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The role of cloud structural parameters in modulating cloud radiation
王晓聪
中国科学院大气物理研究所LASG
The importance of accurate radiative effects of clouds in general circulation models (GCMs) can never be overemphasized. In this study, the dependence of cloud radiation on several cloud subgrid-scale structures that are unresolved by conventional climate models is explored, These subgrid-scale structural parameters to be investigated are decorrelation length Lcf for overlapping of cloud fraction, Lcw for overlapping cloud condensate, and shape parameter v for measuring cloud inhomogeneity. It is found the decorrelation length Lcw has a similar role to the inhomogeneity parameter v in modulating cloud radiative effects (CREs), with increasing (decreasing) Lcw generally leading to a weakening (enhancing) of CREs, which is equivalent to increasing (decreasing) v in individual layers. However, the uncertainty of CREs caused by changes in Lcw is relatively smaller than that induced by changes in v, with the former about twice smaller than the latter. For fractional clouds with multiple layers, it is found the cloud overlap parameter Lcf exerts impacts on radiative transfer process first and foremost, and the inhomogeneity parameter v is of secondary importance, followed by the alignment parameter Lcw, which renders moderate modulation that cannot be neglected. This study also highlights the importance of the vertically varying structure of v in cloud radiation. The simulated CREs and radiative heating fields can be significantly biased if these varying characteristics are missing. This calls for more attention to the vertical structure of v in parameterization development, which is however neglected or poorly represented in the literature.