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distribution of wind power is also needed for the optimal deployment of floating
wind farms in open sea that are enabled by new technology (Liu et al., 2008a).
Just a few decades ago, almost all ocean wind measurements came from merchant
ships. However, the quality and geographical distribution of these wind reports were
uneven. Today, operational numerical weather prediction (NWP) also gives us wind
information, but NWP depends on models, which are limited by our knowledge of
the physical processes and the availability of data. The scatterometers have provided
observations for important science and operational applications, in the past decades,
as reviewed by Liu (2002) and Liu and Xie (2006). At this tenth anniversary of
QuikSCAT launch, we will go back to the basics of scatterometry and turbulence
transfer to demonstrate the uniqueness of scatterometer stress measurements that
may enable new scientific applications from new perspectives.
For oceanographers, it is stress more than wind that directly drives ocean circulation. The two-dimensional stress field is needed to compute the divergence and
curl (vorticity) that control the ocean vertical mixing. The mixing brings short-term
momentum and heat trapped in the surface mixed layer into the deep ocean, where
they are stored over time. It also brings nutrients and carbon stored in the deep ocean
to the surface, where there is sufficient light for photosynthesis. Horizontal currents,
driven in part by stress, distribute the stored heat and carbon in the ocean. The magnitude of stress affects the turbulent transfer between ocean and atmosphere of heat,
moisture and gases that are critical for climate changes.
The relation between wind and stress, affecting the interpretation of
scatterometer measurements, will be described in Section 6.2. Talking about measuring stress with a scatterometer, but using U N as the actual wind, and explaining
the variation of scatterometer observations from wind theories, would lead to a misinterpretation of physical processes. Hence, the difference between wind and stress
is tackled in Section 6.3. Using a neutral drag coefficient to derive stress from the U N
provided by the scatterometer has inherent deficiency, so a new geophysical model
function (GMF) to retrieve stress is discussed in Section 6.4. With the potential benefit of direct retrieval of stress, which is also driven by smaller-scale ocean surface
parameters such as current and temperature, a re-thinking of the feedback processes
is explored in Section 6.5. A constellation of scatterometers to meet operational and
research needs is presented in Section 6.6.
6.2 Turbulence Parametrization and Scatterometer
Geo-Physical Product
Ocean surface stress (τ ) is the turbulent transfer of momentum between the ocean
and the atmosphere. The turbulence is generated by atmospheric instability caused
both by wind shear (difference between wind and current) and buoyancy (vertical
density stratification resulting from temperature and humidity gradients). In the past,
direct τ measurements, by the so-called eddy-correlation method, have only been
done in a few field campaigns (Smith, 1980). In practice, knowledge of τ is derived
W.T. Liu et al.
distribution of wind power is also needed for the optimal deployment of floating
wind farms in open sea that are enabled by new technology (Liu et al., 2008a).
Just a few decades ago, almost all ocean wind measurements came from merchant
ships. However, the quality and geographical distribution of these wind reports were
uneven. Today, operational numerical weather prediction (NWP) also gives us wind
information, but NWP depends on models, which are limited by our knowledge of
the physical processes and the availability of data. The scatterometers have provided
observations for important science and operational applications, in the past decades,
as reviewed by Liu (2002) and Liu and Xie (2006). At this tenth anniversary of
QuikSCAT launch, we will go back to the basics of scatterometry and turbulence
transfer to demonstrate the uniqueness of scatterometer stress measurements that
may enable new scientific applications from new perspectives.
For oceanographers, it is stress more than wind that directly drives ocean circulation. The two-dimensional stress field is needed to compute the divergence and
curl (vorticity) that control the ocean vertical mixing. The mixing brings short-term
momentum and heat trapped in the surface mixed layer into the deep ocean, where
they are stored over time. It also brings nutrients and carbon stored in the deep ocean
to the surface, where there is sufficient light for photosynthesis. Horizontal currents,
driven in part by stress, distribute the stored heat and carbon in the ocean. The magnitude of stress affects the turbulent transfer between ocean and atmosphere of heat,
moisture and gases that are critical for climate changes.
The relation between wind and stress, affecting the interpretation of
scatterometer measurements, will be described in Section 6.2. Talking about measuring stress with a scatterometer, but using U N as the actual wind, and explaining
the variation of scatterometer observations from wind theories, would lead to a misinterpretation of physical processes. Hence, the difference between wind and stress
is tackled in Section 6.3. Using a neutral drag coefficient to derive stress from the U N
provided by the scatterometer has inherent deficiency, so a new geophysical model
function (GMF) to retrieve stress is discussed in Section 6.4. With the potential benefit of direct retrieval of stress, which is also driven by smaller-scale ocean surface
parameters such as current and temperature, a re-thinking of the feedback processes
is explored in Section 6.5. A constellation of scatterometers to meet operational and
research needs is presented in Section 6.6.
6.2 Turbulence Parametrization and Scatterometer
Geo-Physical Product
Ocean surface stress (τ ) is the turbulent transfer of momentum between the ocean
and the atmosphere. The turbulence is generated by atmospheric instability caused
both by wind shear (difference between wind and current) and buoyancy (vertical
density stratification resulting from temperature and humidity gradients). In the past,
direct τ measurements, by the so-called eddy-correlation method, have only been
done in a few field campaigns (Smith, 1980). In practice, knowledge of τ is derived