## Abstract

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 C_{D10N} × 1000 = 0.56 + 0.063 U_{10N}, for U_{10N} > 6 m s^{-1}. This formula provides C_{D10N} 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 C_{E10N} 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 C_{E10N} × 1000 = 0.82 + 0.02 U_{10n}, for U_{10n} > 6 m s^{-1}.

Original language | English (US) |
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Pages (from-to) | 12-1 - 12-17 |

Journal | Journal of Geophysical Research: Oceans |

Volume | 108 |

Issue number | 3 |

DOIs | |

State | Published - Mar 15 2003 |

## Keywords

- Air-flow over ships
- Heat flux
- Turbulent flux measurements
- Wind stress

## ASJC Scopus subject areas

- Geochemistry and Petrology
- Geophysics
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science
- Atmospheric Science
- Astronomy and Astrophysics
- Oceanography