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Jordi Vilà-Guerau de Arellano

The current understanding of the influence of atmospheric turbulence on chemical reactions is briefly reviewed. The fundamentals of this influence and the consequences for the transport and mixing of the reactants are discussed. A classification of the turbulent reacting flows is proposed in terms of the values of dimensionless numbers. These numbers depend on the characteristic timescale of the dynamics and the chemistry.

The main findings obtained by modeling studies of various atmospheric boundary layer flows and chemical mechanisms, in particular the ones done by means of the large eddy simulation technique, are summarized. Based on the conclusions of these studies, the need to carry out intensive and comprehensive atmospheric field campaigns and laboratory experiments to corroborate the numerical results is discussed. Specific open questions are posed to improve, by combining observational experiments and modeling, our knowledge of the role played by physical processes on the transformations of reactive species in the atmospheric boundary layer.

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Alessandro Dosio and Jordi Vilà-Guerau de Arellano

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The influence of the different scales of turbulent motion on plume dispersion in the atmospheric convective boundary layer (CBL) is studied by means of a large-eddy simulation (LES). In particular, the large-scale (meandering) and small-scale (relative diffusion) contributions are separated by analyzing dispersion in two reference systems: the absolute (fixed) coordinate system and the coordinate system relative to the plume’s instantaneous center of mass. In the relative coordinate system, the (vertically) inhomogeneous meandering motion is removed, and only the small-scale, homogeneous turbulent motion contributes to the dispersion process.

First, mean plume position, dispersion parameters (variance), and skewness of the plume position are discussed. The analysis of the third-order moments shows how the structure and the symmetry of scalar distribution are affected with respect to the turbulent characteristics of the CBL (inhomogeneity of the large-scale vertical motion) and the presence of the boundary conditions (surface and top of the CBL). In fact, the reflection of the plume by the CBL boundaries generates the presence of nonlinear cross-correlation terms in the balance equation for the third-order moments of the plume position. As a result, the third-order moment of the absolute position is not balanced by the sum of the third-order moments of the meandering and relative plume position.

Second, mean concentration and concentration fluctuations are calculated and discussed in both coordinate systems. The intensity of relative concentration fluctuation icr, which quantifies the internal (in plume) mixing, is explicitly calculated. Based on these results, a parameterization for the probability distribution function (PDF) of the relative concentration is proposed, showing very good agreement with the LES results. Finally, the validity of Gifford’s formula, which relates the absolute concentration’s high-order moments to the relative concentration and the PDF of the plume centerline, is studied. It is found that due to the presence of the CBL boundaries, Gifford’s formula is not able to reproduce correctly the value of the absolute mean concentration near the ground. This result is analyzed by showing that, when the plume is reflected by the CBL boundaries, the instantaneous relative plume width z2 r(t) departs from its mean value σ 2 r. By introducing the skewness of the relative plume position into a parameterization for the relative mean concentration, the results for the calculated mean concentration are improved.

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Miranda Braam, Jordi Vilà-Guerau de Arellano, and Monica Górska

Abstract

The multiple-single-column approach is proposed as a new concept to study the boundary layer parameterization scheme in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The results are compared with the Dutch Atmospheric Large-Eddy Simulation Model (DALES). Numerical experiments were performed over homogeneous and heterogeneous surfaces under clear convective boundary layer conditions. Identical simulations using MM5 and DALES were performed, which enabled an evaluation of the MM5 boundary layer scheme with DALES results. From the experiment with a homogeneous surface, MM5 shows a slightly shallower, colder, and moister boundary layer than DALES. This result is produced by an underestimation of turbulent mixing near the surface and less-vigorous entrainment of heat and dry air in MM5. In the heterogeneous surface experiment, the domain is divided into dry and wet patches, with the result that both models produce a mesoscale circulation. However, relative to the homogeneous case, larger differences were found between the models in the representation of the boundary layer dynamics. In DALES, the surface heterogeneity influenced the turbulent motions, making the mesoscale circulation much stronger (w max is 6 times as large) than in MM5. Because of this stronger circulation, the boundary layer height, bulk temperature, and humidity also displayed differences in time and spatial patterns. Because of the land–atmosphere coupling in MM5, the mesoscale circulation strengthened the surface flux heterogeneity. Cold and moist air advection close to the surface from the wet patch to the dry patch increased the sensible heat flux above the dry patch and thus the induced mesoscale flow.

