JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. C8, PAGES 18,159-18,185, AUGUST 15, 1999
The relationship between synoptic weather systems and
meteorological forcing on the North Carolina inner shelf
JayA. Austin1
Massachusetts
Institute of Technology/WoodsHole OceanographicInstitution Joint Program in
Physical Oceanography,Woods Hole, Massachusetts
Steven J. Lentz
Department of Physical Oceanography,Woods Hole OceanographicInstitution, Woods Hole,
Massachusetts
Abstract. A strong relationshipis observedbetween synopticweather systemsand
atmosphericforcing of the oceanas estimated from buoy measurementsmade on the
North Carolina inner shelfduring August and October-November1994 as part of the
CoastalOceanProcesses
(COOP) Inner ShelfStudy. Synopticvariation (timescales
of daysto weeks)in the meteorological
time serieswas primarily associatedwith
the passageof atmosphericfrontal systems. The most common synoptic weather
pattern observedwas the passageof a low-pressurecenter to the north of the study
site, which causedthe associatedcold front to pass over the study region. Before
passageof the cold front, warm, moist northeastwardwinds increasedthe heat flux
intothe ocean,whereas
afterthe coldfrontpassed,
cold,dry southwestward
winds
decreasedthe heat flux into the ocean. In addition, in the presenceof oceanic
stratification, northeastward winds drove coastal upwelling, bringing colder water
to the surface, further increasingthe air-sea temperature contrast and hence the
heat flux into the ocean inshore of the surface front between cool upwelled water
and warmer water offshore. The decreasein surface heat flux during the passage
of a coldfront wasof order400W m-2, due primarilyto a decrease
in latent
heat flux. Although other synoptic patterns were observed,including one warm
front passageand two tropical storm systems,the dominanceof cold fronts as a
sourceof variability resultedin a strongpositivecorrelationbetweenthe along-shelf
component of wind stressand the surfaceheat flux. To addressthe issueof spatial
variation in the surface heat fluxes, data from several different sourceslocated along
a cross-shelftransect were analyzed. This analysissuggeststhat the temperature of
the atmospheric boundary layer undergoesadjustment when warm air blows over
cold water but not when cold air blows over warm water. This producescross-shelf
gradients in the bulk estimatesof turbulent heat fluxes during offshorewinds but
not during onshorewinds.
1.
Introduction
estimates from observations taken off the coast of North
Carolina, north of Cape Hatteras during August and
October/November1994 as part of the CoastalOcean
Processes
(COOP)InnerShelfStudy(ISS)fieldprogram
[Burman,1994]. The primary purposeof this paper is
The surfaceheat flux and wind stressplay a crucial
role in determining the behavior of the upper ocean,
especially in the shallow coastal zone, where the entire
water column can be directly influencedby atmospheric
forcing[WinantandBeardsley,
1979;Leeet al., 1989]. to describethe effectof synopticweathersystemson the
This paper presents surface heat flux and wind stress temporal and spatial variation in meteorologicalforcing.
There have been few previousobservationalstudiesof
the surface heat flux over U.S. continental shelves and
even fewer which use direct in situ measurements of the
1Nowat theCollege
ofOceanic
andAtmospheric
Sciences,
OregonState University, Corvallis
Copyright1999bytheAmericanGeophysical
Union.
Papernumber1999JC900016.
014g-O227/99/1999JC900016509.00
radiative fluxes. On the U.S. west coast, Beardsleyet
al. [1998]estimatedsurfacefluxesduringthe Coastal
OceanDynamicsExperiment(CODE) and the Surface
Mixed Layer Experiment (SMILE). They studiedseasonal and synoptic variation during both experiments.
18,159
18,160
AUSTINANDLENTZ:METEOROLOGICAL
FORCINGONTHENCSHELF
They foundthat variationin shortwaveradiationand either sideof the front, they estimateda sharpdecline,
ofthecomsea surfacetemperature(due to upwelling)were the oftheorderof400W m-2, in bulkestimates
across
most important factorsin the seasonalvariationof the binedturbulentheat fluxes(latentand sensible)
net surface heat flux. SMILE was one of the first coastal the composite
front. They alsoobserved
that the highoceanic field experiments to make direct observations est concentrationof cloudslies alongthe front, another
the surfaceheat flux,
of downwardlongwaveradiation.In the SouthAtlantic importantfactorin determining
Bight, Blantonet al. [1989]estimatedturbulentfluxes and that the wind directionchangesduring the passage
of moisture,heat, and momentumas part of the Gen- of the front.
Thepurpose
oftheinterdisciplinary
CoOPInnerShelf
esisof Atlantic Lows Experiment(GALE). They atof the processes
tributed variation in meteorologicalobservations,and Study was to increaseunderstanding
hence in the estimated fluxes, to synopticweather sys- that affect larval distributions over the inner shelf, as
tems. For instance,the passageof a cold-airoutbreak well as to increaseknowledgeof the physicaloceanog-
on January 27, 1986, was responsiblefor an estimated raphy of the inner shelf,a regionof the oceanwhere
decrease
in the surface
heatfluxof nearly1400W m-2. there have been relatively few physicaloceanographic
Mountain et al. [1996]useddata from mooredbuoys studies. The field programtook place betweenAuand coastal stations to estimate the annual cycle and
gustandDecember1994,whichbracketed
the seasonal
interannualvariability of the net surfaceheat flux in the transitionfrom strongstratificationand surfaceheating
Gulf of Maine between 1979 and 1987. In this study, to weak stratification and surfacecooling. The study
annual variation in the shortwave insolation was pri-
site was located on the shallow shelf between Chesa-
marily responsiblefor the annual variability of the net peakeBay and CapeHatteras(Figure1) and included
heat flux. On a larger scale,Bunker[1976]estimated two mooredbuoysinstrumentedwith the meteorologmonthlymean surfacefluxesoverthe entireNorth At- ical sensorsnecessaryto make bulk estimatesof the
lantic Oceanusingdata from over 8 million shipboard surface heat flux and wind stress.
weather observations and discussed heat flux variabilVariability in the meteorologyon timescalesof days
to
weeksduringthe CoOPInner ShelfStudycanbe atity on annual scalesfor the Mid-Atlantic Bight, among
tributed
to three basic scenarios,listed here in order of
other specificregions. Enriquezand Friehe [1997]infrequency
of occurrence:
the passage
of coldfronts,the
vestigatedthe effect of coastalupwellingoff northern
passage
of
tropical
storms,
and
the
passageof warm
California on the stability of the air column, which in
part determinesthe transfer of heat, momentum,and
moisture between atmosphereand ocean. They found
that the changein the transfer coefficientsdue to upwelling affected estimatesof surfacewind stress,but
had a negligibleeffecton the turbulenttransferof heat.
fronts. The responseof the local meteorologicalvariablesto these weather systemswas distinctive. A key
result of this study is that the predominanceof cold
fronts as sourcesof variation and the particular response
of the surface heat flux and wind stressto their passage
leadsto a strong relationshipbetweenwind direction
consideredin the open ocean. The Frontal Air Sea In- and surface heat flux.
teractionExperiment(FASINEX) wasan observational The rest of the paper is organizedas follows.Section
program that studied the effectsof sea surfacetem- 2 describes the instrumentation and methods used to
The effect of fronts on surface fluxes has also been
perature fronts and atmospheric fronts on open ocean
surface heat flux and wind stress variability. During
estimate surface heat flux and wind stress. In section
3, meteorological
time seriesand surfaceflux estimates
FASINEX, Davidsonet al. [1991]observedsharpde- are used to examine the relationship between synoptic
creases in surface heat fluxes and differences in wind
meteorology
and temporalvariationsin surfacefluxes.
directionduring the passageof coldfrontsoverthe open Section4 is a discussionof cross-shelfgradientsin the
ocean southeast of Bermuda, with decreasesof surface air and sea surfacetemperature fields and the implica-
heatfluxof up to 600W m-•' duringindividual
frontal tions this has for the spatial distributionof surfaceheat
passagesobservedduring January-May 1986. Also dur-
flux. Section 5 is a summary.
ing FASINEX, Friehe et al. [1991]studiedthe effectof
sea surfacetemperature fronts on atmosphericbound- 2. Field Program, Methods
ary layer structure, showingthat warm air blowingover
2.1.
