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P. G. Duynkerke

Abstract

In the E – ε turbulence model an eddy-exchange coefficient is evaluated from the turbulent kinetic energy E and viscous dissipation ε. In this study we will apply the E – ε model to the stable and neutral atmospheric boundary layer. A discussion is given on the equation for ε, which terms should be included and how we have evaluated the constants. Constant cooling rate results for the stable atmospheric boundary layer are compared with a second-order closure study. For the neutral atmospheric boundary layer a comparison is made with observations, large-eddy simulations and a second-order closure study. It is shown that a small stability effect can change the neutral atmospheric boundary layer quite drastically, and therefore, it will be difficult to observe a neutral boundary layer in the atmosphere.

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P. G. Duynkerke
and
A. G. M. Driedonks

Abstract

An observational study of the cloud-topped atmospheric boundary layer (ABL) during a strong gale reveals that the turbulent boundary layer was dominated by shear instead of convection. A one-dimensional ensemble-averaged model is used to study this type of cloud-topped ABL. Turbulence closure is formulated by using an equation for both the turbulent kinetic energy and the viscous dissipation. The radiation model consists of an emissivity model for the longwave radiation and a two-stream model for the shortwave radiation. Both model results and observations indicate that the longwave radiative cooling at cloud top is mainly balanced by entrainment of warm air from above the inversion. A parameterization for the rainfall is included and the effect of this on the liquid water content is studied.

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P. G. Duynkerke
and
A. G. M. Driedonks

Abstract

A multilevel ensemble-averaged model has been developed to study the cloud-topped atmospheric boundary layer (ABL). Turbulence closure is formulated by using an equation for the turbulent kinetic energy and either a diagnostic formulation of the integral length scale or a parameterized version of the dissipation equation. The latter two options are compared. The model is used to study various combinations of physical processes in a cloud-topped ABL and their combined effect on the turbulent structure. The physical processes considered are an upward buoyancy flux at the surface, longwave radiative cooling near cloud top, shortwave radiative heating in the cloud, and wind shear near cloud top. We discuss a case with only a surface buoyancy flux (no radiation) and a case with only longwave radiative fluxes (no surface fluxes). The usual concept that the latter is the upsidedown version of the former is not confirmed by the model results. Furthermore, we apply the model to the datasets of Brost et al. and Nicholls. Tile pronounced differences in the observed turbulent structure of the ABL in these two cases (due to different combinations of physical processes) are well simulated by the model.

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J. W. M. Cuijpers
and
P. G. Duynkerke

Abstract

A large eddy simulation (LES) model, used for studying the dry convective boundary layer, has been extended with an equation for the total water specific humidity and a condensation scheme to simulate the partly cloudy convective boundary layer. A simulation has been made based on the observations gathered near Puerto Rico on 15 December 1972. Starting from a clear air situation, the model evolves to a situation with small cumulus clouds. Vertical profiles of variances and fluxes show satisfactory agreement with the experimental data. It will be shown that layer-averaged fluxes and variances within the cloud layer are related to the amount of cloud water.

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