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Ocean Surface Topography from Space
SCIENCE
A coordinated programme for global calibration/validation of altimeter sea state data
Artists Concept: Jason-1

Authors:


P.D. Cotton
(Satellite Observing Systems, UK),
D.J.T. Carter
(Satellite Observing Systems, UK),
P.G. Challenor
(Southampton Oceanography Centre, UK),
V.Yu. Kareev
(Institute of Applied Physics, Russia)

CORRESPONDING AUTHOR:
David Cotton
Satellite Observing Systems
15, Church Street
Godalming
Surrey GU7 1EL - UK
d.cotton@satobsys.co.uk


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Abstract:

The main aim of this work is to calibrate Jason-1 wind and wave data against a large-scale
buoy database. By applying a single calibration procedure, which includes a proper
consideration of errors, we aim to bring all historical and current altimeter wind
and wave data into agreement. A second aim is to assess the reliability of Jason-1
wind/wave measurements, including an investigation for dependencies on local environmental
conditions. Finally, algorithm developments will increase the range and quality of
wind/wave information that can be extracted from Jason.

Overview

Applications of altimeter wind and wave data range from the operational use of data in
near-real time, through assimilation into forecast models, to the use of carefully
calibrated offline products for climatological studies and predictions of extremes.

Accurate calibration is important for all applications, but particularly so for climate
studies, where any bias in the altimeter wave heights, even of one or two percent, would
affect the statistics. This would then render the database useless for studies of
climate change in which trends of one or two percent per annum in annual mean wave
heights have been found to be of significance [Bacon and Carter, 1991].

Many authors have carried out calibration and validation studies on wind and wave
measurements from spaceborne altimeters, using a variety of data sets and techniques
[Cotton et al, 1997]. However, because these authors have used different procedures
and different validation data sets, it is difficult to combine their results to form
a single combined assessment of the relative accuracies and reliabilities of the
measurements from the different altimeters. This confusing situation exists across
altimeter and buoy data sets, and creates a major obstacle to the full exploitation
of these data.

The calibration carried out within this programme will be consistent with those
performed by the same authors on data from all past and present radar altimeters
(Geosat, ERS-1, ERS-2, TOPEX/POSEIDON, ENVISAT, Geosat Follow-On). The principal
objective is to apply a consistent calibration procedure across all altimeter and
buoy measurements of winds and waves, generate a series of calibration corrections,
and so provide users of wave data with the means to achieve consistent measurements
of sea state across all altimeter and buoy data sets.

In addition the team will assess the reliability and accuracy of Jason-1 wind and
wave parameters under different environmental conditions, and will aim to develop
new algorithms to provide new or improved measurements of sea state.

The results will provide a thorough assessment of the reliability and accuracy
of wind and wave measurements from Jason, generate new and improved algorithms to
increase the range and quality of sea state information retrieved from the altimeter,
and, most importantly, provide the means for users to generate a reliable,
globally-consistent data set across all altimeter and buoy measurements.

Calibration procedures

Many previous calibration studies have applied quite crude methods of regression
to derive relationships between calibration and reference data sets. Very rarely
do these studies consider the nature of the errors in the individual data sets
before considering which techniques are most appropriate.

In the general case, errors are found within both the reference and altimeter
data sets. Indeed, recent studies suggest the accuracy of altimeter significant
wave height data is now close to that of buoy data. If we believe that the errors
in the two data sets are equal, then we can generate the factors defining the
linear relationship by completing two separate one-way regressions (e.g., altimeter
on buoy, buoy on altimeter) and taking the line whose gradient is the geometric
mean of the two slopes and which passes through the centre of gravity of the
colocated data. More correctly, we should take full account of variance in both
data sets by carrying out a full weighted orthogonal distance regression. In
practice this procedure has rarely been followed, partly because of the complexity
involved in generating associated estimates of error. Other approaches can also
include the effect of other environmental parameters, for example the effect of
wave age on the measurement of wave period, by including extra terms in the relationship
[Davies et al, 1997]. Stoffelen [1997] suggests that, where errors are not well- known,
analysis of triple colocations can achieve simultaneous error modeling and calibration.

Altimeter wind/wave algorithms

In recent years, significant effort has been applied to the problem of improving
the accuracy of sea-state data from satellite altimeters.

