A. Druilhet

Ozone fluxes above and within a pine forest canopy in dry and wet conditions

Abstract The physiological and physical processes controlling ozone dry deposition to vegetated surfaces are still not fully understood. In particular, the role of the understorey and the possible action of dew on ozone deposition have not received much attention so far. This paper presents the results of an experiment aimed at quantifying ozone dry deposition to a maritime pine forest in the “Les Landes” area in France. Ozone deposition fluxes were measured using the eddy-covariance technique above and within the canopy. We investigate the factors acting on ozone deposition in both dew-wetted and dry conditions. The values obtained for the ozone deposition velocity are well in the range of previously published measurements over coniferous forests. For the present forest, ozone uptake by the understorey is a significant portion of ozone deposition to the whole pine stand. The understorey contributes more to the overall ozone flux than to the other measured scalar fluxes (sensible heat and water vapour). During dry nights the surface conductance for ozone and the friction velocity are strongly correlated, showing that ozone deposition is largely controlled by dynamical processes. During the day, in dry conditions, the canopy stomatal conductance is the major parameter controlling ozone deposition. However, in winter, when the stomatal conductance is low, the influence of dynamical processes persists during day-time. It is also found that surface wetness associated with dew significantly enhances ozone deposition, during the night as well as in the morning.

Experimental Studies of the Turbulence Structure Parameters of the Convective Boundary Layer

Abstract We present a collection of experimental data concerning turbulent characteristics of the convective boundary layer. These data were obtained by means of instrumented aircraft during numerous experiments carried out above five flight areas from 1973 to 1978. Only dry convective conditions with weak dynamic instability are contained in this report. The computed quantities are vertical turbulent fluxes of sensible and latent heat, second- and third-order moments of vertical wind component, potential temperature and humidity, dissipative and spectral characteristics and length scales. All these quantities are normalized using the convective similarity hypothesis of Deardorff. A particular case is considered for humidity where it is necessary to choose between the two boundary conditions: evaporation and entrainment flux. The latter is chosen.

Monitoring of the Atmospheric Stability above an Urban and Suburban Site Using Sodar and Radon Measurements

Abstract This study presents a method of obtaining the quantitative intensity of vertical diffusion during periods of atmospheric stability. This method associates the continuous measurement of radon concentration at ground level and the use of a monostatic sodar. The value of “equivalent mixing height” he is calculated using the radon variation which can be related to the global exchange coefficient of the inversion layer. The sodar detects the thickness of the nocturnal inversion layer. Both systems operated simultaneously over several months at two sites (urban and suburban). In most cases the nocturnal layer was indicated both by sodar echoes and by a large decrease of the he value. In other cases the use of sodar alone can lead to a lack of detection of stable periods which, nevertheless, are shown clearly by radon. The comparison of measurement at two sites shows a modification of nocturnal stability above the urban site. The equivalent exchange coefficients are ∼0.3 m2 s−1 (at the urban site) and 0...

A Sea-Land Transition Observed during the COAST Experiment

Abstract A sea wind situation was analyzed during the COAST (Cooperative Operations with Acoustic Sounding Techniques) experiment. The thermal internal boundary layer (TIBL) which develops inland from the coast was investigated by an instrumented aircraft and fixed measurements, and by a two dimensional version of a third order turbulence closure model. The two-dimensional structure of the TIBL was demonstrated in the vertical, perpendicular to the shore. The mean quantities (temperature, humidity, and wind), as well as their turbulent moments, were computed and comparison made between experiment and model. The experimental mean fields were well reproduced by the, model. The turbulence fields were reproduced in their general features as well as in their magnitude, but not in local singularities.

Structure of the marine atmospheric boundary layer over an oceanic thermal front: SEMAPHORE experiment

The Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, the third phase of which took place between October 4 and November 17, 1993, was conducted over the oceanic Azores Current located in the Azores basin and mainly marked at the surface by a thermal front due to the gradient of the sea surface temperature (SST) of about 1° to 2°C per 100 km. The evolution of the marine atmospheric boundary layer (MABL) over the SST front was studied with two aircraft and a ship in different meteorological conditions. For each case, the influence of the incoming air direction with respect to the orientation of the oceanic front was taken into account. During the campaign, advanced very high resolution radiometer pictures did not show any relation between the SST field and the cloud cover. The MABL was systematically thicker on the warm side than on the cold side. The mean MABL structure described from aircraft data collected in a vertical plane crossing the oceanic front was characterized by (1) an atmospheric horizontal gradient of 1° to 2°C per 100 km in the whole depth of the mixed layer and (2) an increase of the wind intensity from the cold to the warm side when the synoptic wind blew from the cold side. The surface sensible heat (latent heat) flux always increased from the cold to the warm sector owing to the increase of the wind and of the temperature (specific humidity) difference between the surface and the air. Turbulence increased from the cold to the warm side in conjunction with the MABL thickening, but the normalized profiles presented the same structure, regardless of the position over the SST front. In agreement with the Action de Recherche Programme te Petite Echelle and Grande Echelle model, the mean temperature and momentum budgets were highly influenced by the horizontal temperature gradient. In particular, the strong ageostrophic influence in the MABL above the SST front seems linked with the secondary circulation due to the SST front.

