J7.6
What controls planetary albedo and its interannual variability?
Xin Qu, Department of Atmospheric and Oceanic Sciences, UCLA, Los Angeles, CA; and A. Hall
The ISCCP D-series cloud and flux datasets were used to determine what controls planetary albedo and its interannual variability. Based on an idealized radiative transfer model, climatological planetary albedo is broken down into two components: atmospheric albedo and effective surface albedo. Atmospheric albedo accounts for more than 75% of planetary albedo in all regions except Antarctica, while effective surface albedo accounts for less than 25%. This can be attributed to relatively small surface albedo and the damping effect of the atmosphere on the surface contribution. The observed poleward increase in climatological planetary albedo was also examined. Atmospheric albedo and effective surface albedo contribute approximately equally to this increase in the northern hemisphere. The contribution of effective surface albedo, however, is three times larger than the contribution of atmosperic albedo in the southern hemisphere, due largely to the high values of effective surface albedo in Antarctica.
The sources of interannual variability in planetary albedo were identified through regression analysis. In a global sense, more than 90% of the variability can be linearly related to surface albedo fluctuations and cloud fluctuations. In snow and ice-covered regions, the surface accounts for more than 50% of the variability, due to large surface albedo variability associated with snow and ice fluctuations. Over snow and ice-free areas, however, the cloud contribution overwhelms the surface contribution during all seasons. The importance of surface fluctuations in generating planetary albedo variability in the cryosphere zones implies any change in surface albedo will significantly modify solar radiation. The retreat of northern hemisphere snow cover and Arctic sea ice have been recently observed by satellites. This is associated with a large-scale warming in the northern hemisphere, and the decrease in surface albedo associated with it may result in more warming by increasing the amount of the solar radiation absorbed by the surface-atmosphere system. This supports the idea that surface albedo feedback amplifies the initial perturbation of the climate system and thus acts as a positive feedback in climate change.
.Joint Session 7, Mechanisms of Climate Change (Joint Session with the Eight Conference on Polar Meteorology and the 16th Symposium on Global Change & Climate Variations)
Thursday, 13 January 2005, 3:30 PM-5:30 PM
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