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Logarithmic radiative effect of water vapor and spectral kernels
Author(s) -
Bani Shahabadi Maziar,
Huang Yi
Publication year - 2014
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2014jd021623
Subject(s) - modtran , radiative transfer , water vapor , logarithm , scaling , computational physics , infrared window , materials science , physics , mathematics , optics , infrared , mathematical analysis , meteorology , geometry
Radiative kernels have become a useful tool in climate analysis. A set of spectral kernels is calculated using a moderate resolution atmospheric transmission code MODTRAN and implemented in diagnosing spectrally decomposed global outgoing longwave radiation (OLR) changes. It is found that the effect of water vapor on the OLR is in proportion to the logarithm of its concentration. Spectral analysis discloses that this logarithmic dependency mainly results from water vapor absorption bands (0–560 cm −1 and 1250–1850 cm −1 ), while in the window region (800–1250 cm −1 ), the effect scales more linearly to its concentration. The logarithmic and linear effects in the respective spectral regions are validated by the calculations of a benchmark line‐by‐line radiative transfer model LBLRTM. The analysis based on LBLRTM‐calculated second‐order kernels shows that the nonlinear (logarithmic) effect results from the damping of the OLR sensitivity to layer‐wise water vapor perturbation by both intra‐ and inter‐layer effects. Given that different scaling approaches suit different spectral regions, it is advisable to apply the kernels in a hybrid manner in diagnosing the water vapor radiative effect. Applying logarithmic scaling in the water vapor absorption bands where absorption is strong and linear scaling in the window region where absorption is weak can generally constrain the error to within 10% of the overall OLR change for up to eightfold water vapor perturbations.