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Ocean Surface Topography from Space
SCIENCE
Dual-frequency altimetry for precipitation and wind studies

Figure 1

Authors:


J. Tournadre, B. Chapron
(IFREMER, France)

CORRESPONDING AUTHOR:
Jean Tournadre
IFREMER
BP 70
29280 Plouzané - France
jean.tournadre@ifremer.fr



Abstract:

The planned Jason-1 altimeter will, like the TOPEX altimeter, interleave the nominal Ku-band (2.1 cm) with a C-band (5.5 cm) signal. The primary purpose of the second radar is to provide a collocated ranging measurement to correct for ionospheric path delay in the Ku-band range estimate. Our research plan, which is a direct continuation of the studies undertaken by the proposing team, will further emphasize the capabilities of dual-frequency measurement in two other fields: to study oceanic precipitation and to refine the near-surface wind speed. The available TOPEX data set will be extensively used prior to the Jason-1 launch to initiate the studies.

Scientific and technical program

The Jason-1 altimeter will operate at two microwave frequencies, 13.5 GHz (Ku-band) and 5.3 GHz
(C-band). The primary goal of the dual-frequency operation is to provide a precise ionospheric
correction. Besides a differential ionospheric path delay, Ku- and C-band signals are differentially
and significantly affected by geophysical quantities such as atmospheric precipitation and sea surface
roughness. The potential benefit of having collocated measurements of surface backscatter at two
frequencies to infer atmospheric liquid water and near-surface wind has been investigated in
recent years using the TOPEX data. The promising results of these studies encourage us to develop
the ongoing research effort on the analysis of dual-frequency measurements. The program covers
two themes: first, the analysis of the impact of precipitation and estimation of rain from
altimetry, and second, the refinement of near-surface wind algorithms.

Impact of precipitation

Besides others effects, raindrops absorb the altimeter pulse and attenuate the return pulse power.
This attenuation, which is frequency-dependent, is an order of magnitude larger in Ku than in C-band.
The C-band is only slightly attenuated, except in heavy rain. Furthermore, when an altimeter
footprint is partially filled by rain, the surface echo (waveform) received by the altimeter
is distorted. This causes problem in altimeter processing and errors in both sea surface height
and significant wave height estimates [Quartly et al., 1996, Tournadre, 1998]. For the TOPEX
altimeter, a rain flag has been proposed using a simple criterion based on the detection of a
simultaneous departure from the normal C-Ku backscatter relationship and an excess of liquid
vapor content as estimated from the TOPEX Microwave Radiometer (TMR) [Tournadre and Morland, 1997].

Using a precipitation index based on similar criteria, global monthly rain climatologies have been estimated from TOPEX altimeter data [Chen et al, 1997]. They show a good qualitative and quantitative agreement with those obtained from in situ or other satellite data. Moreover, attempts to determine rain cell characteristics from waveform analysis have been made to obtain a high-resolution description of the rain distribution under various weather conditions.

These investigations will be carried on using Jason-1 data to further assess rain climatologies, to determine rain cell characteristics and to test rain flags from altimetric missions.


Figure 2

  1. The rain flag based on the Ku-C relationship will be validated during the calibration/validation phase and the impact of the rain flagging on the accuracy of the mean surface topography will be tested on selected regions.

  2. Seasonal rain climatologies will be constructed using TOPEX and Jason-1 data as well as ENVISAT dual-frequency (Ku and S band) altimeter data. Particular attention will be paid to intercalibration of the fields during the overlap period.

  3. Based on an analytical model, a method has been defined to invert the altimeter waveform distortion in terms of rain cell characteristics (rain rate, diameter) (see figure 2). This allows a precise description of rain fields in tropical cyclones, for example. The method will be further tested and validated to produce rain cell characteristics climatology.