The Site
cold water leads to a stable, shallow boundary layer,
while cold air blowing over warm water leads to an unThe CoOP Inner Shelf Study took place offshoreof
stable, growingboundarylayer. Mooerset al. [1976] the North Carolina Outer Banks, between Cape Henry
studied the effects of cold fronts on surface heat flux and
(at the mouth of the ChesapeakeBay) and Cape Hatwind stressusing a compositeof 34 low-pressuresys- teras, at the southernend of the Middle Atlantic Bight
tems observedover the Middle Atlantic Bight between (Figure 1). The coastlineis relatively straightin this
1972 and 1975, for use as an idealizedforcingfield for region with an orientation of approximately 340øT. The
ocean models. This compositelow-pressuresystemin- shelf is approximately 80 km wide and 60m deep at the
cluded a trailing cold front with warm, moist air ahead shelf break, increasingin width to the north. On the
of the front and cold, dry air behind the front. On the western side of the Outer Banks lie Currituck Sound,
basis of the air temperature and moisture content on PamlicoSound,and AlbemarleSound,whichare large,
AUSTIN AND LENTZ' METEOROLOGICALFORCING ON THE NC SHELF
18,161
36.28
CoOP Inner Shelf Study
Central line moorings
36.26
I
ß
36.24
340ø-
'.
!
5 km
--Fd3 (26m)
37
36.22
Cape Henry
z
'o
36.2
•\c,..?,LO?•'.
-I-d2
(21m)
36.5
04401
"+dl (13n:i).
36.18
e44006
Region
of
DItlill
Atlantic
36.16
36.14
35.5
ß10m
'.20m
Ca
36.12
35
76
-75.5
-75
-74.5
I
-75.75
-75.7
-75.65
-75.5
-75.55
-75.6
o
Longitude, E
Figure 1. A plan view of the CoastalOceanProcesses
Inner Shelf Study (COOP ISS) observational site, showingthe study area, and the locationsof CoOP ISS buoys DP, dl, d2, and d3,
and National Data Buoy Center buoys44006, 44019, and 44014. The inset is a regionalview.
-5
-10
E
c.•
m -20
-25
-30
-35
0
5
10
15
20
25
30
35
40
45
Offshore distance, km
Figure 2. Cross-shelfconductivity-temperature-depth
(CTD) sectiontaken from R/V Cape
Hatteras on August 19, 1994, during upwelling-favorablewinds. The section extends 50km
offshorefrom the Field ResearchFacility, perpendicular to the shore. The positions of the three
CoOP ISS mooringsare indicated by vertical dashedlines.
50
18,162
AUSTIN AND LENTZ: METEOROLOGICAL
FORCING ON THE NC SHELF
28
'1•'
""/'""
L
(A)
O 26=24-
•2.tr,.]• ••,1•
•
.I
•'22 -
,
t/.•.'..""l-'
'd.
i't ;;.•; '.
.j.:• ....
':r';
I
,
x
FRF
I ...... NDBC
44006
• i' r
........
d2
I ---
NDBC44019
:'.. •.•,, •z.• ½ {.•.l• - ,L•:•'" .: ,•:' :: '•.
f,•., / •::
:•i..7'
;";,'•• cl"'•:
.,.:'...ß.x-....
•
::
:':•'•i::-: '.x ': x
',..... ;: ,. :............x
,'-;x''.x'' ..
ß
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.'..x
, .',:
' .'ß:.x:..j'•'"
x•x
ß 20-
,
o
,,.
"- "x
,
x
c/) 18-
CTD
section
(Figure
2)
z16I
I
i
Aug 8
Aug13
Aug18
14
Aug 3
I
i
I
Aug23 Aug28
Sep2
Se3
7
28
x
o
0 26
d2
(B)
NDBC 44006
NDBC
44019
=24
•'22
e 20
o
0318
Z16
tl
i
i
i
i
I
I
I
I
i
ct 2 Oct 7 Oct 12 Oct 17 Oct 22 Oct 27 Nov 1 Nov 6 Nov 11 Nov 16Nov 21
Figure 3. Near-surfacewater temperaturesfrom the FRF pier, d2, NDBC buoys 44006 and
44019,for (a) Augustand (b) October.FRF valuesare daily, othersare hourly. Arrow indicates
timing of CTD sectionin figure 2.
shallow inland bodies of water, characterizedby high
to 6m thick, at a depth of approximately10m (Fig(northward)windsbrought
surfacetemperatures
duringsummer[Roelofs
andBum- ure 2). Upwelling-favorable
pus,1953].The mooredobservations
focused
on a cross- the thermoclineto the surface,resultingin cross-shelf
shelf section located offshoreof the Army Corps of En- seasurfacetemperaturedifferences
of up to 8øC (Figgineers'Field ResearchFacility (FRF) in Duck, North ure 3a), which play a significantrole in determining
Carolina.
This section extended
16 km offshore and con-
the spatial distribution of the surface heat flux. Dur-
the watercolumnwasrelatively
sistedof three main mooringsites,dl, d2, and d3, two ing October-November,
of which (d2 and d3) wereinstrumentedwith meteoro- well mixed in temperature(as inferredfrom mooring
logical equipment.
data),andcross-shelf
temperature
differences
weretypically lessthan 1øC (Figure3b).
2.2.
Oceanographic
Setting
The CoOP Inner Shelf Study took place during August and October-November1994, which represented
two very distinct oceanicsettings. During August, the
water columnwascharacterizedby a strongthermocline
with a temperature differenceof 5øC to 9øC, usually 3
2.3.
Instrumentation
and
Other
Data
Sources
Many sourcesof data are utilized in this study to understandthe fluxesat the CoOP ISS site duringAugust
and October-November 1994 and to put these observations into a wider regional and temporal context. The
instrumentation
is summarized
in Table 1.
AUSTIN AND LENTZ: METEOROLOGICAL
FORCING ON THE NC SHELF
18,163
Table 1. Meteorological
Instruments
onthe Vector-Averaging
WindRecorder
andAssociated
Moorings
Parameter
Instrument
Height,m
Accuracy
a
2.7
+0.4 ø
dœ VA WR instruments
Air temperature
Thermistor
YellowSprings
(WS > 5 ms-•)
5 K at 25 øC
Barometric
pressure
Longwave
Radiation
Relative
humidity
Paroscientific
2.7
model216-B-101
pyrgeometer
+0.6 mb
(WS < 20ms-1)
3.3
+5%
2.7
+ 5%
Eppley PIR
Vais ala
Humicap 0062HMP
Insolation
Pyr anometer
Eppley 8-48
3.3
4-5%
Wind direction
Integral vane
2.7
4- I bit
w/vane follower
WHOI/EG2•G
Windspeed
R.M. Young
(5.6ø)
2.7
4-6%b
-1.1
•:0.5 øCb
3-cup anemometer
Water
thermistor
temperature(d2)
Water
Seacat
Other GoOP Buoy Instruments
-2.0
+0.5øCc
-4.0
4-1oCc
Air temperature
Water temperature
Wind speed
10
- 1.5
10
4-1øC
4-1øC
Wind direction
10
4-10 ø
Air temperature
Water temperature
Wind speed
5
-1
5
4-1øC
4-1øC
Wind direction
5
4-10 ø
temperature, dl, d3
Water
Seacat
temperature, DP
NDBC
NDBC
NDBC
44006
NDBC Instrumentsd
4-1 ms -1 or 10%
44O19
4-1 ms -1 or 10%
44O14
2.3.1. Moored instrumentation. Meteorologi- midity, downwardshortwaveand longwaveradiation,
cal instrumentationfor the experimentconsistedof two barometricpressure,
seasurfacetemperature,andwind
buoysequippedwith vectoraveragingwind recorders speed and direction to make bulk estimates of wind
(VAWRs,Table1), locatedat d2 andd3 [Alessiet al., stressand surfaceheat flux. The meteorological
mea1996].Thesesiteswere5 and 16.4kmoffshore,
on the surementsweremadeat heightsof 2.7 to 3.3m (Table
21 and 26-misobaths,respectively.
