14 Forecasting Wind-driven Ocean Waves
GERBRAND J. KOMEN
KNMI, Koninklijk Nederlands Meteorologisch Instituut,
De BUt, The Netherlands
14.1 Introduction
The study of wind-generated surface waves on the ocean is important from a
physical, a geophysical and a practical point of view. In these lectures I will concentrate on their dynamics and prediction. Over the last 50 years enormous
progress has been made in this field (see, for example: Phillips, 1977; WMO, 1988;
and Komen et al., 1994), which has resulted in the availability of advanced measurement tools and advanced numerical models, such as the WAM, WAve Model
(WAMDI,1988).
In the not so distant past, wave predictions were made manually. This required
considerable insight in processes responsible for wave growth, propagation and
decay. Now there is a real danger that this experience is lost, as one can use a wave
prediction model as a black box: winds in - waves out. To avoid this unhealthy situation, I will begin with a discussion of some basic phenomenology. I will start by
considering certain simplified cases. However, the main message will be that wave
spectra are usually quite complex in realistic situations.
In the second part 1 will show how our understanding of basic wave dynamics
and airlsea interaction has been used to derive the WAM model, and 1 will discuss
some aspects ofthe operational implementation ofthis model. I will also discuss its
performance and its usefulness for wave forecasting.
14.2 Phenomenology
Our knowledge of waves is based on many observations. Visual observations of
wind speed and direction, significant wave height, wave period, and wave direction
(wind sea and swell) have always been ofimportance, and will continue to be so. In
addition, there are valuable instrumental observations from buoys and platforms of
the (two-dimensional) wave spectrum. Certain areas, such as the North Sea, are
covered reasonably well with instruments. Instrumental observations in other
areas, notably in the Southem Hemisphere are extremely scarce. Fortunately, a
wealth of ocean wave observations has become available from satellites such as
Geosat, Topex-Poseidon and ERS-1 and ERS-2. In particular, the radar altimeter
has proven to be quite reliable for measurement of the significant wave height.
Synthetic aperture radars (SAR) are able to provide two-dimensional information
about the wave spectrum.
GERBRAND J. KOMEN
KNMI, Koninklijk Nederlands Meteorologisch Instituut,
De BUt, The Netherlands
14.1 Introduction
The study of wind-generated surface waves on the ocean is important from a
physical, a geophysical and a practical point of view. In these lectures I will concentrate on their dynamics and prediction. Over the last 50 years enormous
progress has been made in this field (see, for example: Phillips, 1977; WMO, 1988;
and Komen et al., 1994), which has resulted in the availability of advanced measurement tools and advanced numerical models, such as the WAM, WAve Model
(WAMDI,1988).
In the not so distant past, wave predictions were made manually. This required
considerable insight in processes responsible for wave growth, propagation and
decay. Now there is a real danger that this experience is lost, as one can use a wave
prediction model as a black box: winds in - waves out. To avoid this unhealthy situation, I will begin with a discussion of some basic phenomenology. I will start by
considering certain simplified cases. However, the main message will be that wave
spectra are usually quite complex in realistic situations.
In the second part 1 will show how our understanding of basic wave dynamics
and airlsea interaction has been used to derive the WAM model, and 1 will discuss
some aspects ofthe operational implementation ofthis model. I will also discuss its
performance and its usefulness for wave forecasting.
14.2 Phenomenology
Our knowledge of waves is based on many observations. Visual observations of
wind speed and direction, significant wave height, wave period, and wave direction
(wind sea and swell) have always been ofimportance, and will continue to be so. In
addition, there are valuable instrumental observations from buoys and platforms of
the (two-dimensional) wave spectrum. Certain areas, such as the North Sea, are
covered reasonably well with instruments. Instrumental observations in other
areas, notably in the Southem Hemisphere are extremely scarce. Fortunately, a
wealth of ocean wave observations has become available from satellites such as
Geosat, Topex-Poseidon and ERS-1 and ERS-2. In particular, the radar altimeter
has proven to be quite reliable for measurement of the significant wave height.
Synthetic aperture radars (SAR) are able to provide two-dimensional information
about the wave spectrum.