AirSea Interaction Measurements From FLIP

1
Air-Sea Interaction Measurements From FLIP Carl
A. Friehe University of California, Irvine
Critical Layer MBL II The simultaneous
measurement of wave height (SIO wave wire),
turbulence (sonic anemometer) and wind profile
(cups) made it possible to examine wind-wave
coupling. The problem is compounded due to the
asynchronousosity of ocean waves. Normal
conditional sampling, as used in laboratory
studies with mechanically-generated waves, is not
possible. Hristov et al. used the Hilbert
transform to obtain the instantaneous wave phase,
and averaged conditionally-sampled wind and
turbulence data. The wind pattern is out of
phase winds are high in the troughs and low at
the crests. Further, the existence of Miles
critical layer was found.
Fred Spiess, Bob Fleagle and Bob Stewart
recognized that FLIPs stability could provide an
atmospheric flux platform for the Barbados
Oceanographic and Meteorological Experiment.
FLIP was towed through the Panama Canal, and Russ
Davis, Carl Gibson, Gil Stegen and others
measured waves, turbulence, fluxes and profiles.
Eric Mollo-Christensen of MIT ran a model of FLIP
in a wind tunnel and determined that the deck
side should face into the wind for least flow
distortion. A tug downwind served as a sea
anchor. Fine-scale air temperature measurements
were anomalous (cold spikes), different from
those over land in the 1968 Kansas experiment,
with distorted spectra and heat fluxes.
FLIP was the centerpiece of the North Pacific
Experiment, where additional observations of
fine-scale temperature measurements revealed the
cause as erroneous fluctuations due to
hygroscopic action of humidity fluctuations on
sea salt aerosols naturally deposited on the
temperature probe surfaces. Simpson and Paulson
reported mid-ocean observations of atmospheric
radiation.
Wave Height, m
Gibson, Stegen and Davis
FLIP with Meteorological Mast
Wind Speed, m/sec
Sync Wave Avg
Phase Wave Avg
After BOMEX, several experiments were performed
locally off San Diego. A new port boom was
constructed that allowed personnel access to the
end. The effect of humidity fluctuations on
refractive index at optical wavelengths was
determined.
Sync Wind Avg
Phase Wind Avg
Marine boundary-layer measurements were made off
of Pt. Conception (MBL I) and Monterey (MBL II)
to study the interaction of wind and waves.
Ultrasonic anemometers for high-fidelity
measurements of u,v, and w had improved and fast
temperature was obtained from sound speed, free
from the salt problem. Rieder and Smith examined
wind stress/wave directionality in MBL I. The
first measurements of the vertical divergence of
wind stress were made in MBL II. Nominally
negligible over land, large divergences were
found when waves were high (significant wave
height gt 3m).
The Hilbert Transform was used to conditionally
sample the vertical component of the turbulent
wind vector relative to the phase of the waves.
The phase shifts by 90 deg at the critical layer.
20-m Accessible Port Boom
Wind (blue) and Wave (red) Directions
Wind Speed (blk) and Sig Wave Hght (red)
Ultrasonic Anemometer on Port Boom
4-m (blu) and 18-m (red) Wind Stress
Phase of the vertical wave-induced velocity
fluctuations at 4m relative to the phase of the
waves. The solid black line is the wind speed
which also marks the separation of wave modes
faster (red) and slower (blue) than the
critical phase speed of the wave.
Rayleigh equation streamlines. Solid line is the
ocean-air interface. Dashed line is Miles
critical layer.
Ratio of 4-m to 18-m Stresses
Air-Sea Interaction research on FLIP has been
supported by the Office of Naval Research and the
National Science Foundation. Clayton Paulson and
Gilbert Stegen provided the photos from BOMEX.
Many others, not mentioned here for lack of
space, have contributed to air-sea research on
FLIP. We would like to thank the many crew
members who have made the FLIP cruises
successful, lead by the late Rich Silva, DeWitt
Efrid (retired), and currently Tom Golfinos.
FLIP is owned by the US Navy and managed by Capt.
Bill Gaines of the Marine Physical Laboratory of
Scripps Institution of Oceanography.