Only data fromthe 1), and near-surface
watertemperaturemeasurements
d2 VAWR were recovered. The d3 VAWR was lost during Hurricane Gordon on November 18. Each VAWR
recordedmeasurements
of air temperature,relativehu-
were made at a depth of 1.1m. Data were recorded ev-
ery 7.5 min and weresubsequently
block-averaged
into
hourly values. Each buoy was equippedwith an Ar-
18,164
AUSTIN
AND LENTZ:
METEOROLOGICAL
FORCING
ON THE NC SHELF
Table 1. (continued)
Parameter
Instrument
Height,m
Accuracy
a
Air temperature
Water temperature
Wind speed
5
-1
5
•-1øC
•-1 øC
4-1ms -• or 10%
Wind direction
5
4-10 ø
20
19.5
19.5
n/a
n/a
n/a
n/a
FRF
Instruments
Water temperature
bucketthermometer
Air temperature
Wind Speed
Wind direction
YSI thermistor
F420 anemometer,
NWS
Air temperature
R/V CapeHatterasInstruments
R.M. Young41372C
15.25
n/a
Water temperature
YSI 701
2øCe
0
Abbreviationsare WS, wind speed;PIR, precisioninfraredradiometer;WHOI, WoodsHole Oceanographic
Institution;
COOP,CoastalOceanProcesses;
NDBC, National Data Buoy Center;FRF, Field ResearchFacility; n/a, not available;
NWS, National Weather Service.
aValues denote estimated instrument accuracy;manufacturer'sspecificationsunlessotherwisenoted.
bValueis from Welleret al, [1990].
cAlthoughthe sensors
themselves
havegreateraccuracythan shownhere,the valuerepresents
their estimatedaccuracy
as a measure of surface water temperature.
dData are takenfromthe NationalData BuoyCenterwebsite,http://seaboard.ndbc.noaa.gov.
eValue denotesresolution, not accuracy.
gostransmitter to transmit the meteorological
data and
16.4km offshorein 25 m of water. All three moorings
buoy position to shore.
were instrumented
The d2 and d3 meteorologicalbuoysweredeployedon
August 6, and recovery was planned for early December. However, failure of the surface moorings during
severe storms and failure of the d3 Argos transmitter
systemresulted in only 2.5 months of data from the d2
vertical structure of the water column. Hourly water
temperature measurements were made near the end of
the FRF pier at a depth of 4.0 m usinga SeabirdInstru-
site and almost no data from the d3 site. The d2 surface
mooring failed on September4 during a tropical storm
and came ashore with
all of its instrumentation
intact.
The mooring was refurbishedand redeployedon October 4. It failed again on November 18 during Hurricane
Gordon and came ashore. Although the tower with the
VAWR was torn off the buoy as it came ashore, it was
eventually recoveredwith all data intact. This sequence
of events determined the two time periods considered
here, which are designatedthe "August" time period,
0100 UTC on August 7, 1994, to 0000 UTC on September 4, 1994, and the "October" time period, from 1500
UTC on October 4, 1994, to 0800 UTC on November
18, 1994. The d3 surface buoy stayed in place until
Hurricane Gordon on November 18, when it also came
ashore.Unfortunately,the VAWR on the d3 buoy (and
with
thermistors
to determine
the
mentsSeacat(referredto asDP, for "DuckPier"). This
is rather deep for estimating surfacetemperatures, as
the mean temperature difference at dl between 4.6-m
and 1.5-m depth is approximately0.6øC during August
(maximumdifferenceof 3.2øC), sothis measurement
is
most likely an underestimateof the sea surfacetemperature of order 1øC during August. During October, no
data were available at the surface at dl, but the difference between the temperature at 4.6 and 1.1m at d2
was, on average,of the order or 0.02øC (with a maximuminstantaneous
differenceof 1.1øC),suggesting
that
the temperature at 4-m depth is a reasonableproxy for
the surface temperature.
2.3.2. Field Research Facility measurements.
Wind speed and direction, air temperature, barometric pressure,and water temperature measurementshave
been taken almost continuouslysince1982 at the Army
Corpsof Engineer'sFieldResearch
FacilityFRF [Birkehenceall of the meteorological
data from the d3 site) meier et al., 1985]. Wind speedand directionwere
was lost as the buoy came ashore. The d3 Argostransmitter failed on August 12, was repaired on September
1, and failed again on September8. The amount of data
recoveredvia the d3 Argostransmitter was too small to
be useful for this study.
Other measurementsfrom the moored array consisted
of near-surface
(2-m depth)watertemperatureat the dl
site, 1.4 km offshorein 13 m of water, and at the d3 site,
measured using an anemometer located 19.4m above
sealevel at the end of the FRF pier (560m offshore).
Air temperature was measuredin an instrument housing 40m onshoreand appearedto sufferfrom a diurnal
instrument heating problem. Therefore only nighttime
valueswere used. During onshorewinds, nighttime values typically differed from measurementsat d2 by less
than 1øC, suggestingmeasurementerrors of the order
AUSTIN AND LENTZ: METEOROLOGICAL FORCING ON THE NC SHELF
18,165
of IøC, though no actual uncertainty valuesare avail- tum, heat, and moisturefluxes,the appropriateness
of
able. Sea surface temperature was measured using a the bulk formulasof Fairall et al. [1996]cannotbe
bucket thermometer at the end of the pier, once per judged. The uncertaintyof the meanflux estimatesdue
day, typically around 0700 local time. These data were to measurement uncertainty will be discussedin section
highly correlated with buoy measurementsduring the 2.5.
August and October 1994 periods, and were used to
The surfacewind stressis estimatedusinga stabilitygain someperspectiveon the regional seasonaland in- dependent transfer coefficientthat relates the measured
terannual variability, as well as qualitative information wind speed to the wind stress,
on the cross-shelfvariation during the experiment.
T ----pACd(Ua-- u)lu - u•l,
(1)
2.3.3. NDBC buoy array. The National Data
Buoy Center (NDBC) maintainedthree meteorological
where PA is air density, Ca is a stability-dependent
buoys,designations
44006,44019,and44014(Figure1), transfer coefficient, ua is wind velocity measured at a
acrossthe shelfnear the FRF, which provedusefulto
specifiedheight (in this case2.7m), and us is the surthis study. Thesebuoysrecordedhourly wind speed
face velocity of the water. The direction of the wind
anddirection,air temperature,andwatertemperature,
stressis assumedto be the same as the velocity differwith quoted accuracieslisted in Table 1. While insufence vector Ua --Us.
ficient for completeheat flux calculations,these data
The net surfaceheat flux may be consideredthe sum
were used to examine cross-shelf variations in the me-
teorology,estimate sensibleheat flux, and infer latent
heat flux.
2.3.4. Shipboard data.
The R/V CapeHatteras
of four terms [Gill, 1982]:
QTOT-- QSWNE
T + QLWNE
T + QLAT
+ QSEN,(2)
performed
conductivity-temperature-depth
(CTD) tranQSWNET
isnetshortwave
radiation,
QLWNET
is
sectson the North CarolinainnershelfduringAugust where
net longwaveradiation (the differencebetweenupward
andOctober1994aspart of the CoOPInnerShelfStudy
radiation),QLAT is latentheat
[Waldorfet al., 1995,1996].It made16transects
of the anddownwardlongwave
flux, and QSEN is sensibleheat flux. In this paper,
shelfalongthe FRF to d3 centralmooringline in Au-
positive flux values always denote heat flux into the
gust and 20 in October. In addition, cross-shelftran- ocean.
sectsnorth and southof the centralmooringline were
Thenetshortwave
radiation
fluxQSWNET
duetosomadeto quantifyalong-shelf
variationin the hydrogralar
insolation
between
0.28/zm
and
2.8/zm,
is
estimated
phy. During eachcruise,air temperature,surfacewater
temperature, and wind velocity were measuredevery using
15 s. The air temperature measurementssufferedfrom
QSWNET
-- QSWDow
N(1- Ab),
(3)
very low (2øC) resolution,sothe air temperaturemeawhere
QSWDow
N is measured
using
anEppley
8-48
surementsare usedonly for qualitative comparisons.
pyranometer and Ab is the albedo of the sea surface,
which is determinedempirically as a function of the so2.4. Estimation
of the Surface Heat Flux and
Wind
Stress
lar angleand atmospheric
transmissivity[Payne,1972].
