E1 Surface Radiative Balance

Topic

The surface radiation balance is the net exchange of energy—in the form of electromagnetic radiation—between the Earth's surface and the overlying atmosphere. Shortwave solar radiation reaching the surface (with wavelengths ranging from 0.3 to 4 micrometers) is partially reflected, depending on the specific surface's albedo, while the remainder is absorbed. The surface itself emits longwave radiation in accordance with the Stefan-Boltzmann law—increasing with the fourth power of its temperature—while the atmosphere returns longwave counter-radiation to that same surface; the difference between total incoming and outgoing radiation represents the net available energy, which is subsequently partitioned into sensible heat, latent heat, and heat flux into the ground, as previously described in the discussion of surface fluxes.

Surface albedo varies widely depending on the type of surface: fresh snow reflects between 80% and 90% of incident solar radiation; dense forests and dark bare soil reflect only 10% to 15%; light-colored soil reflects 25% to 35%; crops reflect 20% to 25%; and liquid water reflects a mere 6% to 10%.

It is precisely this vast variation in albedo across different surfaces that makes the radiation balance extremely sensitive to any change in land cover: deforestation in boreal regions—which replaces dark forest with the bright snow exposed beneath—can actually result in net regional cooling, even though the deforestation process itself releases carbon dioxide into the atmosphere. The diurnal cycle of this radiative balance is, moreover, the primary driver of the hydrological cycle occurring at the surface: the net balance shifts from negative at night—when the surface loses longwave radiation without any offsetting solar gain—to strongly positive at midday, reaching up to eight hundred watts per square meter on a tropical summer day. It is precisely this positive net balance that simultaneously drives surface heating, evapotranspiration, and atmospheric convection.

The expected behavior of this entire system involves a positive correlation between available net energy and evapotranspiration, as well as a partitioning between sensible and latent heat—determined, as previously described via the Bowen ratio, by the amount of water available at the surface at any given moment. Thus, from the shortwave and longwave radiation exchange that opens this document to the sensible-latent heat partitioning that concludes it—and encompassing the vast variability of albedo and its accompanying diurnal cycle—the surface radiative balance ensures that the energy available at the Earth's surface, at any given time and location, ultimately determines both the local climate and the water cycle itself.

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