In their Jan., 1997 article, “On Solar Energy Disposition: A perspective from Observation and Modeling,”
Zhanqing Li, Louis Moreau and Albert Arking discussed the amount of solar energy earth's surface absorbs
and what the atmosphere absorbs and reflects. Appearing in the Bulletin of the American Meteorological
Society, vol. 78, the article explained key variables which support the atmospheric column upon which
earth's solar energy budget, or solar energy disposition, (SED), is based. They also discussed
discrepancies which have occurred between observations of energy balance and results of certain global
climate models, (GCM's). In all, four observational data sets and four GCM's were compared for accuracy
in representing the planet's energy budget, and, thus, climate variability.
According to observations and models discussed in the article, clouds are a key variable in the partitioning of solar energy which strikes our planet. Important factors are the amount of, vertical distribution of and optical properties of clouds. The four observational data sets considered here, (one ground-based and three satellite-based geostationarily), proved GCMs’ surface absorption values too high.
The models found that clouds had more absorbing effect than did observations dating back more than one hundred years. Li, et. al. wrote, however, that such discrepancies could be due to models’ insensitivity to increasing amounts of CO2, aerosols and other greenhouse gases in the atmosphere. The article takes the neutral attitude that large existing discrepancies are “wanting” of more information on SED before more aptly built models can run successfully.
“Thanks to satellite observations, the earth's radiation budget (ERB) at the top of the atmosphere is reasonably well known. Current GCM's manage to reproduce a reasonable global and annual mean ERB, but often fail to simulate the variations in ERB associated with certain cloud regimes such as tropical convection and storm tracks,” (Li, et. al., p.53).
Analyses of the observational data sets showed that the earth probably reflects 30% of its incident solar radiation back to space. It absorbs 24% of the energy in the atmosphere and the remaining 46% at the surface.
“The discrepancies in SRB, (surface radiation budget), between satellite-based estimation and model simulation are of the order of 20-25 W m-2,” (Li, et. al., p.67).
Clouds being the largest contenders in uncertainties which accompany model results, one might assume that the entirety of SED's role in the hydrological cycle is not yet well-understood. The article noted that while SED plays an active role in energizing the planet, and, thus climatic patterns, it also is important to the hydrological cycle. Half the solar energy absorbed at the surface of the earth is utilized in evaporating water. This evaporation process eventually forms clouds. Latent heat released in the cloud formation is a major source of energy driving the atmospheric circulation. This is especially true in the tropics, and is as important to heating the planet as what incident solar radiation the atmosphere immediately absorbs.
While models have had trouble spelling out the true SED of the planet, observations also require improvements, according to the article. The major shortcoming of observational data has historically been insufficient sampling. More sophisticated data gathering techniques have also lacked information on some variables affecting “the radiative transfer process, and dependence, directly or indirectly, on radiative transfer models,” (Li, et. al., p.53).
Reference