Transcript Slide 1

CLIMATOLOGY OF AIR-SEA ENERGY EXCHANGE
SW
Sensible heat
LW
Latent heat
CLIMATOLOGY OF AIR-SEA ENERGY EXCHANGE
Ocean surface heat balance:
H = SW - LW - Qh - Qe
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Net heat flux
CLIMATOLOGY OF AIR-SEA ENERGY EXCHANGE
Ideally the globally integrated surface net flux should converge to zero.
Uncertainties of the heat exchange through the ocean bottom and heat
inflow/outflow with rivers and underground water are small.
Major features of the net flux fields:
Spatial patterns are more comparable with surface sensible and
latent fluxes, which are not zonal in contrast to SW and LW
radiation.
How to produce the flux field?
 Individual variable information
 Correction of data, use of metadata
 Selection of schemes
 Computation of fluxes
 Composition of the fields (averaging)
Basic surface meteorological parameters
Sea surface temperature
A general definition of sea surface temperature (SST) is that it is the
water temperature at 1 meter below the sea surface. However, there are
a variety of techniques for measuring this parameter that can potentially yield
different results because different things are actually being measured.
The sea surface skin temperature (SSST) is the temperature that
physically determines the surface heat fluxes. It may be measured
radiometrically from ships and other in situ platforms, and by satellite-borne
radiometers provided the atmospheric effects are properly corrected. The
cooling due to sensible and latent heat fluxes and the longwave emission
occurs at the skin, whereas the shortwave heating is distributed over a
greater depth. Thus, most of the time, the SSST is colder than the water just
beneath the skin, typically by a few tenths °C. This difference increases with
increased surface cooling and decreases with increasing wind speed.
For traditional bulk formulae the transfer coefficients have been
determined with respect to the bulk SST, so for application of these
formulae, this would be a more appropriate temperature to use. For the
bulk formulae which use the transfer coefficients derived from surface
renewal theory the skin temperature is the appropriate value.
Bucket SST
measurements
Infrared radiometer
Ship SST data are obtained mostly from Engine Room Intake
(ERI) thermometers or (about 1/3 of the modern data) from SST
buckets. A small but increasing number of ships use hull contact
sensors which, if carefully calibrated, appear to give the most
consistent SST data (Kent et al., 1993a; Emery et al., 1997).
ERI SST data are warmer under most conditions, on average by 0.35C
although there is significant scatter about this typical value. Bucket
measurements are found only to be biased compared to hull values
during sunny daytime conditions when they gave on average SST
values about 0.3°C warmer. This is more likely due to the buckets
heating on deck prior to use rather than to near surface ocean
heating.
VOS SST correction:
All engine intake samples, when identified, should be
reduced by 0.35C, otherwise - default reduction by 0.2C
Air temperature
For atmospheric temperature and humidity, the most accurate
instrument is a psychrometer (wet and dry bulb thermometer)
whose measurements are based on well-established thermodynamic
theory.
The most critical requirements to attain its potential accuracy are:
 adequate ventilation of air past the sensing elements
(3 - 4 ms-1 flow rate),
 to ensure the full wet bulb depression, and adequate shielding
from solar radiation.
This usually means a double shield with the space between also
ventilated. Basic accuracy depends on the type of sensing element
used; for the familiar sling and Assman psychrometers this is the
precision of the particular mercury-in-glass thermometer, 0.1°C at best.
The exposure of thermometer screens on the VOS varies from good
(e.g. screens hung on stanchions on the outboard rails of either bridge
wing) to very bad (e.g. "the screen is made of brown varnished wood
and fitted to the side of the wheelhouse in the 'porch' of the bridge wing
on the port side").
The poorly exposed sensors are about 0.5°C warm. During the day all
the sensors showed increasingly warm readings with increasing solar
radiation. For the better exposed sensors this bias was up to 2°C; for
the poorly exposed sensors the mean bias reached over 4°C.
Air temperature correction:
Ta = (2.7-0.064urel) SW / 1000
Humidity
The wet and dry bulb psychrometer is the traditional meteorological
instrument for the routine measurement of temperature and humidity.
In general, however, psychrometers are not suitable for continuous
routine measurement of atmospheric humidity at sea in stand-alone
or automatic mode because of their need for attention (e.g. washing
salt from the wick, replenishing the water reservoir).
Dewpoint hygrometers are also based on sound thermodynamic
theory, measuring the temperature at which a film of dew forms on a
cooled mirror, but are generally too complex to serve as operational
instruments. Their main use in air-sea studies is as a reference
standard; accuracy of 0.2°C in dewpoint is readily achievable
(corresponding to 0.2g/kg-1 at about 22°C).
