Search Results
You are looking at 1 - 4 of 4 items for
- Author or Editor: Pierre Queffeulou x
- Refine by Access: All Content x
Abstract
In 1983 the project “Travaux d'Océanographie Spatiale: Capteurs Actifs dans l'Atlantique Nord-Est” (TOSCANE) was initiated in view of calibration, validation, and use of satelliteborne scatterometer and altimeter measurements in terms of wind and wave data, with special emphasis on the ERS-1 satellite to be launched in 1991 by the European Space Agency. The wind measurements from an array of buoys and ship and land stations, deployed during the last experiment within the program, TOSCANE-2, are described, and a method is given to evaluate their accuracy. When comparing wind data from two measuring stations at sea, the averaging time has to be selected according to the separation distance between the two locations to filter out the space-induced wind variability. For buoy and ship comparisons, for distances less than 3 km, a 10-min averaging time was found to be a lower limit. For pairs of buoys separated by 25 or 35 km, the standard deviation of differences decreases with averaging time, in the same manner for speed and direction: by about 40% when passing from 10 min to 6 h averaging (from 1.3 to 0.8 m s−1 for speed, and from 14° to 9° for direction) with a plateau value (50%) after 12 h. Data from the two independent wind systems on board N/O Le Noroit show a high consistency: mean values of differences for direction and speed are 5.9° and 0.29 m s−1, with standard deviations of 4.5° and 0.40 m s−1, the standard deviation of speed differences increasing (0.10 to 0.49 m s−1) over the 1–30 m s−1 speed range; 99.9% of data are within ±2 m s−1. When calibrating anemometers with coefficients from wind tunnel tests, differences between buoy speeds are large, as much as 1.4 m s−1 ± 0.8 m s−1, resulting from, among other causes, the inability of sensors to correctly filter high-frequency fluctuations of wind speed. The buoys were then tuned independently, relative to ship data from dedicated runs, which allowed a reduction of differences to maximum values of 0.5 m s−1 ± 0.8 m s−1 and 4° ± 9°. These results are compatible with an assessment of the foreseen 2 m s−1 and 20° accuracy of the ERS-1 scatterometer wind data. In contrast, wind data from a 10-m mast on a flat island are shown not to be accurately representative of wind at sea, even for onshore wind.
Abstract
In 1983 the project “Travaux d'Océanographie Spatiale: Capteurs Actifs dans l'Atlantique Nord-Est” (TOSCANE) was initiated in view of calibration, validation, and use of satelliteborne scatterometer and altimeter measurements in terms of wind and wave data, with special emphasis on the ERS-1 satellite to be launched in 1991 by the European Space Agency. The wind measurements from an array of buoys and ship and land stations, deployed during the last experiment within the program, TOSCANE-2, are described, and a method is given to evaluate their accuracy. When comparing wind data from two measuring stations at sea, the averaging time has to be selected according to the separation distance between the two locations to filter out the space-induced wind variability. For buoy and ship comparisons, for distances less than 3 km, a 10-min averaging time was found to be a lower limit. For pairs of buoys separated by 25 or 35 km, the standard deviation of differences decreases with averaging time, in the same manner for speed and direction: by about 40% when passing from 10 min to 6 h averaging (from 1.3 to 0.8 m s−1 for speed, and from 14° to 9° for direction) with a plateau value (50%) after 12 h. Data from the two independent wind systems on board N/O Le Noroit show a high consistency: mean values of differences for direction and speed are 5.9° and 0.29 m s−1, with standard deviations of 4.5° and 0.40 m s−1, the standard deviation of speed differences increasing (0.10 to 0.49 m s−1) over the 1–30 m s−1 speed range; 99.9% of data are within ±2 m s−1. When calibrating anemometers with coefficients from wind tunnel tests, differences between buoy speeds are large, as much as 1.4 m s−1 ± 0.8 m s−1, resulting from, among other causes, the inability of sensors to correctly filter high-frequency fluctuations of wind speed. The buoys were then tuned independently, relative to ship data from dedicated runs, which allowed a reduction of differences to maximum values of 0.5 m s−1 ± 0.8 m s−1 and 4° ± 9°. These results are compatible with an assessment of the foreseen 2 m s−1 and 20° accuracy of the ERS-1 scatterometer wind data. In contrast, wind data from a 10-m mast on a flat island are shown not to be accurately representative of wind at sea, even for onshore wind.