Thenetlongwave
radiation
fluxQLWNET
isthedifference
between
the
upward
longwave
radiation
QLWu
P
heatfluxwereestimated
usingbulkformulas
developed
The surface wind stress and the turbulent surface
by Fairallet al. [1996].Likemostotherbulkfluxalgo- due to blackbodyradiation from the oceansurface,cal-
rithms,theseweregenerated
usingmeasurements
made culated usingthe Stefan-Boltzmanlaw, and the downin the open ocean, away from boundariesand fronts.
A comparisonof the heat flux and surface stressesti-
wardlongwave
radiation
QLWDow
N duetoradiation
the Largeand Pond[1981,1982]formulationrevealed
is
from moisture in the atmosphere,and is measureddimatedwiththisformulation
withthoseestimated
using rectly. The formula for the net longwaveradiation flux
no qualitative differences.
The appropriateness of these bulk formulas to the
QLW•:T
--e(QLWDow•
--ate),
(4)
wheree - 0.98 [Dickeyet al., 1994]is the radiativeefof e varyfrom0.93to
fewkilometers
of the shoreline.
Theirsuitabilityis es- ficiencyof the water(estimates
1.0
[Fung
et
al.,
1984]),
rr
=
5.67
x 10-s W m-2K-4 is
peciallyquestionable
duringoffshorewinds,whenthe
the
Stefan-Boltzman
constant,
and
Ts is the seasurface
marineatmospheric
boundarylayermustquicklyadjust
to newsurfaceconditions.
In addition,surfacewaves, temperature
indegrees
Kelvin.
QLWDow
Nisdue
toinsteepenedin shallowwater, may affectthe transferco- fraredradiationin the range3.5 to 50/zmemittedby
moistureandis measured
directlyusingan
efficients
andhence
estimates
ofturbulent
fluxes[Large atmospheric
et al., 1995;Geernaertet al., 1986].It shouldbe noted Eppleyprecision
infraredradiometer(PIR) pyrgeomecoastaloceanis not clear, especiallyto siteswithin a
that neither of these effectshas been taken into account
in the estimatespresentedhere. In the absenceof di-
ter. The downwardlongwaveradiation was corrected
forinstrument
heating
bysubtracting
0.035
QSWDow
N
rect (i.e., eddycorrelation)measurements
of momen- [Alados-Arboledas
et al., 1988].
18,166
AUSTIN AND LENTZ: METEOROLOGICALFORCING ON THE NC SHELF
The latent heat flux QLAT representsthe heat releasedor gained when water evaporatesfrom or condenseson the oceansurface.Althoughoften interpreted
as representingonly evaporation,there is a significant
portionof the Augusttime seriesduringoffshorewinds
when it appearsthat heat was being gained owing to
as possibleusing manufacturer'sspecifications(Table
1) and literature valuesof uncertaintyto providea context for interpreting the surfaceflux estimates.This error analysisrevealswhat terms of the surfaceflux (and
what instruments)contributemost significantlyto uncertainty in the estimate of the total surfaceheat flux
condensation
at the seasurface[Beardsley
et al., 1998]. and wind stress, given what is known about the unThe latent heat flux is related to the moisture gradient
at the ocean surface and the air-water velocity differenceusing the bulk formula
=
certainties of the measurements. For a more thorough
treatment of uncertainty in the VAWR measurements
and how they apply to flux estimates, see Weller et al.
[1990]and Beardsley
et al. [1998].Resultsof the error
- q0)lu - ul,
analysisare summarized in Table 2.
The quoted accuracy of the wind speed measurewhere L is the latent heat of evaporation; qz is the spe- ment is 2%, but there have been indications that cup
cific humidity measuredat height z abovesealevel (in anemometersare prone to overspeedingby as much as
this case,z - 2.7 m); q0 is the estimatedhumidity at
the sea surface, calculated by assumingthe air at the
water surfaceis the same temperature as the water and
6% [Weller et al., 1990]. Usinga nominalvalueof 6%
for the uncertainty of the wind speedestimate and assuming that error in the estimate of wind stressis due
that the air is saturated(assumingthat the saturation primarilyto this measurement
(as opposedto the meahumidity for air over salt water is 0.98 of the saturation
humidity of air over fresh water of the same temper-
surement of surface ocean currents, which are relatively
small),overspeeding
couldbe responsible
for uncertainature); and Ce is a stability-dependent
transfercoeffi- ties in the stressestimatesof up to 12%.
cient,estimatedusingthe Fairall et al. [1996]formulaAn analysis of error propagation in the latent heat
tion.
flux bulk equation suggeststhat the main sourceof in-
The sensible
flux QSENis relatedto the temperature strument error is the measurementof relative humidity,
difference between the air and sea surface and the airassumedto be •- 5% [Weller et al., 1990]. This error
water velocity differenceusing the bulk formula
=
causesan uncertainty in the estimate of the mean latent
heat flux of order 12 W m -2.
The largest sourceof uncertainty in the sensibleheat
- Ts)lu - ul,
whereCpis the heat capacityof water,T$ is the seasur-
flux is the determination
of the difference
between
air
face temperature, TA is the air temperature, and Ch is and surfacewater temperatures. The uncertainty in the
a stability-dependent transfer coefficient,alsoestimated differenceis greatest during strong insolationand weak
winds, when the air temperature sensor suffers from
usingthe Fairall et al. [1996]formulation.
overheatingand the water temperature sensorunderestimates the surfacetemperature owing to formation of
2.5. Estimation
of Instrument-Induced
Flux
stratification in the upper meter of the water column.
Uncertainty
During strong winds, both of these effects are small.
Althoughit is impossible
to takeinto accountall po- Sincethe sensibleflux is proportional to the wind speed,
tential sources
of error inherentin makingsurfaceflux uncertainty in the temperature differenceduring wcak
estimates,it is essentialto make as good an estimate
winds leads to small uncertainties
in the sensible flux.
Table 2. Statisticsof Augustand OctoberTime Seriesof Heat Flux ComponentsMeasuredat d2.
Mean
Aug.
Standard
Oct.
Deviation
Subtidal
Standard
Aug.
Oct.
Aug.
Oct.
QSWNE
T
QLWD
OWN
QLWu
P
QLWNE
T
2204-11
377
4-19
-4224-7
-454-20
1444-7
325
4-16
-3994.5
-744-17
301
27
8
28
222
37
8
37
83
38
31
29
59
39
23
35
QSEN
QLAT
QTOT
54-4
-334-13
1474-26
-154-4
-944-12
-394-22
19
72
317
31
98
258
22
68
135
30
93
138
Deviation. a
Uncertaintiesreflect instrument-baseduncertaintiesonly.
Allvalues
areinWm-2. Subscripts
denote
SWNET,
netshortwave
flux;LWDOWN
, downward
longwave
flux;LWup,
upwardlongwaveflux; LWNET, net longwave
flux;SEN, sensible
heatflux;LAT, latent heatflux; TOT, total heatflux.
aThe subtidal standard deviationrepresentsthe standarddeviationof the low-passedtime series.
AUSTIN AND LENTZ: METEOROLOGICAL FORCING ON THE NC SHELF
18,167
1000 (A)Net .................
•-..t............................. •.. •... •........
500...........
•...........
•............
I
I
I
I
I
II
I
I
I
.
.,
.
