Pierre Durand

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.

Experimental investigation of atmospheric boundary layer turbulence

Abstract This paper describes how to measure turbulence in the atmospheric boundary layer (ABL) in order to address certain problems in modern atmospheric physics. These problems mainly relate to the Earths energy budget (including the hydrological cycle) and biogeochemical cycles. Starting from the main characteristic numbers and the basic equations of atmospheric turbulent flow, we show what turbulence parameters are important to measure. Special attention is given to the various methods used to compute the turbulent fluxes. We analyse the range of scales which has to be measured to properly capture the eddies contributing to the turbulent transfers. This range of scales determines what sensors can be used in the atmospheric surface layer and in the ABL. We describe the most widely used instruments and their performances. The principal platforms used to deploy these instruments are examined. Aircraft are described in more details, because they allow a thorough exploration of the ABL. In the last section, some examples of ABL turbulence signals measured in various conditions are presented. These examples illustrate horizontally homogeneous turbulence as well as inhomogeneous signals for which standard analysis techniques cannot be used. We show how some recent techniques, like wavelet transforms, can help to investigate this kind of signal. At the end, we present what would be interesting to do in the near future for the study of ABL turbulence.

Aircraft to aircraft intercomparison during SEMAPHORE

During the Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, performed in the Azores region in 1993, two French research aircraft were simultaneously used for in situ measurements in the atmospheric boundary layer. We present the results obtained from one intercomparison flight between the two aircraft. The mean parameters generally agree well, although the temperature has to be slightly shifted in order to be in agreement for the two aircraft. A detailed comparison of the turbulence parameters revealed no bias. The agreement is good for variances and is satisfactory for fluxes and skewness. A thorough study of the errors involved in flux computation revealed that the greatest accuracy is obtained for latent heat flux. Errors in sensible heat flux are considerably greater, and the worst results are obtained for momentum flux. The latter parameter, however, is more accurate than expected from previous parameterizations.

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.

Turbulence intensities and bulk coefficients in the surface layer above the sea

The structure of the atmospheric surface layer above the sea is analysed from aircraft turbulence measurements. The data are issued from two experiments performed in 1990 above the Mediterranean sea: Crau and PYREX, and correspond to moderately unstable conditions and to wind velocities ranging from 6 to 20 m/s. Low-altitude straight and level runs were used to compute the variances of the wind components, as well as of the temperature and moisture. Their dependence on the stability index —z/L is analysed. The turbulent fluxes of momentum, sensible heat and latent heat, calculated by the eddy-correlation technique, are used to estimate the neutral bulk coefficients: drag coefficient, Stanton number and Dalton number. The neutral drag coefficient clearly exhibits a dependence on the windspeed, which could be well fitted by the Charnock relation, with a constant of 0.012.

Spatial variability in airborne surface flux measurements during HAPEX-Sahel

Abstract The spatial variability of sensible and latent heat flux measured by aircraft over a 90-km × 75-km area, near the Central Supersites of HAPEX-Sahel (Hydrologic and Atmospheric Pilot Experiment, Sahel) is discussed. The data from six flights are presented four of which were obtained during the rainy season; the others being obtained at the beginning of the dry season. A basic difference in the behaviour of the latent heat transfer is revealed when the measurements under dry and wet conditions are compared: the latent heat flux is far more heterogeneous under drying conditions so that the estimation accuracy is reduced if the same integration length is used. It was found that under dry conditions the contribution of low frequency eddies is more important than that of local turbulence: the surface moisture is probably less homogeneous than in the wet period, but it is mostly the interaction between the marine and continental air masses linked to the closeness of the inter-tropical convergence zone that seems to drive the transfers. Two-dimensional fields of fluxes are constructed to study their spatial variability according to the hydrological conditions. These fields are systematically compared with those of the mean parameters likely to drive the transfers. Some characteristics, common to several fields, are revealed that can be considered specific to the climate in this region, at this time of year: an east-west gradient of albedo, a south-north gradient of the sensible heat flux, surface temperature and air temperature and a north-south gradient of specific humidity. However, the aerodynamic formula that relates flux to mean parameters, fails at a 25-km × 25-km scale but gives good results at a larger scale (90 km × 75 km): at this scale, the Dalton number is around 2.5 × 10 −3 .

