Second‐moment budgets in cloud topped boundary layers: A large‐eddy simulation study

Abstract

A detailed analysis of second-order moment budgets for cloud topped boundary layers (CTBLs) is performed using high-resolution large-eddy simulation (LES). Two CTBLs are simulated-one with trade wind shallow cumuli, and the other with nocturnal marine stratocumuli. Approximations to the ensemble-mean budgets of the Reynolds-stress components, of the fluxes of two quasi-conservative scalars, and of the scalar variances and covariance are computed by averaging the LES data over horizontal planes and over several hundred time steps. Importantly, the subgrid scale contributions to the budget terms are accounted for. Analysis of the LES-based second-moment budgets reveals, among other things, a paramount importance of the pressure scrambling terms in the Reynolds-stress and scalar-flux budgets. The pressure-strain correlation tends to evenly redistribute kinetic energy between the components, leading to the growth of horizontal-velocity variances at the expense of the verticalvelocity variance which is produced by buoyancy over most of both CTBLs. The pressure gradient-scalar covariances are the major sink terms in the budgets of scalar fluxes. The third-order transport proves to be of secondary importance in the scalar-flux budgets. However, it plays a key role in maintaining budgets of TKE and of the scalar variances and covariance. Results from the second-moment budget analysis suggest that the accuracy of description of the CTBL structure within the second-order closure framework strongly depends on the fidelity of parameterizations of the pressure scrambling terms in the flux budgets and of the third-order transport terms in the variance budgets.

The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives

“…Cloudy boundary layers have been simulated using bulk cloud microphysics for cold-air outbreaks by Gryschka et al (2008) as well as for the analysis of second-order budgets in cloudtopped boundary layers for BOMEX and DYCOMS-II 10 (see Stevens et al, 2005) experiments by Heinze et al (2015). Recently, the embedded LCM has been employed for studying the effect of turbulence on the droplet dynamics and growth (Lee et al, 2014;Riechelmann et al, 2015), and for investigating the entrainment (of aerosols) at the edges of cumulus clouds (Hoffmann et al, 2015(Hoffmann et al, , 2014.…”

“…Over the last 15 years, PALM has been applied for the simulation of a variety of boundary layers, ranging from heterogeneously heated convective boundary layers (e.g., Raasch and Harbusch, 2001;Letzel and Raasch, 2003;Maronga and Raasch, 2013), urban canopy flows (e.g., Park et al, 2012;Kanda et al, 2013), and cloudy boundary layers (e.g., Riechelmann et al, 2012;Hoffmann et al, 2015;Heinze et al, 2015). Moreover, it has been used for studies of the oceanic mixed layer (OML, e.g., Noh et al, 2010Noh et al, , 2011 and recently for studying the feedback between atmosphere and ocean by Esau (2014).…”

Evaluation of large-eddy simulations forced with mesoscale model output for a multi-week period during a measurement campaign

“…Typically, higher-order moments from LES are deduced from a spatial (horizontal) average (e.g., Heinze et al, 2015) as opposed to lidar measurements, which define turbulence as departure from a temporal mean. To account for this difference, variances from LES are shown in two different ways in Fig.…”

Analysis of pressure‐strain and pressure gradient‐scalar covariances in cloud‐topped boundary layers: A large‐eddy simulation study

“…[], and Heinze et al . []. In the Reynolds‐stress budget, for example, the pressure terms act to redistribute the kinetic energy produced by shear and/or buoyancy between the velocity‐variance components, thus reducing the turbulence anisotropy generated by shear, buoyancy, and rotation.…”

“…Heinze et al . [, hereinafter H15] analyzed the second‐moment budgets in cloud‐topped boundary‐layer flows generated with LES and demonstrated that the pressure‐scrambling terms are of paramount importance in maintaining the Reynolds‐stress and the scalar‐flux budgets. A step forward is made in the present paper, where a detailed analysis of the pressure‐scrambling terms is performed.…”