•
• •
I
o
50
N
.I
I
I
I
I
II
I
I
...... •...
o .(B) et.longwave
-50
..
-100
....
I
-150
200
,
,
(C)Latent •
-200-'
ß,
100
.
.
,
••
I
.,
,
•
• , ..........,,........... •.....il .-•........
I
I
....
(D)Sensible
......
- 100
1000
500
0
-5OO
0.2 (F)Wind
Stress
[
•
I
I
[
I[
[
I
I[
Aug 8
I
ß
.................
, ....
,,........
.........
I
-0.2 •< •h•re
Aug 3
]
[
Aug13
[ i ,
Aug18
[•
,
Aug23
Aug28
•
I
i,
Sep 2
i
Sep 7
Figure 4. Termsof theheatfluxandthewindstress
fortheAugusttimeperiod,measured
at
d2' (a) netshortwave,
(b) netlongwave,
(c) latent,(d) sensible,
and(e) netsurface
heatflux
(hourly
(solid)
andlowpass
filtered
(dashed))
andwindstress
(lowpassed
andsubsampled
every
6 hours).Thewindstress
is defined
withalong-shelf
upandoffshore
to theright.Thevertical
dashedlinesmark the meteorological
eventslistedin Table4.
With an estimatedtemperaturedifference
uncertainty ationis about7Wm-2 in August. In October,the
of order 1.0øC,muchlargerthan the air temperature uncertainty in the estimation of e is dominant, and the
andwatertemperatureinstrumenterrorsalone,the cor- uncertainty
is around5 W m-2 .
respondinguncertainty in the sensibleflux is of order
The downward longwaveradiation was measureddi12 W m -2.
rectly, and the instrumentuncertaintyis 5%. Beardsley
Upward longwaveradiation is a function of the es- et aL. [1998]comparedrecordsfrom two PIR records
timated seasurfacetemperatureand the emissivityof and founda differenceof 4.2%, but recentstudiessugthe seasurface.Assumingthe uncertaintyin seasur- gest that even this may overestimatethe uncertainty
facetemperaturein Augustis IøC (due primarilyto [Fairall et al., 1999]. Using5% as an estimatein the
near-surfacevertical temperaturegradients;the value uncertaintyof the measurement,the uncertaintyin the
for Octoberis mostlikelysmaller)andthe uncertainty downwardlongwaveflux estimateis of order19W m-•
in the emissivity
to be +0.01 [Dickeyet al., 1994],the in Augustand 16W m-• in October.
total uncertaintyin the meanupwardlongwaveradiIt will be assumed that most of the error in the es-
18,168
AUSTIN AND LENTZ' METEOROLOGICAL FORCING ON THE NC SHELF
-,ooo
"'(A)'sh0rave,
500........
I
,
,
I
o
,
,
I
'•
l',
lOO
'
I
'
I
'
'
I '
(B) Net Lengwave
ß
':
o
'
'1
'
I
] I
I'
•[
'I
'
I
'
I
-100
-200
0
( [
'
-200
' '
I ]
[
I'
'1
'I
I
I
'
I •
' I
' I
,
•
I[
•
I[
'
ß
-400
100
'
'
J
(D)Sensible ,I....
.....
-100
•
/•,
,I
. [.
[
..,,
-ß
I
I
(E) N
500
0
lUx
I
[
I
. .
I
[
I
.
I
[
I
I
ß
'
-500
L
[
i
i
i
I [
I [
o.2j(F)Wind
•tress
,
-0.2J--.......
J
Oct 2
. •//•/.////.
I '/,,/•/I
Oct 7
Oct12
[
:.1....................
I
I
Oct17
I
Oct 22
I[
,
/./VJ[....
I I
Oct 27
[I
,
I.
I I
Nov 1
I I
i
I[
'
.• /.! ./../..I..
. ' ..
, ,
I I"
Nov 6
II
Nov11
i
Nov16
Nov 21
Figure 5. Sameas figure4 but for the Octobertime period.
timation
of shortwave radiation
is in the measurement
3.
Observations
and Results
itself,asopposed
to the altitude-dependent
albedo.The
During the Augustand Octobertime periods,the
uncertaintyin the measurement
for an ungimballed
sen- surfaceheat flux (Table2 and Figure4 (August)and
soris 5% [Wetteret at., 1990],corresponding
to mean Figure5 (October))variedon two distincttimescales:
uncertainties
of 11Wm-2 in Augustand7Wm-2 in diurnal(daily)andsynoptic
(timescales
of 2-7 days).
October.
As the instrumentsthat are primarily responsiblefor
the uncertaintyare differentfor eachterm on the net
surfaceheat flux, the uncertaintiescan be considered
independent. Making this assumption,the estimated
uncertainty in the mean net surface heat flux due to
This study focuseson the synopticvariability,which
is evidentin all of the meteorological
time series(Ta-
ble 3 andFigure6 (August)andFigure7 (October)),
and hencethe surfaceheat flux components.Diurnal
variabilitywas due almostentirelyto the daily shortwaveradiationcycle.This variabilitywasremovedfrom
measurement
uncertainty
isapproximately
26W m-2 in the data usingthe p164lowpass-filter[Beardstey
et at.,
Augustand22W m-2 in October.Thelargestsources1985].However,thereis alsoa largedifference
between
of error, givenwhat is known about the error character- the mean surfaceheat fluxesduring August and Octoistics of the measurements,are the downward longwave ber, presumablyassociatedwith seasonalvariationin
and latent
heat flux estimates.
the meteorology,
whichis discussed
first.
AUSTIN AND LENTZ: METEOROLOGICAL FORCING ON THE NC SHELF
18,169
Table 3. Statisticsof Augustand October-November
Meteorological
Time Series
Mean
Deviation
Oct.
Aug.
23.1
1018
393
86.2
236
4.9
17.1
1019
338
78.6
161
6.2
1.7
3.8
28
8.0
310
2.4
2.16
5.7
36
11.0
234
3.3
21.2
21.7
22.2
23.3
18.0
18.6
1.8
1.7
1.3
0.9
1.4
1.1
Air temperature, øC
Barometric pressure,mbar
Downwardlongwave,W m- •'
Relative humidity, %
Shortwaveradiation, W m- •'
Wind speed,m s-1
WT (DP, 4m depth), øC
WT (dl, 2m depth), øC
WT (d2, 1.1m), øC
WT (d3, 2m), øC
Standard
Aug.
Subtidal
Oct.
Standard
Aug.
Deviation
Oct.
1.5
3.7
25
6.9
89
2.1
2.0
5.6
33
10.6
64
3.1
1.7
-
1.6
-
1.2
0.9
1.4
1.1
WT is water temperature.
30
I
I
I
.
I
25 -(A)
Water Temperature •
Air Temperature
15
I
I
,,
20
•
•'
I
-I
ii
I
i
..
,
,
,
,
,
, •
,
,
,
,
0.015
"'0.01
0.005
1030
I
(C)Barometric
,øressure
I
il
I
'1
,_ 1020
E
1010
I
I
I
I
I
I
I
1000
180
90
0
(D) WindDirecti
,'
'
I
..........
-90
-180
..
5
10
i
i
(E)Wind
Speed
''
i
[[
I
i
I
-ii.•I -•I
ß
i
[
I
4
I
[
[ ./
I
I
[
I
5
i
I
I
[ ,,•.[I
I
'
ß
_
I
I
I
o
Aug 3
Aug 8
Aug 13
Aug 18
Aug 23
Aug 28
Sep 2
Se)
7
Figure 6. Meteorological
time seriesfor August'(a) air andwatertemperature,
(b) specific
humidity,(c) barometricpressure,
(d) wind direction,and (e) windspeed.Wind directionis
defined
suchthat 0ø represents
windsblowing
directlyoffshore
and90ørepresents
windsblowing
alongshelfpoleward.
18,170
AUSTIN AND LENTZ' METEOROLOGICAL FORCING ON THE NC SHELF
30
I
I(A)
I
'
I
I
I
'
I
'
- Water Temperallure
25 L-'.
/ .... - :' '• ..... i.... • .... •..................
i..............
