Jordi Vilà-Guerau de Arellano

Diurnal and vertical variability of the sensible heat and carbon dioxide budgets in the atmospheric surface layer

The diurnal and vertical variability of heat and carbon dioxide (CO2) in the atmospheric surface layer are studied by analyzing measurements from a 213 m tower in Cabauw (Netherlands). Observations of thermodynamic variables and CO2 mixing ratio as well as vertical profiles of the turbulent fluxes are used to retrieve the contribution of the budget terms in the scalar conservation equation. On the basis of the daytime evolution of turbulent fluxes, we calculate the budget terms by assuming that turbulent fluxes follow a linear profile with height. This assumption is carefully tested and the deviation from linearity is quantified. The budget calculation allows us to assess the importance of advection of heat and CO2 during day hours for three selected days. It is found that, under nonadvective conditions, the diurnal variability of temperature and CO2 is well reproduced from the flux divergence measurements. Consequently, the vertical transport due to the turbulent flux plays a major role in the daytime evolution of both scalars and the advection is a relatively small contribution. During the analyzed days with a strong contribution of advection of either heat or carbon dioxide, the flux divergence is still an important contribution to the budget. For heat, the quantification of the advection contribution is in close agreement with results from a numerical model. For carbon dioxide, we qualitatively corroborate the results with a Lagrangian transport model. Our estimation of advection is compared with traditional estimations based on the Net Ecosystem-atmosphere Exchange (NEE).

Entrainment process of carbon dioxide in the atmospheric boundary layer

Received 2 March 2004; revised 7 June 2004; accepted 23 June 2004; published 23 September 2004. [1] Aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide (CO2) were taken above a grassland in a convective atmospheric boundary layer. The observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dioxide in the boundary layer. From the observations we were able to estimate the vertical profiles of the fluxes, the correlation coefficients, and the skewness of the virtual potential temperature, the specific humidity, and the carbon dioxide. These profiles indicate that important entrainment events occurred during the observed period. The data were also used to estimate the budgets for heat, moisture, and carbon dioxide. By studying this observational data we find that the entrainment of air parcels containing lower concentrations of water vapor and carbon dioxide significantly dries and dilutes the concentration of these two constituents in the boundary layer. This process is particularly important in the morning hours which are characterized by a rapidly growing boundary layer. The observations show that the CO2 concentration in the boundary layer is reduced much more effectively by the ventilation with entrained air than by CO2 uptake by the vegetation. We quantify this effect by calculating the ratio of the entrainment flux of CO2 to the surface flux of CO2(bc = � (wc)e/(wc)o). A value of bc equal to 2.9 is estimated at around 1300 UTC from the vertical profile of the carbon dioxide flux. We corroborate this observational evidence by reproducing the observed situation using a mixed layer model. The mixed layer model also yields the variation in time of bc. During the morning the ventilation process is more important than the CO2 uptake by the vegetation (bc > 1), whereas in the afternoon the assimilation by grass at the surface becomes the dominant process (bc < 1). This research points out the relevance of the entrainment process on the budget of carbon dioxide in the lower troposphere and the relevance of boundary layer dynamics in controlling the diurnal variation of carbon dioxide. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0368 Atmospheric Composition and Structure: Troposphere— constituent transport and chemistry; KEYWORDS: entrainment carbon dioxide, mixed layer model Citation: de Arellano, J. V.-G., B. Gioli, F. Miglietta, H. J. J. Jonker, H. K. Baltink, R. W. A. Hutjes, and A. A. M. Holtslag (2004), Entrainment process of carbon dioxide in the atmospheric boundary layer, J. Geophys. Res., 109, D18110,

The Contribution of Shear to the Evolution of a Convective Boundary Layer

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 220% 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 ( b), which includes the main buoyancy and shear contributions, is in good agreement with the LES results. Some aspects of the parameterization of b, for instance, the absence of entrainment flux and its behavior during the transition between convective to neutral conditions, are discussed.

Effects of turbulence and heterogeneous emissions on photochemically active species in the convective boundary layer

Photochemistry is studied in a convective atmospheric boundary layer. The essential reactions that account for the ozone formation and depletion are included in the chemical mechanism which, as a consequence, contains a wide range of timescales. The turbulent reacting flow is modeled with a large-eddy simulation (LES) code. The deviations from chemical equilibrium that are caused by turbulent motions are investigated in terms of the intensity of segregation. For the studied cases it is found that the volume-averaged concentrations calculated with the LES code agree well with the concentrations calculated with a box model. The reaction rate between RH (a generic hydrocarbon emitted at the surface) and OH is most strongly affected (3% slower than in the box model). However, if RH is emitted nonuniformly at the surface, or if the RH-OH reaction rate is increased, the volume-averaged RH destruction by OH may be slowed down by as much as 30% compared to a box model. Sensitivity studies showed that the intensity of segregation between RH and OH not only depends on the strength and spatial distribution of the RH emissions but also on the way NO is emitted in the model atmosphere. The results obtained indicate that the assumption that localized emissions of reactive hydrocarbons, for example, isoprene or terpenes, are instantaneously mixed may lead to an underestimation of their atmospheric lifetime.

