THE ANNUAL COURSE OF ZONAL MEAN ALBEDO AS DERIVED FROM ESSA 3 AND 5 DIGITIZED PICTURE DATA

Abstract

Temporal and meridional variations of zonal means of albedo and absorbed solar radiation, derived from digitized satellite pictures, are portrayed for the period February 1967?February 1968 over a broad range of latitudes. The variations in a number of prominent features of the zonal mean albedo?the maxima associated with the intertropical convergence zone, the minima associated with the subtropical anticyclones and equatorial dry zone, and the poleward ascendants of albedo from the subtropics?are revealed in more detail than possible heretofore. The annual courses of zonal mean albedo in three component sectors of 120° longitude each are also shown. The sector covering the zone 0°?115°E differs the most from the overall mean and from the other two sectors covering the Pacific and Atlantic-Americas regions. These differences are most pronounced in the period June?August when the summer monsoon is dominant over Southeast Asia. Harmonic analysis of the overall zonal mean albedo values for a full year at individual latitude circles between 0° and 45°N indicates the relative prominence of the annual, semiannual, and other long-period oscillations at the various latitudes. The annual cycle is dominant in latitudes 25°?45°N and 10°?15°N. At 0°, 5°, and 20°N, the semi-annual cycle is stronger than the annual cycle. In the Tropics and subtropics, amplitudes of frequencies corresponding to periods of 28?30 days are more prominent than all but the very long-period oscillations (i.e., 90?365 days). The annual course of the absorbed solar radiation associated with these albedo values exhibits some interesting features brought about by the asymmetries of albedo in the Northern Hemisphere relative to the equinoxes and summer solstice. For example, the consistently higher albedos at all latitudes northward of 20°N and lower values at 10°?15°N in March than in September result in a poleward gradient of absorbed solar radiation considerably stronger at the vernal than at the autumnal equinox. Comparison of albedos and absorbed solar radiation for Northern and Southern Hemispheres near their respective summer solstices reveals that the poleward ascendant of albedo is much stronger in the Southern Hemisphere and that, therefore, the poleward gradient of absorbed solar radiation is considerably stronger too.