I---- Air/emDeraturE•
I
'
'
,
,
-
'1
,
i
i ..........
i
I
'
I
i ...... i................
20
15
/
0.02 l
i
I
i
I
i
i
I •f •
i I•1 •
i I
i•
I
i
I
i
I
I
i I
I i
I
i
ii
,
,
,
,
I
I
I
I
I
I
I
' .
'1
(B) Specific,Humidity
•
I
0.015
E
"'
0.01
/•i
I
...
0.005
1030
I
,... 1020
E
1010
I
.....
-
I
I'
I
I
i
I
I
I
I
I I
180(D)Wind•l•'rectiol•l •,
90
,
I
I
I.......
- v..-•. .......
/(C) Barometric
Pressure
i
I
' I................
/ ..........................
•
1 ooo I
I
' ',........
I .......
I '
'• '•/'......
/'l
I!
i ..... •'
I
•
[ I
Ii
I k I
,....
I-
I
•
, ....
•
I
-90
-180
.............
i..........................
......
15
(E WindS
10
I
5
0
Oct 2
,
I ,
I , I
¾' •,l,
I,
Oct 7 Oct 12 Oct 17 Oct 22 Oct 27 Nov 1 Nov 8 Nov 11 Nov 18 Nov 21
Figure 7. Sameasfigure6 but for the Octobertime period.
3.1.
Seasonal
Variation
The mean net surface heat flux was 147 W m -2 in
ence. A decreasein relative humidity also contributed
to the increased
latent
heat loss from the ocean.
To put the August and October 1994 observationsin
the
context of the seasonalcycle, 13 year recordsof air
39Wm-2 in October(Table2). About40%of this
temperature and wind data collectedbetween 1982 and
decrease was due to reduction in shortwave radiation.
Comparison of measured shortwave radiation with cal- 1994 and an 11 year record of near-surfacewater tem-
August (positive indicatesflux into the ocean) and-
culatedaverage
clear-sky
radiation(321W m-2 in Au- perature collected between 1984 and 1994 at the FRF
gustand 184Wm-2 in October[U.S.NavalObserva-wereaveragedinto monthlyvalues(Figure8). In gentory, 1978]) indicatesthat the reductionin shortwave eral, August is a period of weak winds and is one of
the warmest months in terms of both air and water,
of incidenceof sunlight,as opposedto increasedcloud with the air temperature 1.1øC warmer, on average,
radiation was due to the seasonalreduction in the angle
cover. About 33% of the decrease in the net surface
heat flux was due to an increase in latent heat flux loss,
than the water temperature(3.1øC warmerin 1994).
October, on the other hand, occursduring the period
associatedprimarily with a decreasein air temperature of most rapid cooling of both the water and air, and
the air temperature is 1.8øC colder than the water temrelative to the decrease in the surface water temperature
(Table3), whichincreases
the specific
humiditydiffer- peratureon average(3.5øC colderin 1994). At the d2
AUSTIN
30
AND LENTZ:
METEOROLOGICAL
.(A)
I.Ai.r.
Temperature
I
I
I
I
I
FORCING
I
•
ON THE NC SHELF
xI
I
x
I
18,171
I
x
20
10
x
•
I
I
I
I
I
Jan Feb Mar Apr May Jun
I
I
I
I
I
I
I
I
I
I
I
I
Jul Aug Sep Oct Nov Dec
I
I
I
I
I
I
(B) Water Temperature
2O
10
Jan Feb Mar Apr May Jun
Jul Aug Sep Oct Nov Dec
10
x
-5
x
x
x
I
-10
I
I
I
I
I
Jan Feb Mar Apr May Jun
10
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
(D) WindSpeexd
'7
•
"
x
•
•
•
6......x..-•
x
•
x
I
x
x
.•.ß-..•
x
I
Jul Aug Sep Oct Nov Dec
•
x
................
•
x
X
Jan Feb Mar Apr May Jun
x
x
•x
•
.....,...........
•
Jul Aug Sep Oct Nov Dec
•i•ure 8. Monthly •ver•es of •R• •rchived d•t• (198•-1994). Solid line represents1994
d•t•. VMuesof (•) •ir temperature(outliersbetweenAugust•nd November•re from firs• year
of op•tio• •d m• •p•s•t
i•st•m•t
•o•), (b) •t•
t•mp•t•,
(c) •i•
minusw•ter temperature,•nd (d) wind speed.
site, the air temperature- water temperaturedifference
was of the same sign but smaller in magnitude during
both months(Table 3). Winds in Octoberare typically
strongerthan in August, as was observedin 1994. The
seasonalcyclesin Figure 8 and the mean heat fluxes
for August and October are consistentwith the analysisof Mid-Atlantic Bight climatologyby •t•r•/•er[1976].
These results suggestthat the differencesin the meteorologicalvariablesbetweenthe August and October
time periods,and hencein the heat flux terms, are due
to seasonalvariations and that 1994 was a typical year
in this sense.
3.2.
Synoptic Variation
The standarddeviationof the low-passednet surface
heatflux wasapproximately
135Wm-2 (Table2) in
boththe Augustand Octobertime series.The largest
contributionsto this variability came from the latent
heat flux and shortwave radiation. Shortwave radiation
variability was largestin August, and the latent heat
18,172
AUSTIN AND LENTZ' METEOROLOGICAL
FORCING ON THE NC SHELF
Table 4. Characteristicsof MeteorologicalEvents
Date
EventType
AT,
ASH x 10a,
øC
Aug. 15
Clouds
WindDirection,
kgm-a
øT
MaximumWinds,
A SHF
ms-x
Wm-2
Aug.16
warmfront
coldfront
-4.5
-4.5
4
yes
no
45 -•-135
180-->0
8
10
-160
Aug. 22
Aug. 29
Sept. 1
Sept. 4
coldfront
coldfront
coldfront
tropicalstorm
-3.5
-4
-4.5
?
-7
-5
-7
-
yes
yes
yes
yes
0 -->-135
30 -->-150
0 -+ -135
-170 -+ -135
9
7
8
16
-300
-250
-370
-320
Oct. 10
Oct. 15
Oct. 26
cold front
tropicalstorm
cold front
-6
-4
-6
-6.5
-5
-6
yes
yes
yes
45 -+ -135
-170 -+-135
0 --> -170
13
15
11
-610
-360
-490
Nov. 1
Nov. 7
cold front
cold front
-4.5
-6.5
-8
-6.5
no
no
45 --> -45
45 -->-135
14
14
-530
-470
Nov. 10
cold front
-6.5
-6.5
yes
30 -->-135
14
-530
4
(+100)
The changes
in air temperature
AT, specific
humidityASH, andsurface
heatfluxASHF, wereestimated
byjudging
the maximumchange
in theseparameters
in a 24 hourwindowaroundthe frontalpassage.
Lowpassfilteredsurface
heat
flux wasusedfor this purpose.Wind directionis definedsuchthat 0ø is directlyoffshore,
90ø is alongshore
poleward.
flux variability was largest in October. The most common source of variability on synoptic timescales was
the passageof atmosphericfronts associatedwith lowpressuresystems. The net surfaceheat flux and the
pattern of variability depend on the track of the lowpressurecenter relative to the study site. During the
CoOP Inner Shelf Study, variability associatedwith
three basic storm tracks was observed.
The most com-
mon pattern of variation was due to low-pressuresystems passingfrom west to east-northeast,to the north
of the study site. In this case,a trailing cold front associated with the low-pressuresystem passedover the
and 5 times during the Octobertime period (October
10 and 26 and November1, 7 and 10). The secondpattern was associatedwith tropical storms, low-pressure
systemsmoving north to the east of the site, which occurred on September 4 and October 15. The third pattern consistedof the passageof a warm front associated
with a low-pressurecenter developingover the southeastern United States and moving north, to the west
of the site, which occurredonce (August 18). Each of
these cases had a distinct pattern of variation in the
meteorology,surfaceheat flux, and wind stress. Table
4 containsbasiccharacteristicsof eachsynopticweather
study site. This occurred 4 times during the August event. Historical data from the FRF suggestthat the
time period (August15, 22, and 29, and September1), number of low-pressuresystemsobservedduring 1994
••
!