Humidity - more
The VSOP-NA results showed that psychrometers produced lower
(and therefore presumably more accurate) dew point readings
compared to screens. Since the ship may often be a source of
heat but is rarely a significant source of water vapour, shipboard
humidity readings may be of better quality than the temperature
data.
Correction of the humidity (dew point temperature):
For unaspirated screen measurements
Tdew’ = 1.029Tdew - 1.080,
where prime denotes the corrected dew point temperature. Onethird of this correction is applied if no information is available
about the method of humidity measurements
Wind speed
1. Anemometer winds.
In general are considered to be most accurate, if:
 The anemometer is properly installed onboard the ship
 True wind is properly computed from the relative wind
(both requirements normally are not the case)
2. Beaufort estimates of wind
Less accurate, but more homogeneous
Flow distortion by the ship superstructure:
RV Darvin
Laboratory and numerical modeling
can help identify biases in the
measured wind speed. It is difficult
to derive flow distortion effects for
all ships.
RV Knorr
Beaufort wind estimates
Rear-Admiral, Sir Francis Beaufort, Knight
Commander of the Bath, was born in Ireland in
1774. He entered the Royal Navy at the age of 13
and was a midshipman aboard the Aquilon.
Beaufort is said to have had an illustrious career
on the seas and by 1800 had risen to the rank of
Commander. In the summer of 1805 Commander
Beaufort was appointed to the command of the
Woolwich, a 44 gun man-of-war. It was at this time
that he devised his wind force scale. An early
surviving form the scale is replicated below. By
1838 the Beaufort wind force scale was made
mandatory for log entries in all ships of the Royal
Navy. Beaufort last served as Hydrographer to the
Admiralty. He died in 1857 two years after his
retirement.
In 1854 the English and French were
entrenched in fighting at Sevastopool.
The fleets carrying almost all their
winter supplies was struck by an
intense, early winter storm on the
morning of November 14. In response
to the losses and with the hope that
there might be some way to forecast
future storms, the British Admiralty
and the French Marine jointly
sponsored a weather network -- the
ancestor of the World Meteorolgical
Organization -- to provide storm
warnings. And here then is when Sir
Beaufort's scale begins its protean
growth.
Figures to Denote the Force of the Wind
1
Light Air
Or just sufficient to give steerage way.
2
Light Breeze
3
Gentle Breeze
4
Moderate Breeze
Or that in which a
man-of-war with all
sail set, and clean
full would go in
smooth water from.
5
Fresh Breeze
Royals, &c.
Strong Breeze
Single-reefed
topsails and topgal. sail
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7
Moderate Gale
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Fresh Gale
9
Strong Gale
Or that to which a
well-conditioned
man-of-war could
just carry in chase,
full and by.
1 to 2 knots
3 to 4 knots
5 to 6 knots
Double reefed
topsails, jib, &c.
Treble-reefed
topsails &c.
Close-reefed
topsails and
courses.
10
Whole Gale
Or that with which she could scarcely
bear close-reefed main-topsail and
reefed fore-sail.
11
Storm
Or that which would reduce her to
storm staysails.
12
Hurricane
Or that which no canvas could
withstand.
EQUIVALENT SCALES in m/s
Observational height corrections:
Typical observational heights vary on different ships from
the first meters to several tens of meters. On oil and
drilling platforms observational heights can approach 6080 m. Corrections should be applied using the same bulk
parameterizations which are expected to be used for the
flux computations.
Iteration scheme:
Coefficients and
flux – profile
relationships for
“uncorrected”
heights
Correction of winds,
temperatures and
humidity according
to the derived profiles
Re-computation
of the coefficients
and fluxes
Flux averaging and climatological fields:
Averaging of the computed fluxes in the space-time
coordinates may suffer from the number of observations
(inadequate sampling).
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Nature of sampling bias in VOS fluxes: time
dependent biases
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number of reports
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0
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YEARS
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NCEP/NCAR
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net flux, W/m*m
Qh, W/m*m
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VOS
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-100
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days, January 1978
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1960
1965
1970
1975
1980
YEARS
Precipitation:
The use of conventional rain-collecting instruments, designed for land
use, results in uncertainties which are of the same order of magnitude
as the mean precipitation estimates.
 the effect of the flow around the ship’s overall structure which can
lead to undercatch or overcatch depending on the location of gauge;
 the effect of the flow in the close vicinity of the rain-gauge, which
tends to carry the rainabove the orifice of the gauge and leads to a
wind speed dependent undercatch.
Parameterization of precipitation using the weather code information:
Tucker (1961), Dorman and Bourke (1978):
x=1.85 mm, y=5.66 mm, z=8.13 mm (per 3 hours).