Hélène Dupuis

Impact of flow distortion corrections on turbulent fluxes estimated by the inertial dissipation method during the FETCH experiment on R/V L'Atalante

[1]xa0The FETCH campaign was for a large part devoted to the measurement and analysis of turbulent fluxes in fetch-limited conditions. Turbulent measurements were performed on board the R/V LAtalante, on an ASIS spar buoy and on aircraft. On the R/V LAtalante, turbulent data were obtained from a sonic anemometer and from a microwave refractometer. The main focus of this paper is to present results of momentum and heat fluxes obtained from the R/V LAtalante, using the inertial-dissipation method and taking into account flow distortion effects. Numerical simulations of airflow distortion caused by the ship structure have been performed to correct the wind measurements on the R/V LAtalante during the FETCH experiment. These simulations include different configurations of inlet velocities and six relative wind directions. The impact of airflow distortion on turbulent flux parameterizations is presented in detail. The results show a very large dependence on azimuth angle. When the ship is heading into the wind (relative wind direction within ±38° of the bow), the airflow distortion leads to an overestimation of the drag coefficient, associated with a wind speed reduction at the sensor location. For relative wind directions of more than ±38° from the bow, flow distortion causes the wind to accelerate at the sensor location, which leads to an underestimate of the drag coefficient. The vertical displacement of the flow streamlines could not be fully established by numerical simulation, but the results are in qualitative agreement with those inferred from the data by prescribing the consistency of momentum flux as a function of azimuth angle. Both show that the vertical elevation of the flow can be considered as constant (1.21 m from numerical simulations) only within about ±20° from bow axis. Values of vertical displacements up to 5 m are found from the data for high wind speeds and beam-on flows. Our study also shows that the relative contributions of the streamline vertical displacement and the mean wind speed underestimate or overestimate vary significantly with relative wind direction. The relative contribution due to vertical streamline displacement is higher for heat flux than for momentum flux. The consistency of our correction for airflow distortion is assessed by the fact that the correction reduces the standard deviation of the drag coefficient: only if this correction is taken into account, do the curves of the drag coefficient versus wind speed become similar for data corresponding to wind in the bow direction and from the side. When the complete numerical airflow correction is applied to the data set limited to relative wind directions at ±30° from the bow axis, the drag coefficient formula is CD10N × 1000 = 0.56 + 0.063 U10N, for U10N > 6 m s−1. This formula provides CD10N values comparable to the ones found from the ASIS buoy data for wind speeds of about 13 m s−1. They are however smaller by 9% at higher winds (>15 m s−1). This formula is also similar, within a few percent, to the parameterizations of Smith [1980], Anderson [1993], and Yelland et al. [1998]. The exchange coefficient for evaporation is found to be 1.00 × 10−3 on average with a small standard deviation of 0.31 × 10−3. A slight increase of CE10N value with wind speed is, however, observed with a variation of about 20% (0.2 × 10−3) for wind speeds between 6 and 17 m s−1, following CE10N × 1000 = 0.82 + 0.02 U10n, for U10n > 6 m s−1.

Toward a Better Determination of Turbulent Air–Sea Fluxes from Several Experiments

An accurate determination of turbulent exchanges between the ocean and the atmosphere is a prerequisite to identify and assess the mechanisms of interaction that control part of the variability in the two media over a wide range of spatial and temporal scales. An extended dataset for estimating air‐sea fluxes (representing nearly 5700 h of turbulence measurements) has been collected since 1992 during six dedicated experiments performed in the Atlantic Ocean and the Mediterranean Sea. This paper presents the methodology used through the successive experiments to progress in this field. The major developments concern (i) flux instrumentation, with the deployment of a microwave refractometer to get the latent heat flux in most meteorological conditions; (ii) the analysis of airflow distortion effects around the ship structure and sensors through both computational fluid dynamics and physical simulations in a water tank, then the derivation of correction for these effects; (iii) the application of both inertial dissipation and eddy-correlation methods from the various experiments, allowing the authors to assess and discuss flux-determination methods on ships, and particularly bulk parameterization; (iv) the validation and analysis of mesoscale surface flux fields from models and satellites by using ship data, showing some deficiencies in operational model fields from ECMWF, the need of high-quality fluxes to interpret ocean‐ atmosphere exchanges, and the potential advantage of satellite retrieval methods. Further analysis of these datasets is being performed in a unique database (the ALBATROS project, open to the international scientific community). It will include refinement of airflow distortion correction and reprocessing of earlier datasets, the investigation of fluxes under specific conditions (low wind), and the effect of sea state among others. It will also contribute to further validation and improvements of satellite retrievals in various climatic/meteorological conditions.

