摘要(英) |
Greenhouse gases (GHGs) absorb the longwave radiation which is emitted by the earth’s surface and atmosphere, reduce the outgoing longwave radiation, and cause the greenhouse effect. Increasing the concentrations of GHGs ?CO?_2, ?CH?_4 and N_2 O has an impact on the Earth’s climate since 1750. Because these gas concentrations increase in the atmosphere and it makes the radiation unbalance at the top of atmosphere.
Two radiative transfer models, LBL and CLIRAD, are used in the study to calculate the radiation fluxes and cooling rate. Radiative forcing due to a doubling of the GHG concentrations in three atmospheric profiles (tropical, mid-latitude summer and sub-arctic winter) are investigated. In this study, we also have discussions on the difference in fluxes and cooling rate between 1750 and 2012.
Doubling the concentration of 2012 GHG, ?CO?_2 would decrease much longwave radiation more than the other two GHGs to leave the Earth, which means that ?CO?_2 has the strongest radiative forcing among the three GHGs. The weakest radiative forcing is ?CH?_4. Increases of these GHGs concentration increasing have positive radiative forcing at the tropopause. This means that increasing the concentration of GHGs will warm the troposphere and the surface.
Comparing fluxes between 1750 and 2012, the radiative forcing at the top of the atmosphere of the tropical, mid-latitude summer and sub-arctic winter is 2.89W m^(-2), 2.53W m^(-2) and 1.33W m^(-2), respectively. For the cooling rate, there is a little difference in the stratosphere, which is primarily by the ?CO?_2 concentration change.
Comparing the radiative transfer models LBL and CLIRAD, it is found that there are some difference in radiation fluxes at each spectral band. Because the GHGs are not considered at each band to calculate the fluxes. Only at the GHGs major and minor absorption band will consider the GHGs and calculate the fluxes. In cooling rate, two model doesn’t have obviously different on ?CO?_2, but ?CH?_4 and N_2 O has. The cause of this result is the use of different transmittance parameterizations; ?CO?_2 uses table lookup, ?CH?_4 and N_2 O use K-distribution method.
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