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Ackerman, A.S., 2008: GCSS stratocumulus large-eddy simulation intercomparisons targeting climate model uncertainties. GEWEX News, 18, no. 2, 3-5.
The GEWEX Cloud System Study (GCSS) Boundary Layer Cloud Working Group (BLCWG) has conducted a number of workshops devoted to idealized case studies of low-lying clouds simulated with a range of models. While the importance of drizzle on the stratocumulus-topped boundary layer has been long acknowledged, only recently has the significance of cloud-water sedimentation been recognized in large-eddy simulations of stratocumulus (Ackerman et al., 2004; Bretherton et al., 2007). The BLCWG workshop focused on the roles of cloud-water sedimentation and drizzle in a final ensemble of eleven LES models, two of which used bin microphysics while the rest used parameterized cloud microphysics. The simulation specifications were based on an idealization of the second research flight of DYCOMS-II, which sampled a bimodal cloud population with pockets of heavily drizzling open cells among a deck of closed-cell stratocumulus that was drizzling lightly (vanZanten and Stevens, 2005; Stevens et al., 2005b; Petters et al., 2006). Six-hour simulations were run with and without drizzle, each with and without cloud-water sedimentation.
The results reinforce recent findings regarding the importance of cloud-water sedimentation to boundary-layer entrainment (and thereby cloud liquid water path), in this case reducing the entrainment rate by approximately 25 percent or more (with or without drizzle included). It was also found that differences in model dynamics dominate the spread in entrainment rates, which is of the same order as the changes in entrainment rates induced by cloud-water sedimentation. Models that entrain rapidly with microphysics omitted tended to entrain rapidly with microphysics included.
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