Transcript Surface Exchange Processes

Air-Sea Exchange : 1
SOEE3410 : Lecture 4
Ian Brooks
Interaction with the Ocean
Surface
• Unlike land surfaces, the ocean surface
roughness changes with wind speed.
– Higher winds  larger waves
 rougher surface
• The wave height that can be supported by
a given wind speed is limited by gravity &
the weight of water in the wave, and the
loss of energy to the ocean mixed layer via
turbulent mixing.
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Charnock’s Relation
• A simple parameterization of ocean surface
roughness as a function of the wind stress was
formulated by Henry Charnock (1955).
2
*
u
z0  a
g
where a is a constant (~0.015), and g is
gravitational acceleration (9.8 ms-2).
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We have seen that the surface wind
stress can be parameterized in terms
of a drag coefficient and wind speed
at a given height:
u  CDU
2
*
2
this can be related to the sea-surface
roughness by Charnock’s relation
2
*
u
z0  a
g
The problem of parameterizing the
wind stress over the ocean thus
becomes one of determining the drag
coefficient as a function of variables
that are easily measured or modelled.
CD has most often been
parameterized as a simple function of
wind speed with the assumption that
wind and waves are in equilibrium.
– Waves fully developed
– Wind constant and with long
‘fetch’
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Mean annual wind stress magnitude (N m-2) derived from satellite radar scatterometer measurements.
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Wind-Wave Interaction
• The ocean surface roughness depends on the
wind stress over it. A feedback process exists
between wind speed, stress, and wave height
+
wind speed
+
wind stress
+
wave height
+
-
Energy transfer
to ocean
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• The assumption of a steady
state is an oversimplification
• The rapid motion of synoptic
weather systems means winds
are constantly changing speed
and direction
• Fetch is often limited: e.g. in
flow off-coast
• Time required to achieve
steady state depends on wind
speed
– At low winds (~5 ms-s) steady
state is reached in about 2 to
3 hours.
– At high winds (~15 ms-1) it
may take 24 hours
• Waves generated in one
location propagate to areas
with different wind conditions
– Waves not all generated by
local wind
– Wind and wave directions
NOT necessarily the same
– Direction of mean wind and
wind stress not necessarily the
same!
– Waves of different
wavelengths may have
different orientations and
directions of travel.
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Significant wave height (contoured), wave direction at spectral peak (arrows), wind speed (barbs)
(NOAA OceanModelling Branch)
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• Waves of different
wavelengths interact in a nonlinear fashion, transferring
energy to wavelengths both
higher and lower then the
initial waves.
– Short wavelength (high
frequency) waves break
easily, dissipating energy in
white-capping
– Long wavelength (low
frequency) waves dissipate
very little energy, and thus can
travel far from their point of
origin; known as swell
• One approach is to
parameterize CD as a function
of ‘wave age’.
• The wave age is the ratio
between the friction velocity
and phase-speed of the waves
at the peak in the wave
spectrum.
Transfer of energy to longer
wavelengths shifts the
dominant wavelength with
time. Longer waves travel
faster than short, so that
propagating waves disperse,
becoming separated by
wavelength.
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Evolution of wave spectrum with fetch (km)
80
0.6
52
0.4
37
0.3
0.2
After Hasselmann et al., 1973.
Wave Energy (m2Hz-1)
0.5
20
0.1
9.5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Wave frequency (Hz)
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• The motion of water
associated with waves and
swell on the ocean surface is
felt down to a depth
approximately equal to the
wavelength of the wave. If the
water depth is much greater
than this the waves are known
as deep water waves. Wind
driven waves on the open
ocean are deep water waves.
• Shallow water waves occur
when the water depth is less
than the wavelength. In this
case the waves feel the effect
of the sea bed, and different
physics apply.
• Shallow water waves occur
over the continental shelves
(depths ≤50 m) and near
coasts.
• Wind stress–wave relationship
is different for shallow water
waves.
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LOW WINDS
• Most measurements of fluxes on the atmospheric side of
air-sea interface made in wind speeds of 2-15 ms-1.
• As mean wind  zero, parameterizations based on
mean wind speed will fail.
• Zero mean wind does not necessarily mean no air
motion: localised gusts organised around individual
convective cells (primarily in tropics).
– Local airflow towards base of rising convection cell
– Spatial/temporal average of gusts  zero mean wind
• A gustiness factor needs adding to flux parameterization
for convective conditions.
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HIGH WINDS
• At high winds (>~15 ms-1) measurement becomes
increasing difficult: motion of ships or buoys is severe;
spray wetting of instruments; instruments on fixed towers
risk damage from waves.
• Significant quantities of spray droplets in near-surface air
– Increases drag on wind: Spray droplets accelerate to local wind
speed extracting momentum from near-surface wind. As they fall
back into ocean they transfer this momentum to the ocean
surface layer
– Surface heat flux modified: Evaporation of droplets removes heat
from air above the surface
– Moisture flux modified: Evaporation adds water vapour to air
above surface
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Powell, M. D., P. J. Vickery, and T. A. Reinhold, 2003: Reduced drag coefficient for high wind speeds in
tropical cyclones. Nature, 422, 279-283. doi:10.1038/nature01481.
