Evidence for Hemispheric Spectral Albedo Inequality

<p>In this study, we examine whether the interhemispheric symmetry observed<br />in broadband shortwave albedos also applies to the<br />hemispheric-mean visible and near-infrared albedos. &#160;While<br />several recent exploratory studies have examined this question<br />using climate models, we explore this<br />question using direct observations of the visible and<br />near-infrared albedos collected by the Nimbus-7 satellite. This<br />study builds upon earlier intercomparisons of cloud spectral<br />albedos from Nimbus-7 and from climate models using the same<br />combinations of Nimbus-7 measurements used here (Collins, 1998).</p> <p>We find that the hemispheric-mean spectral partitioning of albedo is<br />consistently and statistically significantly different between the two<br />hemispheres. &#160;Consistent with prior studies, the origin of these<br />differences is due to interhemispheric differences in cloud cover.<br />Over oceans, the regional daily-mean differences between visible and<br />near-IR albedos are closely correlated with cloud amount. &#160;The<br />relative differences are maximized for clear-sky conditions and<br />minimized for overcast conditions.</p> <p>Background: The shortwave broadband albedo is a weighted sum of the albedos<br />in the visible and near-infrared bands. &#160;Under condensate-free<br />conditions, the interactions of solar insolation in these bands<br />with the atmosphere and surface are quite different. &#160;To an<br />excellent approximation, the condensate-free atmosphere is a<br />conservative Rayleigh-scattering medium in the visible. &#160;Solar<br />radiation that is not reflected back to space is, to leading<br />order, transmitted to the surface. &#160;In the near-infrared, the<br />interactions of sunlight with the atmosphere are dominated by<br />absorption, primarily with water vapor. &#160;Additional absorption is<br />contributed by well-mixed greenhouse gases, oxygen, and other<br />gaseous constituents. The solar radiation<br />reaching the surface has therefore been reduced both by<br />reflection to space (from atmospheric condensates and the surface<br />albedo) and by absorption in the atmosphere. Hence, the relative<br />partitioning of net TOA insolation between the visible and<br />near-infrared bands will affect the relative partitioning between<br />atmospheric absorption and transmission to the surface.</p> <p>&#160;</p>