Mr. Robert C. Beal
The Johns Hopkins University
Applied Physics Laboratory (APL)
Johns Hopkins Road
Laurel, MD 20707-6099
Co-Investigators:
Frank M. Monaldo (APL)
Thomas Gerling (APL)
Global Wave Forecasting in the Southern Ocean
OBJECTIVES
The goal of this project is to demonstrate the potential value of spaceborne Synthetic
Aperture Radar (SAR) for operational ocean wave monitoring and forecasting.
This project complements the Space Radar Laboratory (SRL) investigation "Optimization
of
SAR
Parameters for Wave Spectra" (PI: F. Monaldo, CI: R. Beal), with somewhat
overlapping tasks and similar goals, that is, the understanding and application of
spaceborne
SAR
to operational ocean wave monitoring and forecasting.
PROGRESS
Using the
APL
real-time
SAR
processor with both the
SRL-1
and
SRL-2
missions, we have
acquired a
SAR
wave imaging and comparison data set more than two orders of magnitude
greater than in any previous U.S.
SAR
mission. This SRL data set is all the more
unique and valuable, since it was acquired at both a low altitude and a low off-nadir
angle. Both of these are necessary conditions for any future free flyer dedicated
to global ocean wave monitoring. The data set for our SRL investigation consists
of:
1) Over 100,000
SAR
Image Spectra over the Southern Ocean from the real-time
APL
SAR
Processor: 45,000 from April, and 55,000 from October.
2) Eighteen precision, high resolution
SAR
imagery data-takes correlated by
JPL,
both
over the Southern Ocean and over the North Atlantic.
3) Numerical wave model (WAM) nowcasts for the periods covering both April and October
missions, from both the U.S. Navy Fleet Numerical and Meteorological Center (Monterey)
and the European Centre for Medium Range Forecasting (ECMWF) (Reading).
4) European Remote Sensing Satellite (ERS-1)
SAR
derived wave spectra from the Max
Planck Institute covering the entire globe for the time intervals of both the April
and October missions.
Aboard Endeavour, an APL-built processor correlated
SAR
imagery from the
SIR-C
C-band
HH-polarization signal and produced image spectra which were transmitted to the ground
in real time. More than 100,000 image spectra from the APL-processor were stored
from 120 data takes in
SRL-1
and 126 data takes in
SRL-2.
The wave spectra cover most
of the Southern Ocean from 45S to 60S. The data set includes measurements where the
ocean significant wave height (SWH) varied from near zero to over 12 m, spanning
nearly the full range of naturally occurring sea states.
During
SRL-1
and
SRL-2,
the Navy's Fleet Numerical Meteorology and Oceanography Center
(FNMOC) retained the nowcast spectra made from their version of the operational
WAM
forecast model. An analogous comparison model data set is also available from
ECMWF
through collaborators at the Max Planck Institute (Hamburg).
Given the broad geographical coverage of data from the
APL
processor, there are at
least 30 times in the Southern Ocean during each mission when
ERS-1
wave spectra
and APL-processor image spectra were located less than 50 km apart and acquired within
one hour of one another. Loosening the colocation criteria only slightly will produce
more than 100 such comparisons sets.
SIGNIFICANT RESULTS
Using real-time APL-processor data received at Johnson Space Center, we merged
SAR
wave vector estimates over the Southern Ocean with wave vector forecasts from
FNMOC,
and daily placed the combined products on a World Wide Web site. During the mission,
the site was visited more than 150 times by investigators from as far away as South Africa
and Australia. This effort demonstrated the potential of providing wave information
from spaceborne
SAR
quickly enough to be usefully assimilated into wave forecast
models. The site
currently shows recent data and analyses from the SRL missions. In the past year,
we have processed image spectra with only a simple
SAR
modulation transfer function.
Even these results show quality retrievals of wave direction and wavelength. In
the future, we will employ more sophisticated retrieval schemes that may permit the accurate
measurement of ocean
SWH
as well.
