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Peter G. Duynkerke

Abstract

The stability of a uniformly saturated cloud layer separated from an overlying nonturbulent unsaturated layer by a thin inversion is considered. The stability of the interface can be described by entraining a parcel of air from above the inversion into the cloud layer below and by subsequently studying the effect of the mixing on the buoyancy of the parcel. From the relevant momentum equation for the parcel it is shown that the important quantity to consider is the total buoyancy (total mass of the parcel times the virtual potential temperature difference between parcel and environment) of the parcel per unit mass of entrained air. The total buoyancy is a more general and useful concept than all other parameters discussed in the literature.

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Peter G. Duynkerke

Abstract

A one-dimensional model designed to study the formation, growth, and dissipation of radiation fog is described. The model is compared with detailed observations made at the 200-m tower at Cabauw in the Netherlands. This study we use observations made in a shallow radiation fog that formed on the night of 16/17 August 1988. The model seems to be able to describe the most important mechanisms occurring during the fog evolution. In this study special attention is given to the parameterization of the vegetation, which is important for a good representation of the (minimum) air temperature. The influence of turbulence transport, longwave radiative cooling, and gravitational droplet settling on the fog evolution is described.

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Peter G. Duynkerke

Abstract

Observations are presented that were made in the lower 2 m of the atmosphere and in the soil near the Cabauw mast in the Netherlands. The surroundings of the mast are horizontally homogeneous and the soil is covered with short grass. In the air, the wind, temperature, specific humidity, and different radiation fluxes were measured, whereas in the soil, observations of the temperature and soil heal flux were made.

From the observations, the ratio of the roughness length for heat and momentum has been calculated and is shown to depend strongly upon the friction velocity u *. From a theoretical analysis of the heat transfer from the vegetation towards the air, it is shown that the ratio of the roughness length for heat and momentum is a function of the friction velocity, the leaf-area index (LAI), and the Prandtl number. It is shown that the observational results compare reasonably well with the theoretical prediction. In addition to the ratio of the roughness length for heat and momentum, the roughness length for moisture, albedo, and canopy resistance has also been calculated from the observations.

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Peter G. Duynkerke and João Teixeira

Abstract

The European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA) results for July 1987 are compared with stratocumulus observations for the same period made during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE). The FIRE I experiment, conducted off the coast of California, provided a comprehensive observational set of data on marine stratocumulus during July 1987. The observations reported in this paper and used as a basis for comparison with the ERA results include the mean diurnal variation of cloud cover, liquid water path, cloud-base and cloud-top height, incoming shortwave and longwave radiation at the surface, and monthly mean vertical profiles of (potential) temperature and specific humidity.

It is concluded that the ERA thermal structure in the boundary layer is quite realistic and the ERA data reveal some diurnal variability. However, in ERA the cloud thickness is overestimated and cloud cover and liquid water path are strongly underestimated as compared with the FIRE I observations. As a result the ERA downwelling shortwave radiation at the surface is much larger than observed in FIRE I. It is argued that the large difference between the ERA downwelling shortwave radiation at the surface and that observed in FIRE I can lead to large biases in the sea surface temperature in a coupled ocean–atmosphere model.

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Peter G. Duynkerke and Phillip Hignett

Abstract

A model simulation is presented of the diurnal cycle of a marine stratocumulus-capped boundary layer. The model results are compared with observations obtained during the 1987 First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment marine stratocumulus project, made from San Nicolas Island, off the coast of California. Both the simulation and the observations show a marked diurnal variation in cloud properties, as a result of the decoupling of the boundary layer into a separate cloud and a subcloud layer. In the model simulation the decoupling is caused by the absorption of the solar radiation in the cloud layer.

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Stephan R. de Roode and Peter G. Duynkerke

Abstract

Aircraft measurements made during the “First Lagrangian” of the Atlantic Stratocumulus Transition Experiment (ASTEX) between 12 and 14 June 1992 are presented. During this Lagrangian experiment an air mass was followed that was advected southward by the mean wind. Five aircraft flights were undertaken to observe the transition of a stratocumulus cloud deck to thin and broken stratocumulus clouds penetrated by cumulus from below. From the horizontal aircraft legs the boundary layer mean structure, microphysics, turbulence structure, and entrainment were analyzed. The vertical profiles of the vertical velocity skewness are shown to illustrate the transition of a cloudy boundary layer predominantly driven by longwave radiative cooling at the cloud top to one driven mainly by convection due to an unstable surface stratification and cumulus clouds. During the last flight before the stratocumulus deck was observed to be broken and replaced by cumuli, the total water flux, the virtual potential temperature flux, and the vertical velocity variance in the stratocumulus cloud layer were found significantly larger compared with the previous flights. To analyze the cloud-top stability the mean jumps of conserved variables across the inversion were determined from porpoising runs through the cloud top. These jumps were compared with cloud-top entrainment instability criteria discussed in the literature. It is suggested that enhanced entrainment of dry air is a key mechanism in the stratocumulus–cumulus transition.