•
/
•
Direction
of
propaganon
I -:e:.•<.•.•.•....•....:...........•
[.........
i•'•:'"p-:-'.::,.'•-,•i•i•i.-'•
...........
'.-'.:,.:•eii!•:•-':::•'•-*•..'..'•i•
............
::""'•:':""""•
Front
Cold,
dry
air
/
''.
alr•;•2•3i•:.
.,• .
arm,moist
'
ColdFro?
k Cloud
Band
Figure 9. Schematic of a low-pressuresystem and associatedfronts, plan view.
AUSTIN
AND LENTZ'
25
METEOROLOGICAL
FORCING
ON THE NC SHELF
18,173
WaterT"emP
iAir
Temp /5
20
15
10
0.02
o
Spebific
Hun;lidity
i'..•
0.01
0.015
-100
-2OO
0.005
0
1040
:
..
:
I Bar(•metric
Pressure'
1030 .........
ß........
'.............
: ........
E 1020
-200
-400
100
1010
-100
180
o
•o
90
=
5
500
0
; .;
•Net
Heal
Flux
l
-90
0
-180
-500
20
0.5
15
e10
E
-0.5
Nov 5
Nov 6 Nov 7
Nov 8 Nov 9 Nov 10
Nov 5
Nov 6
Nov 7
Nov 8
Nov 9 Nov 10
Figure 10. Time seriesof meteorology
and surfaceheat flux termsfor the passage
of a cold
front
on November
7.
was not unusual for these time periods, with typically tion during the passageof the front. The structure
three to six low-pressureeventsoccurringin the August of the pressurefield results in an increasein magnitime periodsbetween1982 and 1994 and five to nine oc- tude and changein directionof the wind during the
of a coldfront. During the CoOP Inner Shelf
curring in the October time periods. A descriptionof passage
Study, the wind directiontended to changefrom pre-
each of the three cases follows.
Cold fronts are regions of strong temperature gra- dominantlynortheastwardbefore the passageof the
dient, usually characterized by large spatial variations front to southwestwardafter the passageof the front,
in air temperature, specifichumidity, and wind speed since surface winds blow primarily 20ø to 50ø to the
and direction(Figure9) [Willeft and Sanders,1959]. left of isobars. The change in wind orientation correwinds leadingthe front
As cold, dry air movesfrom west to east, it displaces spondsto upwelling-favorable
windsfollowingthe front.
warm, moist air upward, often creating a cloudline due and downwelling-favorable
This pattern resultsin a dramatic changein the surto adiabatic cooling. During the passageof a front,
face
heat flux during the passageof a coldfront. Warm,
the local changein temperature and humidity usually
moistair precedingthe coldfront (and, in August,cool
pressureaccompaniesthe rapid changein wind direc- nearshoresurfacewater temperaturesdue to upwelling)
occurs in 6 hours or less.
A minimum
in barometric
18,174
AUSTIN AND LENTZ: METEOROLOGICAL FORCING ON THE NC SHELF
Figure 11. Synoptic meteorologymap for 1200 UT, November6. The cold front extending
south from the low-pressurecenter over the Great Lakes passedover the study site on November
7 [fromNational Oceanicand AtmosphericAdministration1994].
leadsto positivelatent and sensibleheat fluxesinto the
ocean, as well as lessupward longwaveradiation loss.
After a frontal passage,colder,drier air resultsin large
sensibleand latent heat lossesfrom the ocean. Strong
winds in the vicinity of the front intensify the varia-
ward, the air temperature dropped about 6øC, and the
tion in the sensible and latent heat fluxes. In addition,
the presenceof clouds behind the front often result in
imately350W m-2 andthe sensible
heatfluxdropped
80Wm-2. Therewasnot muchof an impactdueto
a dramatic
cloudsduring this particular passage,althoughthe few
days precedingthe frontal passageshow some cloudi-
reduction
in the amount
of shortwave
radi-
ation on the day following the frontal passage,further
intensifyingthe decreasein heat flux followingthe front.
However, if the cloudsassociatedwith the front passat
night, they have no impact on the shortwavesignal.
To illustrate the influence of a cold front passageon
the surfacefluxes, the passageof a cold front on Novem-
specifichumiditydroppedabout0.006kgm-3. Most of
the changein the turbulent fluxestook placein approximately 5 hours. Over a 1-daylongperiodbracketingthe
frontal passage,the latent heat flux decreasedapprox-
ness in the shortwave
radiation
time
series.
The
net
longwaveradiationinitially increasedslightlyduringthe
passageof the front, possiblyowing to increasedcloud
cover,but then decreasedon November7 owingto clear
skies and drier air.
As each heat flux term decreased
ber 7 is examinedin detail (Figuresl0 and ll). As the during the passageof the front, there was a net change
low-pressurecenter moved to the northeast, the cold
front passedover the study region, resultingin a significant changein the local meteorology.First, a drop in
pressureon November 6 precededthe oncomingcold
front. As the front passed the study site, the wind
changed direction from northeastwardto southwest-
in thetotal surfaceheatfluxof nearly500W m-2 overa
period of a few hours. This changewasrepresentativeof
the other cold front passageeventsobservedduringthe
field program, which causedthe net surface heat flux
to dropbetween
160and600W m-2 (Table4). In each
case,the largest contributionto this changewas a de-
AUSTIN AND LENTZ: METEOROLOGICAL FORCING ON THE NC SHELF
251
i
;
.'.W.
ater
T"emp
I
20
1000
ß
18,175
:
;ShortwaVe
•E 500
15
0
Net Longwave
10
0.02
•
0.015
E
-50
• -100
E 0.01
-150
0.005
0
1040
1030
•: -200
........
ß
•arome•ric
PresSure
: ......
:.......
• ........
:.......
-4OO
E 1020
1010
lOO
.......................................
;
180
:
ß
'
:
-lOO
;
:Wind Dir.ection
ß
90
ß
-. '
ß
.
.
.
.
5OO
ß
-180
-50O
2O
0.5
10
•
E
Z
5
ß
.
-0.5
0
0ct13
0ct14
0ct15
0ct16
0ct17
0ct18
0ct130c•140ct150c;160c•170ct18
Figure 12. Time seriesof meteorologyand surfaceheat flux terms for the October 15 event.
crease in latent heat flux, which dropped, on average,
thepassage
ofa low-pressure
centerfromsouthto north,
about150Wm-2 in Augustand300Wm-2 in Octo- eastof the site. In addition,cloudspreceded
the pres-
ber. The magnitude of the observedchangesin surface sureminimumfor 2 days,and windsgraduallybuilt to
heat flux are consistentwith the changesestimated by 15m s-1 asthe lowpassed,
with the mostintensewinds
Mooers
et al. [1976](oftheorderof 400W m-2) and beingconcurrent
with the lowestpressure.
The specific
with the open-oceanvalues observedby Davidson et al.
[1991]ofupto 600W m-2 duringfrontalpassages.
humidityslowlydroppedof the orderof 0.005kgm-3,
andthe air temperaturedropped4øC.Owingto the de-
An event occurringon October 15 is presentednext as
an example of the influence of the passageof a tropical
creasesin air temperatureand humidity,the latent flux
droppedapproximately
100W m-2 andthe sensible
flux
storm (Figures12 and 13). A low-pressure
centerdevel- dropped50W m-2. The shortwaveflux decreased
on
oped in the South Atlantic Bight and eventually moved October14 ascloudcoverincreased,
andthe net longnorth, to the east of the study site. In this case, the waveflux dropped75W m-2 asthe cloudcovercleared
site was never in the "warm sector" of the low-pressure on October 15. Overall, the net surfaceheat flux de-
system,and no frontspassedoverthe regionduringthe creased
approximately
200Wm-2 duringthe passage
event. Consequently,
the observedwindsshiftedslowly of the tropical storm.
from southwestwardto southeastward,consistentwith
In the third scenario,whichbeganon August16
18,176
AUSTIN AND LENTZ: METEOROLOGICAL FORCING ON THE NC SHELF
..
o
Figure 13. Synopticmeteorologymap for 1200UT, October15. The tropical storm offshoreof
CapeHatteraspassedto the eastof the studysiteon October15 [fromN0,4,4 1994].
