The cumulus‐capped boundary layer. I: Modelling transports in the cloud layer

• Published in: Quarterly journal of the Royal Meteorological Society 2006-07, Vol.132 (618), p.1385-1403
• Main Author: Grant, A. L. M.
• Format: Article
• Language: English
• Subjects: Cloudy boundary layer Cumulus parametrization Large‐eddy simulation; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1256/qj.04.169

Scalar‐flux budgets have been obtained from large‐eddy simulations (LESs) of the cumulus‐capped boundary layer. Parametrizations of the terms in the budgets are discussed, and two parametrizations for the transport term in the cloud layer are proposed. It is shown that these lead to two models for scalar transports by shallow cumulus convection. One is equivalent to the subsidence detrainment form of convective tendencies obtained from mass‐flux parametrizations of cumulus convection. The second is a flux‐gradient relationship that is similar in form to the non‐local parametrizations of turbulent transports in the dry‐convective boundary layer. Using the fluxes of liquid‐water potential temperature and total water content from the LES, it is shown that both models are reasonable diagnostic relations between fluxes and the vertical gradients of the mean fields. The LESs used in this study are for steady‐state convection and it is possible to treat the fluxes of conserved thermodynamic variables as independent, and ignore the effects of condensation. It is argued that a parametrization of cumulus transports in a model of the cumulus‐capped boundary layer should also include an explicit representation of condensation. A simple parametrization of the liquid‐water flux in terms of conserved variables is also derived. © Crown Copyright, 2006