Abstract
Altimeter significant wave height (SWH) measurement data from six satellite missions covering 14 yr were analyzed over the Mediterranean Sea. First, data correction and screening were performed using the same method for the six altimeters [European Remote Sensing Satellites (ERS-1 and ERS-2), Ocean Topography Experiment (TOPEX), Geosat Follow-On, Jason, and Environmental Satellite (Envisat)]. The data from the TOPEX and Jason missions enabled the construction of seasonal maps of along-track SWH mean values and standard deviations. These reveal the regional short-scale sea state features associated with the specific meteorological patterns of the various geographical basins. Time series of monthly SWH mean values and standard deviations from each satellite and over the whole Mediterranean Sea were calculated and seen to be in good agreement, thus demonstrating interannual variability. The six altimeter missions used together enable the investigation of the monthly annual cycle at the short scales of the various subbasins. Significant differences are observed between the western and eastern parts of the Mediterranean Sea. The annual SWH cycle changes in both shape and amplitude depending on the subbasin. Analysis of the seasonal interannual variability confirms the existence of some degree of independence between the subbasins. Thanks to multisatellite missions and homogeneous corrections of the altimeter data, SWH time and space characteristics were able to be obtained at regional short scales. These results are independent of numerical wind and wave models. This method can be applied to any geographical region.
Abstract
Altimeter significant wave height (SWH) measurement data from six satellite missions covering 14 yr were analyzed over the Mediterranean Sea. First, data correction and screening were performed using the same method for the six altimeters [European Remote Sensing Satellites (ERS-1 and ERS-2), Ocean Topography Experiment (TOPEX), Geosat Follow-On, Jason, and Environmental Satellite (Envisat)]. The data from the TOPEX and Jason missions enabled the construction of seasonal maps of along-track SWH mean values and standard deviations. These reveal the regional short-scale sea state features associated with the specific meteorological patterns of the various geographical basins. Time series of monthly SWH mean values and standard deviations from each satellite and over the whole Mediterranean Sea were calculated and seen to be in good agreement, thus demonstrating interannual variability. The six altimeter missions used together enable the investigation of the monthly annual cycle at the short scales of the various subbasins. Significant differences are observed between the western and eastern parts of the Mediterranean Sea. The annual SWH cycle changes in both shape and amplitude depending on the subbasin. Analysis of the seasonal interannual variability confirms the existence of some degree of independence between the subbasins. Thanks to multisatellite missions and homogeneous corrections of the altimeter data, SWH time and space characteristics were able to be obtained at regional short scales. These results are independent of numerical wind and wave models. This method can be applied to any geographical region.
Abstract
New parameterizations for the spectral dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum and wind speed and direction, in a way consistent with observations of wave breaking and swell dissipation properties. Namely, the swell dissipation is nonlinear and proportional to the swell steepness, and dissipation due to wave breaking is nonzero only when a nondimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short-wave dissipation is introduced to represent the dissipation of short waves due to longer breaking waves. A reduction of the wind-wave generation of short waves is meant to account for the momentum flux absorbed by longer waves. These parameterizations are combined and calibrated with the discrete interaction approximation for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but, overall, the parameterizations probably yield the most accurate estimates of wave parameters to date. Perspectives for further improvement are also given.
Abstract
New parameterizations for the spectral dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum and wind speed and direction, in a way consistent with observations of wave breaking and swell dissipation properties. Namely, the swell dissipation is nonlinear and proportional to the swell steepness, and dissipation due to wave breaking is nonzero only when a nondimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short-wave dissipation is introduced to represent the dissipation of short waves due to longer breaking waves. A reduction of the wind-wave generation of short waves is meant to account for the momentum flux absorbed by longer waves. These parameterizations are combined and calibrated with the discrete interaction approximation for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but, overall, the parameterizations probably yield the most accurate estimates of wave parameters to date. Perspectives for further improvement are also given.
