Velocity Probability Density Functions from Altimetry (original) (raw)
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Probability Density Functions of Large-Scale Turbulence in the Ocean
Physical Review Letters, 1998
Probability density functions (pdfs) of surface velocity and surface velocity gradients in the ocean are calculated using altimetric data from the Topex / Poseidon satellite. These provide information about turbulence in a high-Reynolds-number geophysical flow. Both velocity pdfs and velocity gradient pdfs calculated over small regions are Gaussian but have more exponential shapes as the size of the region increases. We develop a simple explanation for the non-Gaussianity of velocity pdfs based on the inhomogeneity of eddy kinetic energy in the ocean. [S0031-9007(98)07902-2]
Direct measurements of ocean surface velocity from space: Interpretation and validation
Journal of Geophysical Research, 2005
The median Doppler shift of radar echoes is analyzed in measurements by ENVISAT's Advanced Synthetic Aperture Radar (ASAR) over the ocean. This Doppler centroid differs from a predicted signal based on the predicted motion of the satellite and Earth. This anomaly, converted to a surface Doppler velocity U D , appears to be of geophysical origin. Two wide-swath images over the Gulf Stream around Cape Hatteras suggest that U D contains high-resolution information on surface currents, while on a global scale, U D is found to vary with the wind speed in the range direction. A simple quantitative forward model is proposed, based on a practical two-scale decomposition of the surface geometry and kinematics. The model represents the effect of the wind through the wave spectrum, and gives U D ≈ γU 10 ∥ + U c ∥ , with U 10 ∥ and U c ∥ as the 10 m wind speed and quasi-Eulerian current in the line of sight of the radar projected on the sea surface, respectively, and γ as a coefficient function of the wind speed, wave development, and radar geometry. It is found that for an incidence angle of 23°, γ ≈ 0.3 for moderate winds and fully developed seas. This model is validated with a global data set of ASAR Wave Mode observations, with colocated model winds, acquired over the global ocean during the years 2003 and 2004. The Doppler signal therefore provides the signed parameter U D that can be used to reduce the wind direction ambiguity in the inversion of high-resolution wind fields from SAR imagery. A qualitative validation of current effects is shown for the English Channel where tidal currents dominate. Thus it should be possible to combine this previously ignored geophysical Doppler signal with traditional information on sea surface roughness, in order to provide very high resolution wind and current fields.
Velocity Probability Density Functions for Oceanic Floats
Journal of Physical Oceanography, 2000
Probability density functions (PDFs) of daily velocities from subsurface floats deployed in the North Atlantic and equatorial Atlantic Oceans are examined. In general, the PDFs are approximately Gaussian for small velocities, but with significant exponential tails for large velocities. Correspondingly, the kurtoses of the distributions are greater than three. Similar PDFs are found in both western and eastern regions, above and below 1000-m depth, with more significant non-Gaussianity in the North Atlantic than at the equator. Analogously, Lagrangian statistics in decaying two-dimensional turbulence also display non-Gaussian velocity PDFs with approximately exponential tails, in the limit of large Reynolds number.
Journal of Geophysical Research, 1998
A method is presented to calculate mean sea surface dynamic topography from satellite altimeter observations of its temporal variability. Time averaging of a simplified version of the quasi-geostrophic potential vorticity equation for the upper ocean layer results in a differential equation for the averaged relative vorticity in which the mean divergence of the eddy vorticity fluxes acts as a source or sink. The essential part is that these eddy fluxes can be determined from the altimeter observations. Consequently, no parameterisations appear in the averaged vorticity equation. From the average vorticity field, surface geostrophic velocities and related mean dynamic sea surface topography can then simply be derived. The usefulness of the method is established using "perfect" data, namely numerical output from the United Kingdom Fine Resolution Antarctic Model. The method appears applicable to areas of the ocean with strong enough mesoscale variability such as the major western boundary currents and their extensions and to frontal regions of the Antarctic Circumpolar Current. Quite realistic results are presented for active regions of the world ocean. Results are compared with hydrographic observations for the major western boundary current extensions of the Southern Ocean. An important application is to combine the newly derived averaged flow field with the observed eddy field to derive the total time-varying geostrophic surface velocity field. As a striking example this is applied to the Agulhas Current retrofiection, where the repeated shedding of large rings can now be synoptically reconstructed as a continuous process.