An accuracy of retrieval for wind speed of the order of two meters per second
was quickly achieved with the first algorithms [e.g., Brown et al., 1981], but
still has not been significantly improved on. The algorithm currently used to
calculate wind speeds for altimeter geophysical data records (i.e., for ERS-2
and TOPEX) is one generated for use on Geosat [Witter and Chelton, 1991]. This
algorithm was developed empirically, through comparisons with buoy data, and
depends upon an assumed, unique relationship between the wind speed and the
small-scale sea surface roughness caused by wind-generated gravity waves
[e.g., Glazman and Pilorz, 1990]. However, a number of studies have demonstrated
that the radar backscatter is not uniquely dependent on local wind-driven gravity
waves, but also upon larger-scale waves. Authors such as Lefèvre et al. [1995] have
therefore tried to include a degree of dependence on larger scales by including
significant wave height in their wind speed retrieval algorithms. These new forms
of algorithms have brought some limited improvement in accuracy, but they are still
largely empirically-based. In a recent development, Elfouhaily et al. [1997] have
taken advantage of the two frequencies used by the TOPEX altimeter (Ku and C band).
In their study they demonstrated that some of the effects due to non-local sea can
be removed by considering the difference between the Ku and C Band radar backscatter.
A further consideration, adversely affecting the accuracy of all wind speed retrievals
from satellite radar measurements, is the need to estimate wind speed for a
reference height above the sea surface (usually 10 meters). The radar necessarily
makes a measurement exactly at the air/sea interface, and so any measurement must be
corrected to the reference height. This is usually achieved by assuming a neutral
boundary layer stability, but errors may occur when this assumption does not hold.
Indeed, some researchers have found that the accuracy of satellite scatterometer
winds depends on local air and sea temperature [Ebuchi et al, 1996]. Thus greater
accuracy may be achievable by including some local information (instrumental,
climatological or model) in a retrieval algorithm. In fact, it could be argued
that satellite radar data are better used to make a direct estimate of wind stress,
though there would be significant problems in validating such measurements.

References

Bacon S., D.J.T. Carter, 1991: Wave climate changes in the North Atlantic and North Sea. Int. J. Climatol., 11, 545-558.

Brown G.S., 1977: The average impulse response of a rough surface and its applications, IEEE Trans. Ant. Prop., AP-25, 67-74.

Brown G.S., H.R. Stanley, N.A. Roy, 1981: The wind speed measurement capability of spaceborne radar altimetry. IEEE J. Oceanic. Eng., OE-6, 59-63.

Cotton P.D., P.G. Challenor, D.J.T. Carter, 1997: "An assessment of the accuracy and reliability of Geosat, ERS-1, ERS-2 and TOPEX altimeter measurements of significant wave height and wind speed", in Proceedings of CEOS Wind and Wave Validation Workshop, 2-5 June 1997. ESTEC, Noordwijk, The Netherlands, ESA special publication.

Davies C.G, P.D. Cotton, P.G. Challenor, 1997: "Altimeter estimates of wave period", Proceedings of the 3rd ERS Symposium: "Space at the Service of our Environment", Florence, March 1997, ESA Special Publication.

Ebuchi N., H.C. Graber, R. Vakkayii, 1996: Evaluation of ERS-1 scatterometer winds, Technical Report CAOS 96-001, Center for Atmospheric and Oceanic Studies, Tohoku University, Japan, 69 pages.

Elfouhaily T., D. Vandemark, J. Gourrion, B. Chapron, 1997: Estimation of wind stress using dual-frequency TOPEX data. J. Geophys. Res. (submitted)

Glazman R.E., S.H. Pilorz, 1990: Effects of sea maturity on satellite altimeter measurements. J. Geophys. Res., 95, C3, 2857-2870.

Lefèvre J.M., J. Barckicke, Y. Ménard, 1994: A significant wave height dependent function for TOPEX/POSEIDON wind speed retrieval. J. Geophys. Res., 99, 25, 035-25,049.

Stoffelen A., 1997: In search of the true wind: Scatterometer calibration and error modelling, Proceedings of CEOS Wind and Wave Validation Workshop, 2-5 June 1997. ESTEC, Noordwijk, The Netherlands, ESA special publication.

Witter D.L., D.B. Chelton, 1991: A Geosat altimeter wind speed algorithm and a method for wind speed algorithm development, J. Geophys. Res., 96, 18853-18860.


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