Mobile version

TOPEX/POSEIDON and the wind

J.M. Lefèvre and J. Barckicke (Météo France, France) - Y. Ménard (CNES, France)

Météo France is involved in the TOPEX/POSEIDON program because it is interested in operational use of satellite data. Near-global coverage by the onboard altimeters is an advantage for assimilating the data into operational numerical models.

The data is also useful as a short-term weather forecasting aid. Our main investigations aim at improving estimates of two geophysical parameters: surface wind speed modulus (U) and significant wave height (H).

The wind modulus is also useful for determining sea level: we use it in parametrizations of electromagnetic bias, needed to correct altimeter ranges.

The radar altimeters receive a complex signal due to specular reflection of the pulse on a surface of some 4 km in diameter.
The effective backscattering cross-section (sigma) can therefore be seen as depending on the shape of the sea-surface, or statistical moments of the slope. As a first approximation, the most significant parameter is the mean quadratic slope. Capillary slopes are mainly due to the wind, while those of the large-scale components are due to waves. It therefore makes sense to express the wind by a relation U=f(sigma,H). We use a second-order polynomial in H and sigma:

U = a00 + a10H + a01sigma + a11Hsigma + a20 H2 + a02sigma2

The number of spatial data values co-located with surface measurements is generally limited to a few hundred over periods of a few years. However, using wind/wave data from operational numerical models to calibrate the function gave us a large data set of around 17,000 values. We filtered out doubtful model and altimetry data by removing values with too great a discrepancy between model and altimeter wave heights.

By introducing H in the transfer function we reduced the standard deviation of the wind error from 1.74 m/s to 1.67 m/s, using functions depending only on sigma. The data set was large enough for this reduction in variance to be significant with better than 99.9% confidence. The function was calibrated for the ALT altimeter, then validated with SSALT data. Figure 1, a composite image based on the tracks of cycle 20, shows the wind spatial distribution from the new algorithm (LBM) and from POSEIDON data. The same distribution with the Modified Chelton Wentz (MCW) algorithm shows striking differences of up to 2 m/s, particularly in tropical areas.

Reference :

  • Lefevre, J.M., Barckicke, J. and Y. Ménard, 1994, A SWH Significant Wave Height-dependent function for TOPEX/POSEIDON wind speed retrieval, J. Geophys. Res. (in press).
Rss feedsContactPartnersFAQLexiconMobile
You must enable cookies in your browser preferences to save the modifications
You are using an older version of Internet Explorer, please update to take full advantage of Aviso+ features

Customize the home page

View all widgets
Reduce display
Public profile
User profile

Select/Deselect widgets

Discover AVISO+

Customizable home, reading tools, products search guide, etc.. Discover step by step the features of the new site AVISO+.

Customizable homepage

The new home page is composed of blocks called "widgets".

Rearrange the blocks with a "Click & Drop".

Hide a widget with the cross at the top right. Reset or add a widget with the "wrench" icon. The "User" and "Public" profiles offer a predefined organization.

All the modifications are automatically saved as cookies on your machine so as to enable you to have your own specific AVISO+.

Reading tools

Ease your reading by choosing the page background colour (black/white) and/or by modifying the font size.

Browsing menu and breadcrum trail

To ease your browsing through AVISO+, you can use at any time the main banner items with the interactive submenus or use the browsing menu on the left side of the screen (it is open by default). For more readiness of the page contents, you can fold and unfold the browsing menu by clicking on the tab. At any time you can know where you are thanks to the browsing menu or the breadcrum at the top of the page.

Products search guide

The first submenu of the "Data" banner item leads you to a new search form enabling you to find quickier the product that suits your needs. The list of results is updated according to your selection criteria.

AVISO+ Multimedia

The first submenu of the "Multimedia" banner item leads you to AVISO+ collections of images and videos linked to altimetry and its applications. Have a nice visit!