Rieke Heinze

Large eddy simulation using the general circulation model ICON

ICON (ICOsahedral Nonhydrostatic) is a unified modeling system for global numerical weather prediction (NWP) and climate studies. Validation of its dynamical core against a test suite for numerical weather forecasting has been recently published by Zangl et al. (2014). In the present work, an extension of ICON is presented that enables it to perform as a large eddy simulation (LES) model. The details of the implementation of the LES turbulence scheme in ICON are explained and test cases are performed to validate it against two standard LES models. Despite the limitations that ICON inherits from being a unified modeling system, it performs well in capturing the mean flow characteristics and the turbulent statistics of two simulated flow configurations—one being a dry convective boundary layer and the other a cumulus-topped planetary boundary layer.

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

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 vertical-velocity 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.

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

A detailed analysis of the pressure-scrambling terms (i.e., the pressure—strain and pressure gradient—scalar covariances) in the Reynolds-stress and scalar-flux 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. The pressure-scrambling terms are decomposed into contributions due to turbulence-turbulence interactions, mean velocity shear, buoyancy and Coriolis effects. Commonly used models of these contributions, including a simple linear model most often used in geophysical applications and a more sophisticated two-component-limit (TCL) non-linear model, are tested against the LES data. The decomposition of the pressure-scrambling terms shows that the turbulence-turbulence and buoyancy contributions are most significant for cloud-topped boundary layers. The Coriolis contribution is negligible. The shear contribution is generally of minor importance inside the cloudy layers, but it is the leading-order contribution near the surface. A comparison of models of the pressure-scrambling terms with the LES data suggests that the more complex TCL model is superior to the simple linear model only for a few contributions. The linear model is able to reproduce the principal features of the pressure-scrambling terms reasonably well. It can be applied in the second-order turbulence modeling of cloud-topped boundary layer flows, provided some uncertainties are tolerated. This article is protected by copyright. All rights reserved.