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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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A. G. M. Driedonks and H. Tennekes

A general review is given of the characteristic problems encountered in the parameterization of the atmospheric boundary layer in large-scale models. Only those parameterizations that interact with the large-scale flow are considered. Two aspects are especially important and require further study: the interaction of the boundary layer with clouds and the use of integrated parameterizations instead of a multilevel treatment. It is also necessary to carry out further experiments in order to evaluate the effect of a particular parameterization on the results of large-scale simulations or forecasts.

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W. A. A. Monna and A. G. M. Driedonks

Abstract

The dynamic properties of three types of commercially available propeller vanes were investigated in a wind tunnel. The same was done for the cup anemometer and wind vane as used on the 200 m mast at Cabauw, the Netherlands, as well as for the trivane, used for three-dimensional turbulence measurements. For these instruments the distance constant, damping ratio and damped wavelength were determined as well as the propeller response to a constant air speed as a function of the yaw angle. The results show differences from the manufacturer's specifications. Compared with the other instruments, the trivane appeared to be about twice as fast in response to variations of the wind vector. Between the other instruments there were no large differences.

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F. T. M. Nieuwstadt and A. G. M. Driedonks

Abstract

A case study of nocturnal boundary-layer development is presented. The data include observations of turbulence and of profiles of wind and temperature. The measurements were done along a 200 m high meteorological mast.

The observations are interpreted in terms of the results of a one-dimensional boundary-layer model. The model is derived from the full set of equations governing the evolution of the nocturnal boundary layer by neglecting advection. The validity of this approximation is discussed.

From a comparison of observations and calculated results it follows that the influence of advection is important especially in the upper part of the boundary layer. Nevertheless, we find that important characteristics of the nocturnal boundary layer such as its height can be reasonably well simulated by a one-dimensional model.

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