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Gert-Jan Steeneveld and Jordi Vilà-Guerau de Arellano

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Numerical weather prediction models have become widespread tools that are accessible to a variety of communities, ranging from academia and the national meteorological services to commercial weather providers, wind and solar energy industries, and air quality modelers. Mesoscale meteorological models that are used to refine relatively coarse global weather forecasts to finer atmospheric scales have become mainstream. The wide use of mesoscale meteorological models also generates new requirements in undergraduate education concerning the knowledge and application of these models. In this paper, we present teaching strategies, course outcomes, student activities, impacts, and reflections on the possible future direction of the graduate-level atmospheric modeling course using the Weather Research and Forecasting (WRF) Model at Wageningen University, the Netherlands. This information is based on 15 years of experience in teaching the course and the continuous implementation of new educational techniques to adapt to students’ needs and improve their chances in their academic careers and the atmospheric sciences job market.

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

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

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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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M. Jeroen Molemaker and Jordi Vilà-Guerau de Arellano

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The influence of convective turbulence on chemical reactions in the atmospheric boundary layer is studied by means of direct numerical simulation (DNS). An archetype of turbulent reacting flows is used to study the reaction zones and to obtain a description of the turbulent control of chemical reactions. Several simulations are carried out and classified using a turbulent Damköhler number and a Kolmogorov Damköhler number. Using a classification based on these numbers, it is shown that it is possible to represent and to solve adequately all relevant scales of turbulence and chemistry by means of DNS. The simulations show clearly that the reaction zones are located near the boundaries where the species are introduced. At the lower boundary of the convective boundary layer, the reaction takes place predominantly in the core of the updrafts, whereas in the upper part of the domain the chemical reaction is greatest in the center of the downdrafts. In the bulk of the boundary layer the chemical reaction proceeds very slowly, due to the almost complete segregation of the chemical species. From the point of view of chemistry, the mixing across the interface between updrafts and downdrafts in the bulk of the convective boundary layer plays only a minor role.

The amount of chemical reaction in relation to the degree of turbulence is quantified by the introduction of an effective Damköhler number. This dimensionless number explicitly takes into account the reduction of the reaction rate due to the segregation of the chemical species. It is shown that the number approaches an asymptotic value that is O(1) for increasingly fast reaction rates. This shows explicitly that the timescale of the chemical reactions is limited by the integral turbulent timescale. It is suggested how a parameterization could be used to include this effect into one-dimensional atmospheric models.

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David Pino, Jordi Vilà-Guerau de Arellano, and Si-Wan Kim

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Dry convective boundary layers characterized by a significant wind shear on the surface and at the inversion are studied by means of the mixed-layer theory. Two different representations of the entrainment zone, each of which has a different closure of the entrainment heat flux, are considered. The simpler of the two is based on a sharp discontinuity at the inversion (zeroth-order jump), whereas the second one prescribes a finite depth of the inversion zone (first-order jump). Large-eddy simulation data are used to provide the initial conditions for the mixed-layer models, and to verify their results. Two different atmospheric boundary layers with different stratification in the free atmosphere are analyzed. It is shown that, despite the simplicity of the zeroth-order-jump model, it provides similar results to the first-order-jump model and can reproduce the evolution of the mixed-layer variables obtained by the large-eddy simulations in sheared convective boundary layers. The mixed-layer model with both closures compares better with the large-eddy simulation results in the atmospheric boundary layer characterized by a moderate wind shear and a weak temperature inversion. These results can be used to represent the flux of momentum, heat, and other scalars at the entrainment zone in general circulation or chemistry transport models.

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Chiel C. van Heerwaarden and Jordi Vilà Guerau de Arellano

Abstract

The influence of land surface heterogeneity on potential cloud formation is investigated using relative humidity as an indicator. This is done by performing numerical experiments using a large-eddy simulation model (LES). The land surface in the model was divided into two patches that had the same sum of latent and sensible heat flux but different Bowen ratios to simulate heterogeneous land surfaces. For heterogeneity in the meso-γ scale (2–20 km), sensitivity analyses were carried out on the heterogeneity amplitude (Bowen ratio difference between contrasting areas) and the inversion strength of potential temperature and specific humidity. The competition between absolute temperature decrease by ABL growth and dry air entrainment in heterogeneous conditions is analyzed using the LES results. First, it is shown that entrainment is located and enhanced over patches with higher Bowen ratios (warm patches) than their surroundings (cold patches). The heterogeneity-induced strong thermals can further penetrate the inversion at the ABL top, thereby reaching lower absolute temperatures than in homogeneous conditions. Second, because of the heterogeneity-induced circulations the moisture is located over the warm patch, and higher time-averaged RH values at the ABL top (RHzi) than over the cold patches are found here, even for dry atmospheres. These RHzi exceed values found over homogeneous land surfaces and are an indication that surface heterogeneity may facilitate cloud formation. In vertical profiles of RH, few differences are found between the homogeneous and heterogeneous cases, but the essential heterogeneity-induced modifications are within the domain variability.

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Pedro A. Jimenez, Jordi Vila-Guerau de Arellano, Jorge Navarro, and J. Fidel Gonzalez-Rouco
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