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David Pino, Jordi Vilà-Guerau de Arellano, and Peter G. Duynkerke

Abstract

The role of shear in the development and maintenance of a convective boundary layer is studied by means of observations and large eddy simulations (LESs). Particular emphasis is given to the growth of the boundary layer and to the way in which this growth is affected by surface fluxes of heat and moisture and entrainment fluxes. This paper analyzes the processes that drive the latter mechanism, which accounts for approximately 30% of the growth of the mixing layer. Typically, it is found that under pure convective conditions, without shear, the entrainment buoyancy flux at the inversion is about −20% of the surface buoyancy flux. This value is widely used for entrainment rate closures in general circulation models.

The data collected during the Atmospheric Radiation Measurement campaign allow one to introduce realistic vertical profiles and surface fluxes into the LES runs and to compare the simulation results with the observed evolution of the boundary layer height during a convective situation with high entrainment rates and high geostrophic winds. The analysis of the turbulent kinetic energy (TKE) budget shows that the inclusion of geostrophic winds, which produce shear at the surface and in the entrainment zone, modifies the vertical profile of the various terms in the TKE budget. As a consequence, the entrainment flux is enhanced, resulting in increased growth of the boundary layer. The numerical experiments and the observations enable one to validate the efficiency of earlier representations, based on the TKE equation, which describe the evolution of the ratio between entrainment and surface buoyancy fluxes. The proposed parameterization for the entrainment and surface buoyancy flux ratio (β), which includes the main buoyancy and shear contributions, is in good agreement with the LES results. Some aspects of the parameterization of β, for instance, the absence of entrainment flux and its behavior during the transition between convective to neutral conditions, are discussed.

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Peter G. Duynkerke, He Qing Zhang, and Piet J. Jonker

Abstract

Measurements of the microphysical and turbulence structure of stratocumulus made during the Atlantic Stratocumulus Transition Experiment are presented. The measurements made from a C-130 aircraft, belonging to the Meteorological Research Flight, on the night of 12–13 June 1992 show that the convection in the boundary layer is driven both by longwave radiative cooling at cloud top and by the surface buoyancy flux. The turbulence kinetic energy budget, velocity and temperature variance, and vertical fluxes are calculated to discover how the turbulence structure varies with height. The vertical velocity variance profile is found to resemble that of a clear convective boundary layer. The entrainment velocity and entrainment fluxes are estimated. The results show that the entrainment is very efficient in the case studied. As a result, the buoyancy production of turbulent kinetic energy in the cloud layer is considerably reduced. Horizontally averaged droplet spectra are calculated to study the relative contribution of small and large droplets to the droplet concentration, liquid water content, and drizzle rate. The observations show that the water vapor flux, liquid water flux, and drizzle rate are all of the same magnitude and, therefore, are important in the moisture budget.

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Margreet C. vanZanten, Peter G. Duynkerke, and Joannes W. M. Cuijpers

Abstract

Various runs were performed with a large eddy simulation (LES) model to evaluate different types of entrainment parametrizations. For this evaluation, three types of boundary layers were simulated: a clear convective boundary layer (CBL), a boundary layer containing a smoke concentration, and a cloud-topped boundary layer. It is shown that the assumption that the entrainment flux equals the product of the entrainment rate and the jump over a discontinuous inversion is not valid in CBLs simulated by an LES model. A finite inversion thickness (i.e., a first-order jump model) is needed to define an entrainment flux for which this approximation of the flux is valid. This entrainment flux includes not only the buoyancy flux at the inversion, but also the surface heat flux. The parameterization of the buoyancy flux at the inversion is evaluated for different closures, as suggested in the literature (i.e., Eulerian partitioning, process partitioning, and a closure developed by Deardorff), and where needed is extended for use in a first-order jump model. The closure based on process partitioning is found to yield consistent results in all types of convective boundary layers and shows the best agreement with the limit found in LES results if the longwave radiative flux divergence takes place in a much shallower layer than the mixed layer.

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Geert Lenderink, Margreet C. vanZanten, and Peter G. Duynkerke

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The simulation of entrainment by an (E–l) turbulent kinetic energy–diagnostic length scale closure model is investigated in a smoke cloud convective boundary layer. The behavior of the E–l scheme is compared with two simple entrainment closures based on the integral turbulent kinetic energy budget of the boundary layer: one based on Eulerian partitioning and the other based on process partitioning of the buoyancy flux. It is demonstrated that the E–l turbulence scheme strongly supports the entrainment closure based on Eulerian partitioning. However, results of large eddy simulation (LES) models point in the other direction and support process partitioning. This suggests that the representation of entrainment physics in the E–l model is essentially different from that in LES models.

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