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S. Galmarini
,
P. G. Duynkerke
, and
J. Vilà-Guerau de Arellano

Abstract

The nocturnal cycle of nitrogen oxides in the atmospheric boundary layer is studied by means of a one-dimensional model. The model solves the conservation equations of momentum, entropy, total water content, and of five chemical species. The chemical cycle relates to the nighttime conversion of NO, NO2, and O3 into HNO3 via NO3 and N2O5. For simplicity, only homogeneous chemical reactions are considered. The turbulent fluxes of momentum, temperature, and moisture and of the chemical species are determined by means of a second-order closure model. The fluxes of the chemically reactive species are determined by explicitly taking into account the chemical transformation during the transport process. The one-dimensional model simulates a stable boundary layer with typical rural concentrations of the above-mentioned species. To study the effect of heterogeneous mixing due to the strong gradients of temperature and concentrations, the authors compare the one-dimensional model results with the results obtained with a box model. The study demonstrates that the concentration of NO plays a considerable role in the formation of NO3, N2O5, and HNO3. The reduced activity of turbulent transport shows that the chemical activity in the boundary layer can be decoupled from that of the so-called reservoir layer. The stability conditions induce inhomogeneous distribution of the species in the vertical direction and the formation of large concentration gradients. In these conditions, the study of the process by means of a box model can lead to an inaccurate estimate of the concentrations of species like NO and NO3.

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Pedro A. Jimenez
,
Jordi Vila-Guerau de Arellano
,
Jorge Navarro
, and
J. Fidel Gonzalez-Rouco
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Alessandro Dosio
,
Jordi Vilá Guerau de Arellano
,
Albert A. M. Holtslag
, and
Peter J. H. Builtjes

Abstract

Eulerian and Lagrangian statistics in the atmospheric convective boundary layer (CBL) are studied by means of large eddy simulation (LES). Spectra analysis is performed in both the Eulerian and Lagrangian frameworks, autocorrelations are calculated, and the integral length and time scales are derived. Eulerian statistics are calculated by means of spatial and temporal analysis in order to derive characteristic length and time scales. Taylor’s hypothesis of frozen turbulence is investigated, and it is found to be satisfied in the simulated flow.

Lagrangian statistics are derived by tracking the trajectories of numerous particles released at different heights in the turbulent flow. The relationship between Lagrangian properties (autocorrelation functions) and dispersion characteristics (particles’ displacement) is studied through Taylor’s diffusion relationship, with special emphasis on the difference between horizontal and vertical motion. Results show that for the horizontal motion, Taylor’s relationship is satisfied. The vertical motion, however, is influenced by the inhomogeneity of the flow and limited by the ground and the capping inversion at the top of the CBL. The Lagrangian autocorrelation function, therefore, does not have an exponential shape, and consequently, the integral time scale is zero. If distinction is made between free and bounded motion, a better agreement between Taylor’s relationship and the particles’ vertical displacement is found.

Relationships between Eulerian and Lagrangian frameworks are analyzed by calculating the ratio β between Lagrangian and Eulerian time scales. Results show that the integral time scales are mainly constant with height for z/zi < 0.7. In the upper part of the CBL, the capping inversion transforms vertical motion into horizontal motion. As a result, the horizontal time scale increases with height, whereas the vertical one is reduced. Current parameterizations for the ratio between the Eulerian and Lagrangian time scales have been tested against the LES results showing satisfactory agreement at heights z/zi < 0.7.

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Harm J. J. Jonker
,
Jordi Vilà-Guerau de Arellano
, and
Peter G. Duynkerke

Abstract

In this paper variance spectra of chemically active species in a dry convective boundary layer are studied by means of large-eddy simulations (LESs). The aim is to quantify the impact of chemistry on the spatial fluctuations in the concentration fields. The computational domain has a large aspect ratio (width/height = 16) in order to encompass all relevant scales (mesoscale to microscale). Variance spectra are used to calculate a characteristic length scale of the species' concentration variability. By locating the peak in the spectrum, a “variance dominating length scale” is derived.

