Abstract
Impact of flow distortion corrections on turbulent fluxes estimated by the inertial dissipation method during the FETCH experiment on R/V L'Atalante
Département de Géologie et Océanographie, UMR 58 05, Talence, France
Centre d'étude des Environnements Terrestre et Planétaires (CETP)/IPSL/CNRS, Vélizy, France
Centre d'étude des Environnements Terrestre et Planétaires (CETP)/IPSL/CNRS, Vélizy, France
Centre d'étude des Environnements Terrestre et Planétaires (CETP)/IPSL/CNRS, Vélizy, France
Meteo-France/CNRM/Centre d'Aviation Meteorologique, Aerodrome, Bretigny-sur-Orge, France
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA
Universite Pierre et Marie Curie, IPSL, Paris, France
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA
The 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 L'Atalante, on an ASIS spar buoy and on aircraft. On the R/V L'Atalante, 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 L'Atalante, 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 L'Atalante 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
Published 13 March 2003.
Citation: (2003), Impact of flow distortion corrections on turbulent fluxes estimated by the inertial dissipation method during the FETCH experiment on R/V L'Atalante, J. Geophys. Res., 108(C3), 8064, doi:10.1029/2001JC001075.
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