Momentum and heat fluxes via the eddy correlation method on the R/V L'Atalante and an ASIS buoy

[1]xa0In this paper, we present the results obtained with the eddy correlation method applied to data acquired during the Flux, Etat de mer et Teledetection en Condition de Fetch variable experiment onboard the R/V LAtalante. We discuss the corrections made to account for platform motion and for the effects of mean airflow distortion. The data are compared to those obtained from a moored Air-Sea Interaction Spar (ASIS) buoy and from the LAtalante using the inertial dissipation method (IDM). The main results from eddy correlation method on LAtalante are that the momentum flux in-line with the mean wind, −〈u′w′〉, is overestimated by 18%, likely due to turbulent flow distortion around the ship. In contrast, the results for heat flux do not appear to be contaminated by turbulent flow distortion. Indeed, heat fluxes obtained using the sonic temperature on the LAtalante and the ASIS buoy are very similar. The eddy correlation latent heat fluxes obtained on the LAtalante using a refractometer are significantly higher than those obtained from the same sensor using the IDM.

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.

A New Shipborne Microwave Refractometer for Estimating the Evaporation Flux at the Sea Surface

Abstract After a brief description of humidity measurement and a short presentation of methods of microwave refractometry for evaporation flux, a new X-band refractometer system is presented. Based on a new design and a new material for the microwave cavity, it does not need calibration for refractive index variations because of its reduced thermal time constant. The new device has been combined with a sonic anemometer and traditional mean meteorological measurements on a 12-m shipborne mast. It has been found to be very efficient for obtaining humidity fluctuations and fluxes in the CATCH 97 (Couplage avec l’ATmosphere en Conditions Hivernales) and FETCH 98 (Flux, Etat de la mer et Teledetection en condition de fetCH variable) experiments under various wind and stability conditions. The inertial subrange is of very high quality. To first order, the evaporation flux and refractive index flux are very similar. In extreme meteorological conditions, such as those encountered during CATCH, the sensible heat f...

An integrated approach to estimate instantaneous near-surface air temperature and sensible heat flux fields during the SEMAPHORE experiment

A new technique was developed to retrieve near-surface instantaneous air temperatures and turbulent sensible heat fluxes using satellite data during the Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, which was conducted in 1993 under mainly anticyclonic conditions. The method is based on a regional, horizontal atmospheric temperature advection model whose inputs are wind vectors, sea surface temperature fields, air temperatures around the region under study, and several constants derived from in situ measurements. The intrinsic rms error of the method is 0.78C in terms of air temperature and 9 W m22 for the fluxes, both at 0.16 83 0.168 and 1.125 83 1.1258 resolution. The retrieved air temperature and flux horizontal structures are in good agreement with fields from two operational general circulation models. The application to SEMAPHORE data involves the First European Remote Sensing Satellite (ERS-1) wind fields, Advanced Very High Resolution Radiometer (AVHRR) SST fields, and European Centre for Medium-Range Weather Forecasts (ECMWF) air temperature boundary conditions. The rms errors obtained by comparing the estimations with research vessel measurements are 0.38 Ca nd 5Wm 22.

A model to estimate the density, characteristic surface, and coverage of whitecaps using underwater sound

A model of ambient noise is described which permits one to estimate the density of whitecaps at the sea surface as well as the scaling as a function of wind speed of whitecap coverage and characteristic surface. This model relates the statistical properties of underwater sound (first and second moments) to whitecap distribution at the sea surface and to underwater sound propagation. The sound propagation model accounts for seabed reflections. Simulations show that seabed reflection effects can be great in shallow water, implying that the listening radii for hydrophone measurements, as defined by Farmer and Vagle (1988), can often be very large compared to hydrophone depth.

Is Sea Surface Ambient Noise Correlated to Wind Turbulence

Preliminary results from studies of relationships between wind and acoustic noise using an hydrophone system are presented. Using a large band acoustic signal between 100 Hz and 20 kHz, correlation functions between noise and wind and also noise and turbulence are performed and analyzed. Particularly, the problem of an appropriate choice of time averaging to improve the different correlations is pointed out. A time delay in the correlation between wind and noise is found which is suggested to be related to the building of the bubbles field. Furthermore, in the comparison between correlation functions of increasing and decreasing wind with noise, some significant differences have been found indicating the necessity to consider more precisely the fast sampled acoustic signal and related physical mechanisms.