Second-moment budgets in cloud topped boundary layers

A large-eddy simulation study

authored by

Rieke Heinze, Dmitrii Mironov, Siegfried Raasch

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 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. Key Points: Pressure-scrambling terms are crucial in Reynolds-stress and scalar-flux budgets Third-order transport plays key role in TKE and scalar (co)variance budgets TKE dissipation rate is underestimated with a diffusive advection scheme in LES

Organisation(s)

Institute of Meteorology and Climatology

External Organisation(s)

Deutscher Wetterdienst (DWD)

Type

Article

Journal

Journal of Advances in Modeling Earth Systems

Volume

7

Pages

510-536

No. of pages

27

ISSN

1942-2466

Publication date

01.06.2015

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Global and Planetary Change, Environmental Chemistry, General Earth and Planetary Sciences

Sustainable Development Goals

SDG 14 - Life Below Water

Electronic version(s)

https://doi.org/10.1002/2014MS000376 (Access: Open)