Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer

Abstract

Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

“…The horizontal grid spacing of these LES cases is fixed at 50 m, while the vertical grid is stretched, with a minimum spacing of 5 m near the surface and the capping temperature inversion to better resolve small-scale turbulence there. Further details of the model setup for the DYCOMS-II case are provided in Ackerman et al (2009). The ATEX cases are updated model runs with increased spatial resolution that are similar to the cases discussed in Fridlind and Ackerman (2011).…”

Diagnosis of systematic differences between multiple parametrizations of warm rain microphysics using a kinematic framework

“…For example, the intercomparison based on DYCOMS-II demonstrated a substantial range in inter-model surface precipitation rates for the drizzling, stratocumulus-topped boundary layer case (see Ackerman et al, 2009). Ackerman et al (2009) argue that this range results from differences in model dynamics.…”

“…For example, the intercomparison based on DYCOMS-II demonstrated a substantial range in inter-model surface precipitation rates for the drizzling, stratocumulus-topped boundary layer case (see Ackerman et al, 2009). Ackerman et al (2009) argue that this range results from differences in model dynamics. Although evidence is presented for this claim, the fact that the microphysics and dynamics are coupled in these simulations means that it is difficult to say to what extent the spread can be attributed to the different numerical methods for dynamics alone or interactions between dynamics and microphysics.…”

Analytic upscaling of a local microphysics scheme. Part II: Simulations

“…This may explain why the SCM leaves excessive moisture in the sub-cloud layer as compared with SAM LES (see Figure 1(b)). Nevertheless, both SCM profiles of w 2 lie within the range of the LES models (grey shaded region in Figure 1(c)) that participated in the GCSS intercomparison (Ackerman et al, 2009). The vertical velocity variance w 2 indirectly influences rain, but because our simulations specify cloud droplet number it does not directly influence rain.…”

“…SAM has successfully performed LES of a variety of boundary layer cases, such as the DYCOMS-II RF01 marine stratocumulus case (Stevens et al, 2005), a trade-wind cumulus case (Siebesma et al, 2003), and a stable boundary layer case (Beare et al, 2006). Of special relevance is the fact that SAM produced comparable output to the other LES models in the recent DYCOMS-II RF02 intercomparison (Ackerman et al, 2009).…”