Surface wind regionalization over complex terrain: Evaluation and analysis of a high-resolution WRF simulation

This study analyzes the daily-mean surface wind variability over an area characterized by complex topography through comparing observations and a 2-km-spatial-resolution simulation performed with the Weather Research and Forecasting (WRF) model for the period 1992–2005. The evaluation focuses on the performance of the simulation to reproduce the wind variability within subregions identified from observations over the 1999–2002 period in a previous study. By comparing with wind observations, the model results show the ability of the WRF dynamical downscaling over a region of complex terrain. The higher spatiotemporal resolution of the WRF simulation is used to evaluate the extent to which the length of the observational period and the limited spatial coverage of observations condition one’s understanding of the wind variability over the area. The subregions identified with the simulation during the 1992–2005 period are similar to those identified with observations (1999–2002). In addition, the reduced number of stations reasonably represents the spatial wind variability over the area. However, the analysis of the full spatial dimension simulated by the model suggests that observational coverage could be improved in some subregions. The approach adopted here can have a direct application to the design of observational networks.

Relative Humidity as an Indicator for Cloud Formation over Heterogeneous Land Surfaces

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 p...

Atmospheric surface layer similarity theory applied to chemically reactive species

The similarity theory proposed by Monin-Obukhov for the atmospheric surface layer is extended so that it can be used to study the flux-gradient relationships of chemically reactive species. A second-order model is developed in order to calculate the flux-gradient relationships and the (co-)variances of momentum, temperature, and concentration. The equations of the statistical quantities of the chemical species take chemical reactions into account. The closure constants are adjusted for a near-neutral surface layer using micrometeorological measurements. All model equations are made nondimensional and without chemistry can be solved as a function of a nondimensional parameter which accounts for the atmospheric stability z/L only. In the case of an irreversible second-order chemical reaction, three new dimensionless scaling parameters are introduced by the chemical processes: two Damkohler numbers (the ratio of the turbulent timescale to the chemical reaction timescale) and the ratio of the fluxes of the chemical species. Model calculations of the momentum and temperature (co-)variances are in good agreement with the observations over the whole range of the atmospheric stability. If the concentration of one chemical species (abundant) is much higher than the concentration of the other species (scarce), then one finds the largest deviations from the flux-gradient relationships and the other statistical quantities for the scarce chemical species with respect to the flux-gradient relationships and the statistical quantities of temperature. The magnitude of these deviations depends on the Damkohler numbers and on the direction of the transport of the two chemical species, i.e., both deposited/emitted or one deposited and the other emitted. The abundant chemical species behaves as a passive scalar, i.e., temperature or moisture. Therefore in the case of scarce chemical species and Damkohler numbers close to unity (moderate chemistry), one cannot use the same flux-gradient relationship for temperature and for chemically reactive species. A similar behavior is found if one takes into account a backward chemical reaction (photodissociation) to the second-order chemical reaction. However, the deviations of the flux-gradient relationships of the scarce chemical species from the flux-gradient relationship of temperature are less severe if the photodissociation rate is increased. In summary, the model contains all the most important equations for solving the behavior of chemical species over the entire stability range of the atmospheric surface layer. Therefore it can be used to calculate the fluxes and (co-)variances equations of chemical species in atmospheric chemistry models.

Fluxes and (co-)variances of reacting scalars in the convective boundary layer

The effects of chemistry on the transport and the mixing of reacting scalars in the convective atmospheric boundary layer (CBL) are investigated. To do this, we use large-eddy simulation (LES) to calculate explicitly the different terms of the flux and (co-)variance budget equations and to analyse in particular the role of the chemical term with respect to the thermodynamical terms. We examine a set of chemical cases that are representative of various turbulent reacting flows. The chemical scheme involves two reacting scalars undergoing a second-order reaction. In addition, we study a chemical cycle, based on a first and a second order reaction, to study the behaviour of chemical systems in equilibrium in turbulent flows. From the budget analysis, we found that the chemical terms become more relevant when the chemical time-scale is similar to the turbulent time-scale. In order to determine the importance of the chemical terms, we compared these terms to the dynamical terms of the budget equations. For the flux of reactants, the chemical term becomes the dominant sink in the bulk of the CBL. As a result, flux profiles of reacting scalars have non-linear shapes. For the covariance, which accounts for the segregation of species in the CBL, the chemical term can act as a sink or source term. Consequently, reacting scalar covariance profiles deviate considerably from the inert scalar profile. When the chemistry is in equilibrium, the chemical term becomes negligible and therefore the flux and (co-)variance profiles are similar to those of inert scalars. On the basis of the previous budget results, we develop a parameterisation that represents the segregation of reacting species in large-scale models under convective conditions. The parameterisation is applied to an atmospheric chemical mechanism that accounts for ozone formation and depletion in the CBL. We found a good agreement between the parameterisation and the LES results. ∗ Corresponding author address: J.-F. Vinuesa, Meteorology and Air Quality Group, W-UR, 6701 AP Wageningen, The Netherlands; e-mail: vinuesa@hp1.met.wau.nl

Dispersion of a Passive Tracer in Buoyancy- and Shear-Driven Boundary Layers

Abstract By means of finescale modeling [large-eddy simulation (LES)], the combined effect of thermal and mechanical forcing on the dispersion of a plume in a convective boundary layer is investigated. Dispersion of a passive tracer is studied in various atmospheric turbulent flows, from pure convective to almost neutral, classified according to the scaling parameters u∗/w∗ and −zi/L. The LES results for the flow statistics and dispersion characteristics are first validated for pure convective cases against the available results from laboratory and field experiments. Currently used parameterizations are evaluated with the model results. The effect of wind shear is studied by analyzing the dynamic variables, in particular the velocity variances, and their relation with the dispersion characteristics, specifically plume mean height, dispersion parameters, ground concentrations, and concentration fluctuations. The main effect of the wind shear results in a reduction of the vertical spread and an enhancement ...

Representing Sheared Convective Boundary Layer by Zeroth- and First-Order-Jump Mixed-Layer Models: Large-Eddy Simulation Verification

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.