Do Shallow Cumulus Clouds have the Potential to Trigger Secondary Circulations Via Shading?
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The effects on the convective boundary layer (CBL) of shading due to shallow cumulus clouds are investigated. The main question is to see whether clouds are able to produce secondary circulations by shading of the surface (dynamic heterogeneities) and how these dynamic heterogeneities interact with static heterogeneities in terms of the production of secondary circulations. Also the effects of cloud shadows on cloud-field characteristics are analyzed. The effects are studied using large-eddy simulations of a cloud-topped CBL with an idealized surface. Over a homogeneous surface, shadows trigger secondary circulations with different strengths depending on the solar zenith angle \(\vartheta \), with large \(\vartheta \) favouring the development of secondary circulations. Over a static heterogeneous surface with a simple striped pattern, the strength of secondary circulations is effectively reduced by dynamic heterogeneities at small \(\vartheta \). At large \(\vartheta \), however, the effect on secondary circulations depends on the orientation of the striped static heterogeneities to the shadow-casting direction of the clouds. The influence of shadows is only small if they are cast perpendicular to the striped heterogeneity, but if stripes and the shadow-casting direction are parallel, secondary circulations are reduced in strength also for large \(\vartheta \). Shadow effects on the cloud-field characteristics vary with \(\vartheta \) as well. The results show that small \(\vartheta \) favours the development of small clouds with a reduced lifetime while large \(\vartheta \) promotes the development of larger clouds with an extended lifetime compared to non-shading clouds.
KeywordsCloud shadows Cloud-topped boundary layer Large-eddy simulation Secondary circulation Surface heterogeneity
All simulations were performed on the SGI Altix ICE at the North-German Supercomputing Alliance (HLRN), Hanover/Berlin. NCL (The NCAR Command Language (Version 6.1.2) [Software]. (2013). Boulder, Colorado: UCAR/NCAR/CISL/VETS. http://dx.doi.org/10.5065/D6WD3XH5.) was used for data analysis and visualization. We thank the two anonymous reviewers for their constructive and valuable comments that helped to improve the manuscript.
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