Mean Annual of Terrestrial Radiation [TOA] and Albedo
Compiled and Modified by Sandro Lubis from NASA CERES Satellite
Graduate Student of Leipzig Institute for Meteorology, University of Leipzig, Germany
[ All computations are made using NCAR Command Language (NCL)]
Albedo is a fraction of solar energy (SW radiation) reflected from the Earth back into space (Ahrens, 2006). In general, most land areas have higher albedo value than the ocean because of the contribution of clouds over the land (more sunlight is reflected) and also the snow- /ice-covered Arctic/Antarctic leading to much radiation reflects back up into the atmosphere . It can be seen that at the higher latitudes, the albedo of the ocean surface increases significantly, why? Easy, It’s because of low Sun angles or the presence of sea ice.
Variation of albedo with latitude in general showed the fact that the oceans and arid tropical regions have low albedo but the polar regions, and bordering latitudes, have snow cover for much of the year that lead to the high albedo values. As we discussed above, this is a result of the lower sun angle present at the poles and also the higher presence of fresh snow, ice, and smooth open water- all areas prone to high levels of reflectivity.
The distribution of albedo values of mean total sky is higher than annual clear sky albedo due to clouds reflecting more sunlight than land and water. The cloud reflects more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere. In other words, the presence of clouds has a significant contribution to reflect fraction of the incoming solar radiation back to the space. Solar energy gained in the tropical region in clear sky condition (no clouds) is much higher than region covered by sky because much energy is absorbed to the surface than reflected to out of space.
Longwave radiation emitted is a function of the temperature of the emitting surface. The presence of clouds and water vapors has a large influence. The maximum outgoing longwave radiation is found above warm dry areas such as the subtropical deserts. In the wet tropical regions such as Indonesia, South America (Amazon basin), Middle Africa, and in the Western Equatorial Pacific has low OLR. It’s because this region generally emits less radiation than dry tropical areas and also due to large convective clouds and water vapor exist in the atmosphere. Consequently, as we can see above, we can group that wet equatorial areas which has low annual mean OLR values (<200 Wm-2) are associated with deep atmospheric convection. Area outside tropics which has OLR < 200 Wm-2 is associated with the cold temperatures. High annual mean OLR values (>280 Wm-2) are associated with the high temperatures and subsidence area such Sahara and the Arabian Peninsula. This is an explanation why OLR on earth has latitudinal variation.
OLR values in clear sky condition are much higher than full sky/cloudy regions because they (clouds) absorb part of the infra-red radiation emitted by the surface. The net loss in terrestrial radiation in the tropics of all sky is lower than clear sky because most of LWup is absorbed by the water vapor and clouds in the atmosphere, while less is radiated directly into space.
How much energy does the system gain in the tropics in clear sky and without clouds?Total energy was calculated from spatial average of net solar spectrum radiation (SWinc-SWup) in a bounded area (30S-30N) and (180E-180W). Total mean annual energy gains in the tropics is approximately 305 W/m2 *[24*60*60*365] s = 9.631 .10^9 J/m2 and without sky (clear sky) is 349 W/m2 *[24*60*60*365] s = 1.103 .10^10 J/m2.
How much net loss terrestrial radiation does the system gain in the tropics in clear sky and without sky? It was calculated from spatial average of net terrestrial spectrum radiation (LWup-LWinc) of a bounded area (30S-30N) and (180E-180W). Total mean annual net loss terrestrial radiation gains in the tropics is approximately 259.7 W/m2 *[24*60*60*365] s = 8.190 .10^9 J/m2 and without sky (clear sky) is 291W/m2 *[24*60*60*365] s = 9.176 .10^9 J/m2. Assumed that LWinc=0 (data is not available for this analysis).
Ahrens, C. D. 2006. Meteorology Today. An Introduction to Weather, Climate, and the
Environment. Eighth Edition. Thompson, Brooks/Cole. USA.
Chelliah, M., and P. A. Arkin, 1992: Large-scale interannual variability of outgoing
longwave radiation anomalies over the global tropics. J. Climate, 5, 371-389.