Abstract
Abstract. The radiative impacts of horizontal heterogeneity of layer cloud condensate, and vertical overlap of both condensate and cloud fraction are examined with the aid of a new radiation package operating in the GEOS-5 Atmospheric General Circulation Model. The impacts are examined in terms of diagnostic top-of-the atmosphere shortwave (SW) and longwave (LW) cloud radiative effect (CRE) calculations for a range of assumptions and overlap parameter specifications. The investigation is conducted for two distinct cloud schemes, one that comes with the standard GEOS-5 distribution, and another used experimentally for its enhanced cloud microphysical capabilities. Both schemes are coupled to a cloud generator allowing arbitrary cloud overlap specification. Results show that cloud overlap radiative impacts are significantly stronger in the operational cloud scheme where a change of cloud fraction overlap from maximum-random to generalized results in global changes of SW and LW CRE of ~4 Wm−2, and zonal changes of up to ~10 Wm−2. This is an outcome of fewer occurrences (compared to the other scheme) of large layer cloud fractions and fewer multi-layer situations where large numbers of atmospheric layers are simultaneously cloudy, both conditions that make overlap details more important. The impact of the specifics of condensate distribution overlap on CRE is much weaker. Once generalized overlap is adopted, both cloud schemes are only modestly sensitive to the exact values of the overlap parameters. When one of the CRE components is overestimated and the other underestimated, both cannot be driven simoultaneously towards observed values by adjustments to cloud condensate heterogeneity and overlap specifications alone.
Highlights
With recent computationally efficient approaches to treat cloud-radiation interactions, there are fewer reasons to retain the simplistic cloud descriptions that have persisted in General Circulation Models (GCMs) for the last three decades
For the purposes of this study, where the goal is to examine the sensitivity of the cloud radiative effect to a range of decorrelation length specifications and the differences arising when the exact same overlap assumptions are applied to two different cloud schemes, the imperfect matching to observed overlap is acceptable
New capabilities in describing arbitrary cloud fraction and condensate overlaps within GCMs that resemble more faithfully the vertical cloud structures observed in nature, along with progress on how radiation schemes handle these more complex cloud fields, has been improving the current state of affairs
Summary
With recent computationally efficient approaches to treat cloud-radiation interactions, there are fewer reasons to retain the simplistic cloud descriptions that have persisted in General Circulation Models (GCMs) for the last three decades. What should be investigated is whether the effects of cloud complexity on the transfer of solar and thermal infrared radiation matter for the GCM’s climate. Such a study on the full impacts of interactions and feedbacks of the altered radiation fields with the multitude of the GCM’s dynamical and physical processes is left for the future. The availability of two cloud schemes in our GCM combined with our analysis approach provides the opportunity to investigate whether identical assumptions about cloud complexity imposed on different original cloud fields can yield notably distinct radiative impacts The availability of two cloud schemes in our GCM combined with our analysis approach provides the opportunity to investigate whether identical assumptions about cloud complexity imposed on different original cloud fields can yield notably distinct radiative impacts (Sect. 5) and the reasons behind the dissimilar behaviours (Sect. 6)
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