Use of the Inertial Dissipation Method for Calculating Turbulent Fluxes from Low-Level Airborne Measurements

Abstract Airborne measurements are currently used for computing turbulence fluxes of heat and momentum. The method generally used is the eddy correlation technique, which requires sophisticated equipments to calculate the absolute velocities of the air. We used the well-known inertial dissipation method to calculate the turbulent fluxes of heat and momentum from low-level airborne measurements This only requires knowledge of inertial subrange characteristics of velocity and scalars. The method was validated by comparing dissipation fluxes with those computed by the eddy correlation method. The agreement between the two is very good, particularly for heat fluxes. Last, it is shown how the turbulent kinetic energy dissipation rate can be easily calculated, using a single measurement (the attack angle by example), and therefore how turbulent fluxes can be simply calculated from low level airborne measurements.

CONDITIONAL SAMPLING AND SCALE ANALYSIS OF THE MARINE ATMOSPHERIC MIXED LAYER -- SOFIA EXPERIMENT

During the SOFIA experiment, performed in the Azores region in June1992, airborne missions were conducted in the atmospheric boundary layerwith two aircraft instrumented for turbulence measurements. We show howthe conditional sampling technique, applied to the velocity, temperatureand moisture fluctuations, is able to describe the various parcels whichconstitute the turbulent field. Each parcel, so identified, is characterized byits fractional area and by its contribution to the transfers of sensible heat andlatent heat. On the other hand, a scale analysis is conducted by filteringthe turbulent signals in five non-overlapping frequency bands, definedaccording to the characteristic turbulent scales. The contribution of eachband to the turbulent energy and to the transfers is thus presented. Theimportance of the lowest frequencies, which are generally removed fromthe signals by high-pass filtering before computing turbulent fluxes, isshown. In the final section, the conditional sampling technique is applied tothe signals filtered in the various bands. Despite a slight deformation of theeddies due to the filtering technique, the contribution of each parcel can beestimated at the various scales analysed.

Dynamic and thermodynamic structure of the lower troposphere above rain forest and wet savanna during the EXPRESSO campaign

This paper focuses on the energy budget of atmospheric surface and boundary layers over the wet savanna and the rain forest during the Experiment of Regional Sources and Sinks of Oxidants (EXPRESSO). It assesses dynamic features at the transition between savanna and forest at the beginning of the dry season. The experimental zone was still influenced by the monsoon flow with water vapor mixing ratio of about 16 g/kg. Thermodynamic characteristics of the savanna and forest ecosystems are similar. However, for an equivalent net radiation, evaporation is greater over the forest and leads to lower Bowen ratios (0.2 over the forest, 0.45 over the savanna). Low horizontal winds with no mean direction are observed, due to the proximity of the Intertropical Convergence Zone (ITCZ). However, in spite of low horizontal winds, the momentum flux is high, owing to high drag coefficients and roughness lengths. Some differences appear in the turbulent kinetic energy budget: dynamic and thermal production terms over the forest are nearly half those over the savanna. As a consequence, turbulent exchanges are more efficient over the savanna. Entrainment velocities are deduced from the latent heat flux values at the top of the atmospheric boundary layer (ABL) and from the jump in water vapor mixing ratio between the ABL and the free troposphere. Entrainment velocities are of the same order as subsidence velocities deduced from European Centre Medium-Range Weather Forecasts statistical analyses. High pollution levels are observed in the forest ABL (CO concentration is around 280 ppb). Pollutants, produced by savanna fires, are transported from the free troposphere into the forest ABL by entrainment. Furthermore, horizontal exchanges between the savanna and the forest, induced by large eddies, may occur, increasing the pollution of the forest ABL. Through this study, the ITCZ appears to be an exchange zone rather than a barrier to the transfer of pollutants.