Chapter 6
Scatterometer’s Unique Capability
in Measuring Ocean Surface Stress
W. Timothy Liu, Xiaosu Xie, and Wenqing Tang
6.1 Introduction
Ocean surface stress, the turbulent transfer of momentum between the ocean and
the atmosphere, is a vector quantity with a magnitude and a direction. It is closely
related to wind but not solely driven by wind, as it depends also on ocean parameters
such as surface current and temperature. While the general public appreciates wind
as air in motion, very few people know what is stress. Even for oceanographers,
the concept of stress distribution is largely derived from that of wind, because there
was no large-scale measurement of stress over the ocean until the launch of the
first scatterometer. A scatterometer measures the ocean surface roughness that is
supposed to be in equilibrium with stress, and therefore it has the unique capability
of measuring stress over the global ocean.
Although scatterometer measurements were related to stress, in prelaunch studies
of the first scatterometer on Seasat (e.g. Wentz, 1978; Jones and Schroeder, 1978),
and were validated against stress measurements (Liu and Large, 1981), scatterometers have been promoted as wind measuring instruments for the past 3 decades.
The geophysical product of the scatterometer is the equivalent neutral wind (U N ).
U N , by definition, has an unambiguous relation with surface stress, provided that
ocean surface current is negligible, while the relation between actual wind and surface stress depends also on atmospheric density stratification (see Section 6.2). Over
most of the ocean, the atmosphere is generally believed to be near neutral, current
speed is much smaller than wind speed, and U N is assumed to be the actual wind,
particularly in operational weather prediction.
Wind over ocean is much needed for marine weather forecast and to avoid shipping hazard. The significance of wind measurement is clearly felt, for example,
when a hurricane suddenly intensifies and changes course or when the unexpected
delay of monsoon brings drought. Surface wind convergence brings moisture and
latent heat that drives deep convection and fuels atmospheric circulation. Detailed
W.T. Liu (B)
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA
e-mail: w.t.liu@jpl.nasa.gov
93
V. Barale et al. (eds.), Oceanography from Space,
DOI 10.1007/978-90-481-8681-5_6, C
Springer Science+Business Media B.V. 2010
Scatterometer’s Unique Capability
in Measuring Ocean Surface Stress
W. Timothy Liu, Xiaosu Xie, and Wenqing Tang
6.1 Introduction
Ocean surface stress, the turbulent transfer of momentum between the ocean and
the atmosphere, is a vector quantity with a magnitude and a direction. It is closely
related to wind but not solely driven by wind, as it depends also on ocean parameters
such as surface current and temperature. While the general public appreciates wind
as air in motion, very few people know what is stress. Even for oceanographers,
the concept of stress distribution is largely derived from that of wind, because there
was no large-scale measurement of stress over the ocean until the launch of the
first scatterometer. A scatterometer measures the ocean surface roughness that is
supposed to be in equilibrium with stress, and therefore it has the unique capability
of measuring stress over the global ocean.
Although scatterometer measurements were related to stress, in prelaunch studies
of the first scatterometer on Seasat (e.g. Wentz, 1978; Jones and Schroeder, 1978),
and were validated against stress measurements (Liu and Large, 1981), scatterometers have been promoted as wind measuring instruments for the past 3 decades.
The geophysical product of the scatterometer is the equivalent neutral wind (U N ).
U N , by definition, has an unambiguous relation with surface stress, provided that
ocean surface current is negligible, while the relation between actual wind and surface stress depends also on atmospheric density stratification (see Section 6.2). Over
most of the ocean, the atmosphere is generally believed to be near neutral, current
speed is much smaller than wind speed, and U N is assumed to be the actual wind,
particularly in operational weather prediction.
Wind over ocean is much needed for marine weather forecast and to avoid shipping hazard. The significance of wind measurement is clearly felt, for example,
when a hurricane suddenly intensifies and changes course or when the unexpected
delay of monsoon brings drought. Surface wind convergence brings moisture and
latent heat that drives deep convection and fuels atmospheric circulation. Detailed
W.T. Liu (B)
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA
e-mail: w.t.liu@jpl.nasa.gov
93
V. Barale et al. (eds.), Oceanography from Space,
DOI 10.1007/978-90-481-8681-5_6, C
Springer Science+Business Media B.V. 2010