Diurnal asymmetry of surface albedo in a semi-arid grassland over the China's Loess Plateau

In current land surface models or satellite remote sensing retrievals, clear-sky surface albedo (α) is usually assumed to be symmetrical and relies only on the solar elevation angle (SEA). Based on 1-min high-resolution measurements of surface radiation fluxes, this study demonstrated that the diurnal variations of clear-sky surface albedo exhibited a significant asymmetrical pattern in both summer and winter seasons over a semi-arid grassland of the China's Loess Plateau. The results indicated that α values in the morning were generally larger than those in the afternoon at the same SEA, and diurnal asymmetry of surface albedo was distinctly prominent with SEA <40° in summer (before 9:30 a.m.) or SEA <20° in winter (before 10:00 am), and tended to diminish at midday. The averaged morning/afternoon albedo differences under sunny days were 0.05 (30.4%) and 0.09 (37.8%) in summer and winter seasons, respectively. Air relative humidity was positively correlated with the diurnal asymmetry of surface albedo, ascribed to probable formation of dew in the morning. Depression of the dew point was negatively linked to the morning/afternoon albedo differences, which was attributed to the strong scattering of incident sunlight by dewdrops could enhance the morning surface albedo. Such diurnal asymmetry of surface albedo should be included in the parameterization scheme of mesoscale and region-scale climate models in the semi-arid areas of China's Loess Plateau.