Very recent measurements have shown that wave height and surface
roughness (and hence CD and u*) do not continue to increase with ever
higher wind speeds, but level off at about 30-40 m s-1 and may start to
decrease again at higher winds. Several explanations for the physical
processes involved have been proposed, but there is not yet full
explanation.
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A small digression…
•
•
•
•
Wind-driven wave of 27.7m (91ft)
measured during hurricane Ivan in
2004.
Measured by US Navy Research
Lab* via pressure sensors on sea
bed – hydrostatic equation relates
pressure to depth of water above.
Computer models of Hurricane
Ivan suggest the largest waves in
region of strongest wind near eyewall may have been up to 40m
(132ft)!
Measured waves larger than
expected  we still don’t fully
understand wind-wave processes.
Image ©BBC
*Wang,
D. W., D. A. Mitchell, W. J. Teague, E. Jarosz, M.
S. Hubert. 2005: Extreme waves under Hurricane Ivan.
Science, vol. 309, issue 5736, 896.
DOI: 10.1126/science.1112509
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RSS Discovery cruise D313
SHIPPING FORECAST ISSUED 0505
THURSDAY 30 NOV 2006
BAILEY
SOUTHERLY STORM 10 TO
HURRICANE FORCE 12.
PHENOMENAL. RAIN OR SQUALLY
SHOWERS. MODERATE,
OCCASIONALLY POOR
Maximum wave height : 17 m
RRS Discovery
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February 2000, the largest waves ever recorded in
the open ocean were measured by the UK Research
Ship RRS Discovery : 29.1m
Sustained 10m winds speeds of 21 m s-1
Significant wave height Hs = 18.5 m
(Hs = 4standard deviation of wave heights)
Again, wave models under-predicted the maximum
wave heights.
Holliday, N. P., M. J. Yelland,R. Pascal, V. R. Swail, P. K. Taylor, C. R. Griffiths,
and E. Kent (2006), Were extreme waves in the Rockall Trough the largest ever
recorded?, Geophys. Res. Lett., 33, L05613, doi:10.1029/2005GL025238.
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Surfactants modify the wave field because they change the
surface tension of the ocean surface. The surfactant film damps
the wave-field – particularly the shortest wavelengths – reducing
the surface roughness.
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Top-Down Turbulence
• Low-level, stratiform clouds
(stratus and stratocumulus)
absorbs both shortwave solar
and longwave infra-red
radiation, and emits longwave
radiation.
• Over the ocean cloud base
temperature is usually close to
the sea-surface temperature
Strong solar
radiation
daytime only
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Up-welling IR is ~constant
day & night. No down-welling
IR at night.
Up- and down-welling
IR are almost equal
below cloud
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Longwave radiative cooling of
cloud top causes cloud-top air
to become more dense, and to
sink downwards, generating
turbulence  convection
driven by cooling at top of BL
instead of heating at bottom.
Over large areas of ocean,
when wind speed is low, this is
the dominant processes
generating turbulence. It is an
important source of turbulence
even for moderate
windspeeds.
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During the day, strong solar heating of the
cloud offsets the longwave cooling. The
cloud deck may become warmer than the air
below, thus stable, and the cloud and subcloud layers become decoupled.
The cloud is then cut off from the source of
water vapour, and may this or break up.
Turbulence is maintained in-cloud by
radiative forcing – the peak of shortwave
heating is below the peak for longwave
cooling, thus destabilizing the upper part of
the cloud and generating turbulence.
Below cloud turbulence is maintained via
mechanical (wind driven) mixing.
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Sensible Heat Flux
• The heat flux depends upon the
difference in temperature
between the air and sea
surface.
• The relevant water temperature
is the skin temperature, not
the bulk water temperature.
• Skin temperature is maintained
very close to bulk temperature
by turbulent mixing within the
ocean surface mixed layer
except when the wind speed
approaches zero.
Latent Heat Flux
• Water surface provides a
uniform source of water vapour.
• The viscous sub-layer of air
adjacent to water surface can
be assumed to be saturated,
and at the temperature of the
water surface.
• Evaporation from surface
requires input of latent heat
from water – cools water
surface slightly. The large heat
capacity of water and turbulent
mixing within ocean mixed layer
limit the temperature drop.
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Solar radiation
Evaporation
Surface
temperature
< 1 mm
~
~1-10 cm
~
Warm layer
~ metres
ocean
Cool skin
residual
mixed layer
• When wind-driven mixing is very
low, solar radiation warms a thin
layer of water near the surface.
This increases local stability and
further suppresses mixing.
• Evaporation of water at the surface
causes evaporative cooling,
producing a cooler ‘skin’ layer at
the top of the warm layer. This is
convectively unstable, promoting
mixing within the warm layer, and
limiting the extent of the cooling in
the skin layer.
Thermocline
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Additional Reading: Air-Sea Fluxes
• SOLAS (Surface Ocean Lower
Atmosphere Study) Science plan:
http://www.uea.ac.uk/env/solas/aboutsolas/organisationaandstructure/sciplanimpstrategy/sciplanis.html
– Focus 2: Exchange processes at the air-sea interface
– Focus 3: Air-sea flux of CO2 and other long-lived
radiatively active gases
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