For a simple first-order reaction, this length scale demonstrates a clear dependence on the reaction rate: an increase in the reaction rate leads to a significant decrease of the length scale of the species.

For a chemical cycle composed of a second-order reaction and first-order backreaction, the length scales turn out to depend much less on the reaction rate. The value of the length scales of the species involved appears to lie well in the mesoscale range, rather than the microscale range, demonstrating that concentration fluctuations are driven predominantly by scales much larger than the depth of the boundary layer.

External perturbation of the chemical balance can have a direct impact on the variance spectra. For the case where a (hypothetical) passing cloud switches off the chemical backreaction for a while, a dramatic drop in the length scale of the nonabundant species is observed. Once the feedback has been restored, a rapid increase of the length scale is observed.

To better understand these results, a spectral model is developed that incorporates turbulent production and dissipation of variance, chemistry, and spectral transfer. The model gives valuable insight into the relative importance of these processes at each scale separately, and enables one to predict the value of the variance dominating length scale in the limiting cases of very slow and very fast chemistry.

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Alessandro Dosio
,
Jordi Vilà-Guerau de Arellano
,
Albert A. M. Holtslag
, and
Peter J. H. Builtjes

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 −z i /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 of the horizontal dispersion. This effect greatly influences the behavior of the ground concentrations because the tracer is transported by the wind for a longer time before reaching the ground. The vertical dispersion parameter is studied by discussing the two main components: meandering and relative diffusion. Results show that the increasing wind reduces the plume vertical motion. The influence of increasing wind shear on the concentration fluctuation intensity is also analyzed. The limited plume vertical looping in conditions of weak convection results in reduction of the concentration fluctuation intensity. Parameterizations for the dispersion parameters are derived as a function of the flow characteristics, namely, the shear–buoyancy ratio, velocity variances, and wind shear. The parameterizations are partially based on previous studies and are verified for the different buoyancy- and shear-driven flows, showing satisfactory agreement with the model results.

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Influence of Canopy Seasonal Changes on Turbulence Parameterization within the Roughness Sublayer over an Orchard Canopy

M. Shapkalijevski
,
A. F. Moene
,
H. G. Ouwersloot
,
E. G. Patton
, and
J. Vilà-Guerau de Arellano

Abstract

In this observational study, the role of tree phenology on the atmospheric turbulence parameterization over 10-m-tall and relatively sparse deciduous vegetation is quantified. Observations from the Canopy Horizontal Array Turbulence Study (CHATS) field experiment are analyzed to establish the dependence of the turbulent exchange of momentum, heat, and moisture, as well as kinetic energy on canopy phenological evolution through widely used parameterization models based on 1) dimensionless gradients or 2) turbulent kinetic energy (TKE) in the roughness sublayer. Observed vertical turbulent fluxes and gradients of mean wind, temperature, and humidity, as well as velocity variances, are used in combination with empirical dimensionless functions to calculate the turbulent exchange coefficient. The analysis shows that changes in canopy phenology influence the turbulent exchange of all quantities analyzed in this study. The turbulent exchange coefficients of those quantities are twice as large near the canopy top for a leafless canopy than for a full-leaf canopy under unstable and near-neutral conditions. This turbulent exchange coefficient difference is related to the differing penetration depths of the turbulent eddies organized at the canopy top, which increase for a canopy without leaves. The TKE and dissipation analysis under near-neutral atmospheric conditions additionally shows that TKE exchange increases for a leafless canopy because of reduced TKE dissipation efficiency relative to that when the canopy is in full-leaf stage. The study closes with discussion surrounding the implications of these findings for parameterizations used in large-scale models.

Open access
Pedro A. Jiménez
,
J. Fidel González-Rouco
,
Elena García-Bustamante
,
Jorge Navarro
,
Juan P. Montávez
,
Jordi Vilà-Guerau de Arellano
,
Jimy Dudhia
, and
Antonio Muñoz-Roldan

Abstract

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.

