Longwave Radiation

Longwave radiation is also known as infrared, thermal, or terrestrial radiation. It is electromagnetic energy in the spectral band from roughly 3 to 100 mm. Earth surface temperatures produce emissions in this range, with peak terrestrial radiation occurring at a wavelength of about 10 mm. Spectrally integrated longwave radiation can be estimated from the Stefan-Boltzmann equation, Ql = eoT4, where e is the thermal emissivity, o is the Stefan-Boltzmann constant, 5.67 X 10-8 W m-2 K-4, and T is the absolute temperature of the emitting surface. By definition, e is the ratio of emitted longwave radiation to that which would be emitted by a perfect blackbody (a perfect emitter or absorber); e = 1 for a perfect blackbody.

Snow emits as a near-perfect blackbody at infrared wavelengths, with an emissivity es of 0.98-0.99. To good approximation, then,

for surface temperature Ts. This is a loss of energy from the snow or ice surface. For saturated snow and ice, es " 1 (a perfect blackbody), so for a melting snow or ice surface, Ts = 273.15 K and QL- . 315 W m-2.

Incoming longwave radiation is more variable and is difficult to predict without knowledge of lower-troposphere water vapor, cloud, and temperature profiles. We are all empirically familiar with the experience of warm, cloudy nights versus cold, clear nights; we are feeling the impact of variations in QL*. The Stefan-Boltzmann equation still holds in the atmosphere, but the longwave flux to the surface comes from different heights (temperatures) in the atmosphere, and the air is made up of an ensemble of gases with different infrared emissivities. Water vapor and C02 are strong absorbers/ emitters and are the dominant gases that influence QL*, although other greenhouse gases and aerosols contribute. A spectrally and vertically integrated radiative transfer calculation is needed to rigorously predict the longwave radiation incident on the surface. In snow studies, QL* is usually parameterized at a site, assuming that the atmosphere emits longwave radiation with an effective emis-sivity, ea, such that where Ta is the near-surface air temperature. Various pa-rameterizations of ea have been proposed, typically as a function of atmospheric humidity and cloud cover.

In many terrestrial environments, vegetation creates another potential longwave heat source. Trees in particular can provide local sources of both thermal radiation and sensible heat; the tree wells that surround tree trunks attest to this. In coastal mountain settings such as western North America and Norway, such tree wells can be several meters deep. For valley glaciers, side walls that heat up in the summer sun can also provide a significant source of longwave radiation.

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