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Jordi Vilà-Guerau de Arellano
and
Joannes W. M. Cuijpers

Abstract

The combined effect of ultraviolet radiation and turbulent mixing on chemistry in a cloud-topped boundary layer is investigated. The authors study a flow driven by longwave radiative cooling at cloud top. They consider a chemical cycle that is composed of a first-order reaction whose photodissociation rate depends on the cloud properties and time and a second-order chemical reaction between an abundant entrained reactant and a species with an initial concentration in the boundary layer. This turbulent reacting flow is represented numerically by means of a large eddy simulation. The simulation does not take evaporative cooling and aqueous-phase chemistry into account; that is, the authors simulate a dry smoke cloud.

The vertical concentration profiles of the reactants not in excess clearly show the appearance of gradients due to the chemical sources and sinks in the cloud. Moreover, the vertical-flux profiles depart from a linear profile. Fluxes that, in the absence of chemistry, are directed upward could change direction due to the different chemical reaction rate constants inside and below the cloud and because of the dominant downward motions generated by radiative cooling. The flux-budget analysis shows the relevance of the chemical term for the nonabundant species inside of the cloud. The exchange flux between the free troposphere and the boundary layer also depends on the chemical transformation above and in the cloud. An expression for the exchange velocity of reactive species is proposed in terms of an in-cloud flux, the production–depletion chemical rates, and the concentration jump at the inversion height. The calculated exchange velocity values for the smoke and the reactants differ considerably.

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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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Harm J. J. Jonker
,
Peter G. Duynkerke
, and
Joannes W. M. Cuijpers

Abstract

This study has determined energy spectra of turbulent variables in large eddy simulations of the penetrating dry convective boundary layer (microscale convection). The simulated domain has a large aspect ratio, the horizontal size being roughly 16 times the boundary layer depth. It turns out that both the turbulent velocities and the potential temperature exhibit “classic” energy spectra, which means that the dominant contribution to the variance originates from a scale of the order of the boundary layer height.

Surprisingly, the authors find that energy spectra of passive scalars in the convective boundary layer can behave completely differently from the velocity and temperature spectra. Depending on the boundary conditions of the scalar, that is, the surface flux and the entrainment flux, the spectrum is either classical in the aforementioned sense or it is dominated by the smallest wavenumbers, implying that the fluctuations are dominated by the largest scales. Loosely speaking the results can be summarized as follows: if the scalar entrainment flux is a negative fraction (about −½) of the surface flux, the scalar fluctuations are dominated by relatively small scales (∼ boundary layer depth), whereas in most other cases the scalar fluctuations tend to be dominated by the largest scales resolved (∼ tenths of kilometers, i.e., mesoscales). The latter result is rather peculiar since neither the velocity components nor the temperature field contains these large-scale fluctuations.

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