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G. J. Steeneveld
,
T. Mauritsen
,
E. I. F. de Bruijn
,
J. Vilà-Guerau de Arellano
,
G. Svensson
, and
A. A. M. Holtslag

Abstract

This study evaluates the ability of three limited-area models [the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), and the High-Resolution Limited-Area Model (HIRLAM)] to predict the diurnal cycle of the atmospheric boundary layer (ABL) during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) experimental campaign. Special attention is paid to the stable ABL. Limited-area model results for different ABL parameterizations and different radiation transfer parameterizations are compared with the in situ observations. Model forecasts were found to be sensitive to the choice of the ABL parameterization both during the day and at night. At night, forecasts are particularly sensitive to the radiation scheme. All three models underestimate the amplitude of the diurnal temperature cycle (DTR) and the near-surface wind speed. Furthermore, they overestimate the stable boundary layer height for windy conditions and underestimate the stratification of nighttime surface inversions. Favorable parameterizations for the stable boundary layer enable rapid surface cooling, and they have limited turbulent mixing. It was also found that a relatively large model domain is required to model the Great Plains low-level jet. A new scheme is implemented for the stable boundary layer in the Medium-Range Forecast Model (MRF). This scheme introduces a vegetation layer, a new formulation for the soil heat flux, and turbulent mixing based on the local scaling hypothesis. The new scheme improves the representation of surface temperature (especially for weak winds) and the stable boundary layer structure.

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X. Pedruzo-Bagazgoitia
,
H. G. Ouwersloot
,
M. Sikma
,
C. C. van Heerwaarden
,
C. M. J. Jacobs
, and
J. Vilà-Guerau de Arellano

Abstract

Guided by a holistic approach, the combined effects of direct and diffuse radiation on the atmospheric boundary layer dynamics over vegetated land are investigated on a daily scale. Three numerical experiments are designed that are aimed at disentangling the role of diffuse and direct radiation below shallow cumulus at the surface and on boundary layer dynamics. A large-eddy simulation (LES) model coupled to a land surface model is used, including a mechanistically immediate response of plants to radiation, temperature, and water vapor deficit changes. The partitioning in direct and diffuse radiation created by clouds and farther inside the canopy is explicitly accounted for. LES results are conditionally averaged as a function of the cloud optical depth. The findings show larger photosynthesis under thin clouds than under clear sky, due to an increase in diffuse radiation and a slight decrease in direct radiation. The reduced canopy resistance is the main driver for the enhanced carbon uptake by vegetation, while the carbon gradient and aerodynamic effects at the surface are secondary. Because of the coupling of CO2 and water vapor exchange through plant stomata, evapotranspiration is also enhanced under thin clouds, albeit to a lesser extent. This effect of diffuse radiation increases the water use efficiency and evaporative fraction under clouds. The dynamic perturbations of the surface fluxes by clouds do not affect general boundary layer or cloud characteristics because of the limited time and space where these perturbations occur. It is concluded that an accurate radiation partitioning calculation is necessary to obtain reliable estimations on local surface processes.

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J. C. H. van der Hage
,
W. Boot
,
H. van Dop
,
P. G. Duynkerke
, and
J. Vilà-Guerau De Arellano

Abstract

An instrument to measure ultraviolet actinic flux in and outside clouds was constructed and calibrated. Characteristics of the instrument axe: 1) it is equipped with gallium phosphide photodiodes, 2) its isotropic directional response is almost perfect due to the special design of the optical diffusers, and 3) it is a lightweight construction (150 g) allowing the use of a balloon or a kit as an observation platform.

Ground-level observations and vertical profiles, measured with this UV photometer in clear air and in stratocumulus during the Atlantic Stratocumulus Transition Experiment in the Azores, confirm that the Madronich radiation transfer model is valid for clouds.

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