Vertical ozone profiles in a pine forest

Abstract Vertical profiles of ozone concentration have been measured in a pine forest located in the Landes region, in south-west France. In the daytime, ozone concentrations are nearly constant between ground level and an altitude of 35 m, the higher level of measurement. At night, important vertical variations of ozone concentration are observed in relation with temperature profiles. Analysis of the ozone profiles shows that the main ozone sink is the soil. When the ozone concentration near ground level is around zero, the ozone profile changes slowly, even in the main vegetation layer between 15 and 25 m height. This means that the chemical destruction of ozone at night is weak in comparison with its dry deposition at ground level.

UNE METHODE DE MESURE DE LA STABILITE VERTICALE DE L'ATMOSPHERE PRES DU SOL

For a ground-level continuous release of a trace substance like 222Rn, for which the vertical profile of concentrations is exponential, a quantity, ‘equivalent mixing height’ h, is defined in this paper in the following way. If the substance were to be redistributed uniformly through a layer such that the concentration were to be equal to that at the surface, then the top of the layer would be h.Relative values of h can be obtained from surface measurements of Radon concentrations, but absolute values require estimates of the vertical fluxes of Radon. A comparison is given of the estimates of h obtained from surface measurements with estimates derived from the vertical profile of Radon from the surface to 100 m.Practical methods are proposed for estimating the surface flux of Rn.

Comparison of sea surface flux measured by instrumented aircraft and ship during SOFIA and SEMAPHORE experiments

Two major campaigns (Surface of the Oceans, Fluxes and Interactions with the Atmosphere (SOFIA) and Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE)) devoted to the study of ocean-atmosphere interaction were conducted in 1992 and 1993, respectively, in the Azores region. Among the various platforms deployed, instrumented aircraft and ship allowed the measurement of the turbulent flux of sensible heat, latent heat, and momentum. From coordinated missions we can evaluate the sea surface fluxes from (1) bulk relations and mean measurements performed aboard the ship in the atmospheric surface layer and (2) turbulence measurements aboard aircraft, which allowed the flux profiles to be estimated through the whole atmospheric boundary layer and therefore to be extrapolated toward the sea surface level. Continuous ship fluxes were calculated with bulk coefficients deduced from inertial-dissipation measurements in the same experiments, whereas aircraft fluxes were calculated with eddy-correlation technique. We present a comparison between these two estimations. Although momentum flux agrees quite well, aircraft estimations of sensible and latent heat flux are lower than those of the ship. This result is surprising, since aircraft momentum flux estimates are often considered as much less accurate than scalar flux estimates. The various sources of errors on the aircraft and ship flux estimates are discussed. For sensible and latent heat flux, random errors on aircraft estimates, as well as variability of ship flux estimates, are lower than the discrepancy between the two platforms, whereas the momentum flux estimates cannot be considered as significantly different. Furthermore, the consequence of the high-pass filtering of the aircraft signals on the flux values is analyzed; it is weak at the lowest altitudes flown and cannot therefore explain the discrepancies between the two platforms but becomes considerable at upper levels in the boundary layer. From arguments linked to the imbalance of the surface energy budget, established during previous campaigns performed over land surfaces with aircraft, we conclude that aircraft heat fluxes are probably also underestimated over the sea.

Sources and sinks of ozone in savanna and forest areas during EXPRESSO: Airborne turbulent flux measurements

An airborne study of ozone concentrations and fluxes in the lower layers of the atmosphere was conducted over the Central African Republic (CAR) and northern Congo in November/December 1996, within the framework of the Experiment of Regional Sources and Sinks of Oxidants (EXPRESSO). The first 4 km of the atmosphere above savanna, rain forest, and the transitional area between them, were investigated with the French research aircraft Avion de Recherche Atmospherique et de Teledetection (ARAT). Turbulent fluxes and deposition velocities of ozone were determined using the Eddy Correlation (EC) method. A specific methodology was developed to obtain accurate airborne turbulent flux measurements. This methodology is linked to the turbulence stationarity. The average values of ozone fluxes and ozone deposition velocities in the Atmospheric Boundary Layer (ABL) increase appreciably from savanna to forest. Near the ground, the ozone fluxes range between −0.115 +/−0.073 ppbv m/s above savanna and −0.350 +/−0.115 ppbv m/s above forest; for the deposition, the ranges are 0.0042 +/−0.0018 m/s and 0.015 +/−0.004 m/s. A simple empirical relationship between deposition velocity and Leaf Area Index (LAI) is proposed, giving an estimation of the deposition velocity for a whole latitudinal band. Vertical inputs of ozone to the ABL are estimated according to entrainment fluxes. The role of advection is neglected for horizontal transport of ozone in the ABL. The photochemical ozone production is deduced from the photo-stationary state deviation, and compared to the net ozone increase in the ABL during the flights performed above the forest. A tentative ozone budget based on the aircraft measurements is proposed in the ABL of the rain forest. Around noon, the photochemical production dominates with a net production of about 10 ppbv/h.

Analysis of turbulent transfers in vegetation: Use of thoron for measuring the diffusivity profiles

Analysis of turbulent transfer inside a vegetative canopy can be realized by means of several methods. In our case, we have used principally the aerodynamic and the thoron methods. The thoron diffusivity profiles show an important distortion in the middle zone of the canopy, below the maximum of leaf area density. This distortion is associated with a maximum source of sensible heat flux at these levels, which partially invalidates the aerodynamic approach.