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Radiation: Clear Sky and Top of Atmosphere

A question arose in regard to the meaning of certain short wave radiation variables, in particular when they are described at the top of the atmosphere (TOA) or at the surface in clear sky conditions.

A good document describing the TOA and clear sky conditions in the ECMWF model can be found here.  The TOA in this model, corresponds to the 0 hPa pressure level. While NASA say that TOA can be described arbitrarily as 400,000 ft (about 120 km) above the surface of the Earth, or in meteorological terms as the location where pressure is 0.1 mb.

Clear sky is defined in the ECMWF model as as a hypothetical atmosphere where no clouds are present, however the same atmospheric conditions of temperature, humidity, ozone, trace gases and aerosol exist. So the short wave radiation at the Earth’s surface in clear sky conditions is that which would be present if all clouds were removed. In clear sky conditions, when the solar radiation passes through the Earth’s atmosphere, it is reduced only due to scattering and absorption. The radiation that is not scattered or absorbed will reach the Earth’s surface as a direct (beam) radiation, while the scatter radiation that reaches the ground is called diffuse radiation. The scattering and absorption of the solar radiation can be attributed to ozone, water vapor, aerosol and dry-air particles in the atmosphere.

Figure from description of the ECMWF model  below showing the short and long wave radiation entering the top of the atmosphere (TOA) and being scattered and absorbed before reaching the Earth’s surface.

 

Summer daily average accumulated short wave radiation at the TOA and at the Earth’s surface in clear sky conditions are shown below. Data from MERRA2.