Efficient model for evaluation of spectral and vertical distributions of atmospheric longwave radiation


This work presents an analytical method designed to quantitatively evaluate the spectral and vertical distribution of atmospheric longwave radiation (0 ∼ 2500 cm−1) under clear-sky (cloud-free) conditions. A multilayer plane parallel radiative model with spectral resolution of 0.01 cm−1 is used to model the longwave radiation process in the Earth’s atmosphere. An irradiation-radiosity balance for each layer is used to solve for the fluxes for all layers. Broadband contributions of CO2 to surface downwelling flux and top of atmosphere upwelling flux for different values of water vapor content are found to range from 0.3 to 1.2 W m−2 and -0.7 to -0.5 W m−2 per 100 ppm increment, respectively. A plating algorithm is adapted for recursive and expedite calculation of modified transfer factors. This modified transfer factors include aerosol reflectance and take into consideration the vertical distribution of spectral thermal fluxes for each layer, including the ground. With the use of modified transfer factors, we found that for an atmosphere with surface relative humidity of 65% and aerosol optical depth at 479.5 nm equal to 0.1, 64.4% of the surface downwelling longwave irradiation is generated from the nearest atmospheric layer, 15.3% from the second nearest layer, 7.5% from the third nearest layer and the remainder 12.8% from other upper layers. From the first atmosphere layer to the tropopause layer, the largest irradiance contribution to each layer is from the layer itself. Above the tropopause, the largest contributor is the ground layer. Layers above the tropopause contribute less than 4.8% to the longwave radiation received by other layers.

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