FUTURE PLANS
Given the totality of the
SIR-C
image spectra,
ERS-1SAR
spectra, and
WAM
model estimates,
we intend to 1) make a definitive determination of what ocean wave parameters can
be extracted from spaceborne SARs and with what accuracy; 2) specify the
SAR
satellite configuration that optimizes this retrieval; and 3) demonstrate, with a statistically
significant data set, the value of
SAR
for improved global wave forecasting.
We are currently collaborating with William Plant who, while a visiting scientist
at the Max Planck Institute this last year, applied nonlinear retrieval algorithms
to estimate wave spectra from coincident
SIR-C
and
ERS-1
data. We expect to submit
a joint paper in the upcoming year, documenting the comparison of
ERS-1
and SIR-C-derived
image spectra.
We intend to make a systematic comparison, wave system by wave system, to ascertain
systematic differences between
WAM
model wave estimates and
SIR-C
measurements for
the entire Southern Ocean data set. We will then determine whether these differences
are due to input wind field errors, model problems, and/or
SAR
wave imaging limitations.
We expect the results to form the major scientific and operational justification
for a low altitude wave monitoring
SAR
satellite. Such a satellite would easily
fit into the
NASA
"Lightsat" category, and would also be a candidate for the "New Millennium"
spacecraft announcement of opportunity. Both the U.S. Navy and National Oceanic
and Atmospheric Administration (NOAA) have strong interests in the improvement of
operational wave forecast products that would be possible with such a dedicated low altitude
free-flyer. Moreover,
NASA
should have an equally strong interest in the demonstration
of the advanced technology necessary to do the
SAR
on-board processing and the real-time global dissemination of data to a worldwide user network.
PUBLICATIONS
Beal, R. C., S. F. Oden, J. L. MacArthur, and F. M. Monaldo, Real Time Ocean Wave
Monitoring from Space: A Thirty-Year Quest Achieved, Johns Hopkins
APL
Technical Digest
, vol. 15, No. 3, pp. 237-241.
Gerling, T. G and P. A. Wittmann, Comparison of SAR-estimated Wave Spectra with
WAM
Model Estimates During the SRL Southern Ocean Experiment, Proc. 1995 International Geoscience and Remote Sensing Symposium
, Florence, Italy, July 1995.
Monaldo, F. M. and R. C. Beal, SRL Real-Time Wave Forecasting in the Southern Ocean,
Proc. 1995 International Geoscience and Remote Sensing Symposium Florence
, Italy, July 1995.
Monaldo, F. M. and R. C. Beal, Real-Time Observations of Southern Ocean Waves Fields
from the Shuttle Imaging Radar, IEEE Transactions on Geoscience and Remote Sensing
, vol. 33, No. 4, pp. 942-949, 1995.
Plant, W., S. Hasselmann, C. Bruning, R. Beal, and F. Monaldo, Comparison of Ocean
Wave Spectra from a Nonlinear
SAR
Inversion Scheme using
ERS-1
and
SIR-C
Data Sets,
Proc. 1995 International Geoscience and Remote Sensing Symposium
, Florence, Italy, July 1995.
Wittmann, P. A., R. M. Clancy, and R. C. Beal,
FNMOC
Supports Wave Modeling Around
the World and into Space, Naval Meteorology and Oceanography News,
July 1995.
In addition, the following formal presentations have been given:
Beal, R. C., The SRL Real Time Wave Forecasting Experiment and its Implications for a Future Satellite
Design
, invited seminar to NOAA/NESDIS, 15 December 1994.
Beal, R. C., The
SIR-C
Real-Time Southern Ocean Experiment
, invited seminar to the Max Planck Institute for Meteorology, 6 July 1995.
Beal, R. C., Ocean Applications of Spaceborne SAR
, invited presentation at the US-ROC Oceanic Microwave Remote Sensing Workshop, U.
Delaware, 16 August 1995.