Outgoing longwave radiation
Outgoing Longwave Radiation (OLR) is the energy radiating from the Earth as infrared radiation at low energy to Space.
OLR is electromagnetic radiation emitted from Earth and its atmosphere out to space in the form of thermal radiation. The flux of energy transported by outgoing longwave radiation is measured in W/m².
Over 99% of outgoing longwave radiation has wavelengths between 4 µm and 100 µm, in the thermal infrared part of the electromagnetic spectrum. Contributions with wavelengths larger than 40 µm are small, therefore often only wavelengths up to 50 µm are considered . In the wavelength range between 4 µm and 10 µm the spectrum of outgoing longwave radiation overlaps that of solar radiation, and for various applications different cut-off wavelengths between the two may be chosen.
Radiative cooling by outgoing longwave radiation is the primary way the Earth System loses energy. The balance between this loss and the energy gained by radiative heating from incoming solar shortwave radiation determines global heating or cooling of the Earth system (Energy budget of Earth’s climate). Local differences between radiative heating and cooling provide the energy that drives atmospheric dynamics.
Atmospheric energy radiation
OLR is a critical component of the Earth's energy budget, and represents the total radiation going to space emitted by the atmosphere. Earth's radiation balance is quite closely achieved since the OLR very nearly equals the Shortwave Absorbed Radiation received at high energy from the sun. Thus, the Earth's average temperature is very nearly stable. The OLR is affected by clouds and dust in the atmosphere, which tend to reduce it to below clear sky values.
Role in greenhouse effect
Greenhouse gases, such as methane (CH4), nitrous oxide (N2O), water vapor (H2O) and carbon dioxide (CO2), absorb certain wavelengths of OLR adding heat to the atmosphere. Some of this thermal radiation is directed back towards the Earth, increasing the average temperature of the Earth's surface. Therefore, an increase in the concentration of a greenhouse gas may contribute to global warming by increasing the amount of radiation that is absorbed and emitted by these atmospheric constituents. If the absorptivity of the gas is high and the gas is present in a high enough concentration, the absorption bandwidth becomes saturated. In this case, there is enough gas present to completely absorb the radiated energy in the absorption bandwidth before the upper atmosphere is reached, and adding a higher concentration of this gas will have no additional effect on the energy budget of the atmosphere.
The OLR is dependent on the temperature of the radiating body. It is affected by the Earth's skin temperature, skin surface emissivity, atmospheric temperature, water vapor profile, and cloud cover.
Longwave radiation typically refers to radiation in the spectral region from 3 to 100 microns. In the Earth’s climate system, longwave radiation involves processes of absorption, scattering, and emissions from atmospheric gases, aerosols, clouds and the surface. Measuring outgoing longwave radiation at the top of atmosphere and downwelling longwave radiation at the surface are important for understanding how much radiative energy is kept in our climate system, how much reaches and warms the surface, and how the energy in the atmosphere is distributed to affect developments of clouds.
Outgoing longwave radiation (OLR) has been monitored globally since 1975 by a number of successful and valuable satellite missions. These missions include broadband measurements from the Earth Radiation Balance (ERB) instrument on the Nimbus-6 and Nimbus-7 satellites; Earth Radiation Budget Experiment (ERBE) scanner and the ERBE non scanner on NOAA-9, NOAA-10 and NASA Earth Radiation Budget Satellite (ERBS); The Clouds and the Earth's Radiant Energy System (CERES) instrument aboard NASA's Aqua and Terra satellites; and Geostationary Earth Radiation Budget instrument (GERB) instrument on the Meteosat Second Generation (MSG) satellite.
Downwelling longwave radiation at the surface is mainly measured by Pyrgeometer. A most notable ground-based network for monitoring surface longwave radiation is Baseline Surface Radiation Network (BSRN), which provides crucial well-calibrated measurements for studying global dimming and brightening.
OLR calculation and simulation
Many applications call for calculation of longwave radiation quantities: the balance of global incoming shortwave to outgoing longwave radiative flux determines the Energy budget of Earth’s climate; local radiative cooling by outgoing longwave radiation (and heating by shortwave radiation) drive the temperature and dynamics of different parts of the atmosphere; from the radiance from a particular direction measured by an instrument, atmospheric properties (like temperature or humidity can be retrieved, etc. Calculations of these quantities solve the radiative transfer equations that describe radiation in the atmosphere. Usually the solution is done numerically by an Atmospheric radiative transfer code adapted to the specific problem.
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- NOAA Climate Diagnostics Center
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