Analyzing Altimeter Artifacts: Statistical Properties of Ocean Waveforms
Journal of Atmospheric and Oceanic Technology, 2001
In this paper waveforms, that is, returns from the ocean surface, from a number of spaceborne radar altimeter instruments [European Remote-sensing Satellites (ERS-1 and-2), TOPEX, and Poseidon] are examined. This is the first paper to analyze waveform data from a number of altimeters in a consistent manner. Mean shapes and various statistical properties (bin-to-bin correlations, number of independent samples) were determined and the authors comment on their anomalies. The analyses were performed for data over the deep ocean, as that is the best understood surface. However, the determined functional characteristics of the individual altimeters are applicable to their operation over all surfaces. The implications of the existence of these anomalies for the retrieval of geophysical parameters from radar altimeter data are discussed. It is argued that the need for physically based theories, in order to understand radar altimeter returns from the ocean (or indeed any other) surface, implies a need for the engineering and software design of the instrument to be such as to avoid spurious anomalies in the waveforms.
Journal of Geophysical Research, 2004
1] The lack of an accurate geoid still prevents precise computation of the ocean absolute dynamic topography from satellite altimetry and only sea level anomalies (SLA) can be accurately deduced. In the new context of Global Ocean Data Assimilation Experiment (GODAE) where models are assimilating satellite altimetry, the estimation of a realistic mean dynamic topography (MDT) consistent with SLA is a crucial issue. In a first ''direct'' approach, a MDT is computed by subtracting the geoid model EIGEN-2 from the Mean Sea Surface Height CLS01, determined from 7 years of altimetric data (TOPEX and ERS1,2) at spherical harmonic degree 30. To provide the scales shorter than 660 km, the Levitus climatology is merged with the resulting MDT, both weighted by their respective errors. This solution provides a ''first guess'' for the computation of a global and higher resolution MDT. Then, a ''synthetic'' technique is used to combine in situ measurements and altimetric data: TOPEX and ERS1,2 altimetric anomalies are subtracted from in situ measurements of the full dynamical signal (based on buoy velocities from the WOCE-TOGA program and XBT, CTD casts). The resulting values provide local estimates of the mean field, in terms of currents or dynamic topography, which are used to improve the first guess using an inverse technique. The MDT obtained is compared to other mean dynamic fields, and a verification using independent in situ data shows improvements in most areas. It exhibits a more energetic representation of the subtropical and subpolar gyres; sea level gradients associated with the main currents are strongly enhanced. Differences with independent velocity observations are globally lower than 13 cm/s rms. Citation: Rio, M.-H., and F. Hernandez (2004), A mean dynamic topography computed over the world ocean from altimetry, in situ measurements, and a geoid model,
Statistical Characterization of Ocean Vector Winds
Three years of ocean vector wind data are used to evaluate statistics of the wind pseudostress over the ocean. Six different global wind products are compared, including raw swath winds from the QuikSCAT scatterometer, gridded wind products based on scatterometer observations, meteorological analysis winds from the European Centre for Medium-Range Weather Forecasts and reanalysis winds from the National Centers for Environmental Prediction. Buoy winds from a limited number of sites in the Pacific Ocean are also considered. Probability density functions (PDFs) computed for latitudinal bands show that mean winds for the six global products are largely in agreement, while variances differ substantially, by a factor of two or more. Higher moments of the PDFs also differ, with swath winds indicating frequent outliers, particularly at high latitudes. Kurtoses are large for all wind products, implying that PDFs are not Gaussian.
Geophysical Research Letters, 2008
Previous analysis of Advanced Synthetic Aperture Radar (ASAR) signals collected by ESA's Envisat has demonstrated a very valuable source of high-resolution information, namely, the line-of-sight velocity of the moving ocean surface. This velocity is estimated from a Doppler frequency shift, consistently extracted within the ASAR scenes. The Doppler shift results from the combined action of near surface wind on shorter waves, longer wave motion, wave breaking and surface current. Both kinematic and dynamic properties of the moving ocean surface roughness can therefore be derived from the ASAR observations. The observations are compared to simulations using a radar imaging model extended to include a Doppler shift module. The results are promising. Comparisons to coincident altimetry data suggest that regular account of this combined information would advance the use of SAR in quantitative studies of ocean currents.
Using Kolmogorov–Smirnov statistics to assess Jason, TOPEX, and Poseidon altimeter measurements
The Kolmogorov-Smirnov (K-S) test is used to compare probability density functions (PDFs) of geostrophic velocities measured by the TOPEX, Poseidon, and Jason altimeters. Velocity PDFs are computed in 2.5 by 2.5 boxes for regions equatorward of 60 latitude. Although velocities measured by the TOPEX and Jason altimeters can differ, on the basis of the K-S test, the velocities are statistically equivalent during the ¡ 200 day period when the satellites followed the same orbit. Full records from TOPEX, Poseidon, and Jason show less agreement, which can be attributed to temporal variability in ocean surface velocities and differing levels of measurement noise.