Satellite-Based Ocean Surface Turbulent Fluxes - Satellite-based Applications on Climate Change - page 162

functions of the vertical gradient of wind speed, temperature, and moisture near the
surface and are affected by the stability of the surface air. The atmosphere is mostly
under near neutral or stable conditions, and most of the retrieval techniques use drag
coefficients under neutral conditions, in lack of atmospheric stability information.
The expressions of the coefficients for stable and unstable boundary layer can be
found elsewhere (Garratt 1992 ).
Similarity theory predicted that under lowwind conditions the transfer coefficients
of sensible and moisture increase with increasing wind speed, because the increased
roughness facilitates the transfer of heat and vapor (Liu et al. 1979 ). Field and
laboratory measurements suggested that the drag coefficient ( C D ) flattens or decreases
with highwind speed near 22-23 m s 1 (Black et al. 2007 )orat33ms 1 (Powell et al.
2003 ; Donelan et al. 2004 ). Observations also suggest that the transfer coefficient for
water vapor, or the Dalton number, C E is constant with wind speed up to hurricane-
force winds of 33 m s 1 (Black et al. 2007 ).
11.3 Satellite Estimation of Input Parameters
11.3.1 Wind Stress
Winds are measured using active and passive microwave sensors. A scatterometer is
an active microwave sensor. It operates by sending a pulse of electromagnetic energy
and measures the backscatter signal from the sea surface at different azimuth angles.
The return signal is a function of the sea surface roughness, the azimuth angle relative
to the surface wind, and the earth's incidence angle. Surface roughness is a response
of the ocean surface to wind forcing resulting in capillary wave generation. Weather
prediction model provides ancillary information about the wind direction to give both
the direction and the magnitude of the stress (Atlas et al. 2011 ). Scatterometers can
provide global near-surface (at 10-m height) wind speed and direction retrievals.
It should be emphasized that scatterometers measure surface roughness and hence are
measuring the stress ( U U s ) and not the actual atmospheric wind. In the absence of
information about the atmospheric stability, the derived geophysical product is the
wind under neutral conditions (Liu et al. 2010 ).
Seasat is the first satellite mission for ocean monitoring. It carries the first
spaceborne scatterometer - the Seasat-A Scatterometer System (SASS) (Born
et al. 1981 ). Although the mission lasted for only 3 months, it collected a unique
set of synoptic data on ocean winds, waves, temperature, and topography and
served as a proof of concept to follow-on missions that include the Advanced
Microwave Instrument (AMI) on Earth Resource Satellite 1 and 2 (ERS-1 and
ERS-2), NASA Scatterometer (NSCAT) on Advanced Earth Observing Satellite
(ADEOS), SeaWinds on Quick Scatterometer (QuikSCAT), SeaWinds on
Advanced Earth Observing Satellite 2 (ADEOS-2), and Advanced Scatterometer
(ASCAT). These instruments provide almost all-weather wind speed and direction
information except in heavily raining conditions (Freilich and Dunbar 1999 ; Draper