Near-surface wind speed refinement

A mono-frequency altimeter's measurement of ocean radar cross section can be mapped directly onto an
estimate of surface wind speed. The most widely used algorithm is the Modified Chelton and Wentz
look-up table [Witter and Chelton, 1991]. This empirically-derived routine was developed through
comparisons with buoy wind measurements. It provides an estimate of wind speed at 10 or 19.5 metres
above the surface. For such nadir-looking backscatter measurements, studies of physically-based
ocean scattering models have shown that a more direct altimeter inference can be made in terms
of filtered surface mean square slope (mss). This parameter parallels the classical optical
measurements of ocean mss versus wind speed obtained by Cox and Munk [1954]. It has also been
shown that additional attenuation of nadir cross section can be associated with the spectral
density of short gravity-capillary waves. This impact needs to be considered when defining an
effective reflection coefficient [Jackson et al., 1992].

The importance of such altimeter studies is that, while short waves on the sea surface are the
roughness elements responsible for microwave backscatter changes, they also determine to a large
extent the air-sea transfer processes (which are of importance for climate studies).
As a consequence, radar backscatter measurements will be highly correlated to the wind friction
velocity [Elfouhaily et al., 1997].

However, the ocean surface is nominally constituted of both local wind-waves and swell-waves
generated elsewhere that have propagated into the area of interest. Both systems of waves will
influence altimeter radar returns either by affecting the slopes of the wave field or by modulating
the gravity-capillary wave frequency and amplitude. Long gravity waves can also influence the
wind profile to enhance small-scale amplitude variations along the long-wave phases.

The potential of using dual-frequency altimeter measurements to directly assess sea surface
small-scale roughness has been proven [Elfouhaily et al., 1997]. Indeed, these data help provide
evidence of non-wind impacts, such as residual sea state, on mono-frequency altimeter wind
estimates. Now, these observations and a composite model of surface altimeter backscatter are
being used to define an altimeter surface roughness parameter. This parameter can then be
tentatively compared with the aerodynamic roughness length commonly used to describe the stress
of the wind on the ocean surface [Elfouhaily et al., 1998].


Figure 3

Taking the opportunity to compare our derived surface roughness parameter with the NSCAT
scatterometer wind speed estimates, the analysis shown in figure 3 suggests that we can expect
to achieve considerable improvement by using dual-frequency altimeter measurements.

Furthermore, we feel that it is crucial to define the relative weighting scattering from
large- and small-scale structures on the electromagnetic (EM) bias versus wind speed and sea
state. We believe that the respective influences can be documented by using multiple
comparisons between dual-frequency altimeter measurements and scatterometer ones (ERS-1/2, NSCAT).
We also intend to demonstrate through modeling and existing observations a quantitative
link between satellite scatterometer upwind/downwind asymmetry and altimeter EM bias.

References

Chen G., B. Chapron, J. Tournadre, K. Katsaros, D. Vandemark, 1997: Global oceanic precipitation: A joint view by TOPEX and TMR. J. Geo. Res., 102, 10,457-10,471.

Cox C.S., W.H. Munk, 1954: Statistics of the sea surface derived from sun glitter. J. Mar. Res., 13, 198-227.

Elfouhaily T., B. Chapron, K Katsaros, D. Vandemark, 1997: A unified wave spectrum for long and short wind-driven waves. J. Geophys. Res., 102, 15781-15796.

Elfouhaily T., D. Vandemark, J. Gourrion, B. Chapron, 1998: Estimation of wind stress using dual-frequency TOPEX data. J. Geophys. Res., 103, 25101-25108.

Jackson F.C., W.T. Walton, B.A. Walter, C.Y. Peng, 1992: Sea surface mean square slope from Ku-band backscatter data. J. Geophys. Res., 97, 11411-11427.

Quartly G.D., T.H. Guymer, M.A. Srokosz, 1996: The effects of rain on TOPEX radar altimeter data. J. Atmos. Ocean. Technol., 13, 1209-1229.

Tournadre J. and J.C. Morland, 1997: The effect of rain on TOPEX/POSEIDON altimeter data: A new rain flag on Ku- and C-band backscatter coefficients. IEEE Geo. Remote Sens., 35, 1117-1135.

Tournadre J., 1998: Determination of rain cell characteristics from the analysis of TOPEX altimeter echo waveforms. J. Atmos. Ocean. Tech., 15, 387-406.

Witter D.L., D.B. Chelton, 1991: A GEOSAT altimeter wind speed algorithm and a method for altimeter wind speed algorithm development. J. Geophys. Res., 96, 8853-8860.


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