloudy. For the albedo the joined dots are the 30-minute observations and the curves through them are fitted models. The irradiance and model albedo curves for any particular day have the same line style

Local time (hours)

Figure 10. The diurnal cycle of downwelling solar irradiance Si (lower three curves) and albedo (upper three pairs of curves) at Uardry on three days, two of which are clear and one cloudy. For the albedo the joined dots are the 30-minute observations and the curves through them are fitted models. The irradiance and model albedo curves for any particular day have the same line style

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