(Figures 14 and 15), a low-pressurecenter developed 3.3.
over the southern United States and moved north, to
the west of the study site. The associatedwarm front
passedover the study site. This passagewas marked
by a 4øCincrease
in air temperature
anda 0.004kgm-3
increasein specifichumidity. In addition, cloudsassociated with the low-pressuresystem decreasedthe shortwaveflux by approximately50% for the 2 daysfollowing
the frontal passage.Winds associatedwith this system
slowly changed from southwestwardto northeastward
and included the strongest upwelling-favorablewinds
observed during the field program. Starting on August 17, the upwelling-favorablewinds decreasedthe
near-surfacewater temperature 2.5øC by bringing cold,
underlying water to the surface,further increasingthe
temperature contrast between the air and the sea. The
latent flux increased 100Wm -2 and the sensibleflux
increased50 W m -2. Clouds associatedwith the warm
front offset these gains to a certain extent, resulting in
an increase in the low-passednet flux of the order of
100 Wm -•'.
Implications
for the Heat Balance
An interesting consequenceof the subtidal surface
heat flux variability being dominated by the passageof
cold fronts is a strong correlationbetweenthe net sur-
face heat flux and the along-shelfwind stress(Figure
16). The correlation betweenthe subtidal along-shelf
wind
stress and the subtidal
heat
flux is 0.78 in Au-
gust and 0.72 in October, both significantat the 95%
level. This correlation is not causal but simply a consequenceof the structure and orientation of the cold
fronts, which dominate the variation in both the wind
stressfield and the surfaceheat flux. Specifically,the
leadingedgeof cold frontsin this regionare character-
izedbypoleward(upwelling
favorable)windsandstrong
positiveheat fluxes,whereasthe trailing edgeis characterizedby equatorward(downwelling
favorable)winds
and negative heat fluxes. The correlation between the
surfaceheat flux and the alongshelfwind stresshas
the opposite sign of the correlation betweenthe wind
stressand wind-drivenadvectiveheating and cooling
AUSTINANDLENTZ:METEOROLOGICAL
FORCINGON THE NC SHELF
28
26
[k•
24
o
22
'
;
18,177
lOOO
-; ,Water."
Temp
/
ß.:.....
-•-Air-T-•.mp
---•
E
500
20
18
IO0
Netlongwave
J
0.025
Specific
H•umidity
•
E
0.02
-1(•0
'" 0.015
ß
.
0.01
lOO
^
ß
Lat(•nt
1030
1025
-lOO
ß
,
i
i
.a 1020
E
1015
lOO
1010
o
180
-lOO
90
lOOO
-90
•E 500
-180
20
15
'7
•10
E
Aug15 Aug16 Aug17 Aug18 Aug19 Aug20
Aug15 Aug16 Aug17 Aug18 Aug19 Aug 20
Figure 14. Time seriesof meteorology
and surfaceheat flux termsfor the August18 event.
of the shelf, where wind-driven upwelling tends to cool considerablyover the inner shelf. Very little data from
the shelf and downwellingwarms the shelf. This sug- the d3 site were recovered, and only d2 provided us
gests that, depending on the relative strength of the with complete zneteorologicalcoverageduring August
surface heat flux and the wind-driven
advective heat
and October. However, other sourcesof data can be
flux, the heat balance could have two very distinct de- used to estimate the variation in some of the meteopendences on the wind stress. This is considered in rologicalfields, namely, wind velocity, air temperature,
and near-surfacewater temperature. Relative humidity
detail by Austin [thisissue].
was measuredonly at the d2 site, making reliable esti4. Spatial Variation in the Surface Heat mates of latent heat flux at any of the other sites impossible. However, the latent and sensiblefluxes were
Flux
highly correlatedat the d2 site (correlationcoefficient
One of the initial goalsof the moored experiment was 0.82 in August, 0.92 in October), suggestingthat the
to estimate cross-shelfvariation in the surfaceheat flux, sensible heat flux can be used to infer the total turbuas both meteorological and oceanic properties can vary lent heatflux ((•SEN+ (•LAT) at thislocation.No esti-
18,178
AUSTIN AND LENTZ: METEOROLOGICAL FORCING ON THE NC SHELF
7:00
Figure 15. Synoptic meteorologymap for 1200 UT, August 17. The warm front extending
east of the low-pressurecenter over the southeastUnited States passedover the study region on
August17 [fromNOAA 1994].
mates of the cross-shelfgradientsof downwardradiative array of NDBC buoys (includingNDBC 41001, 44004,
flux can be made, as these •vere measuredexclusively CHLV2, and DSLN7, not presentedhere) suggestsa
at d2.
decorrelationlength scalefor the wind field of the order
4.1.
Wind
Field
Wind velocity was measured at all of the NDBC
buoys, the d2 buoy, and the FRF, for a total of five
sites. To facilitate comparison, all of the wind velocities were adjusted to a nominal height of 5 m using a
neutral stability wind profile. The principal axes of the
wind at all of the siteswere approximately40ø counter-
of 600km, whichis consistentwith Weller et aL [1991],
who estimated the scale of synoptic weather systems
to be 500-1000km.
Therefore
the wind field did not
vary appreciably over the spatial scalesof order 100 km
consideredin this analysis.
4.2.
Water
Temperature
Field
Near-surface water temperature measurementswere
clockwise
from directlyoffshore(20øT) (Table 5). Ow- made at seven locations along central line, specifically,
ing to the polarization of the wind field, offshore and DP (near the end of the FRF pier), dl, d2, d3, NDBC
poleward(upwellingfavorable)windsoccurredconcur- 44006, and NDBC 44019, four of which are displayed
rently, as did onshoreand equatorward(downwelling in Figure 3. Most of the observedcross-shelfvariation
favorable)winds. The magnitudeof the major axis at in the near-surfacewater temperature occurred during
eachof the siteswasapproximately
5.5m s-t in August upwelling-favorablewinds in August, when the ther-
and6.0ms-t in October(Table5).
mocline shoaled and created cross-shelftemperature
Although the observedwind wasslightlystrongeroff- differencesof up to 8ø. During non-upwellingcondishore,the differencewas relativelysmall (10% - 20%) tions, the cross-shelfvariation in temperature was rarely
compared to the means. An investigation of a larger more than 1øC, with the water farther offshoreslightly
AUSTIN AND LENTZ' METEOROLOGICAL
500
FORCING ON THE NC SHELF
18,179
,
400 -
x
ß
300 -
ß
x
x
-
.X
X
200-
x
•< •<•<
x•Xx
ix•O
•
x
-
x'x xxo
100 -
o
0 .........
o • 5'ø'6 ............
0 -100 -
oo
o
-200
Xo
O
-
xo
-
o
o
-300-
x
o
x August o October
-400
-
o
-
O
-500
-0.4
'
-0.2
S
Nm-2
•ALONG'
0
0.2
Figure 16. Dailyvalues
of ne•surface
hea•fluxasa function
of •healong-shelf
winds•ress
component,from low passfiltered •ime series.
warmer. During the August time period, the thermo-
surements[Alessiet al., 1996]to moveoffshoreof d3,
gradients in air temperature appeared to be due primarily to adjustment in the marine atmospheric boundary layer and were different during onshoreand offshore
15.5 km offshore.
winds(Figure 17). Cross-shelf
differences
in air temper-
During October and November,the only large crossshelf gradients in surface water temperature occurred
during an intrusion of warm, salty water and subsequent
mixing event, which commencedon November 2. During this event, the surfacewater temperature at NDBC
44019, 54km offshore, attained a maximum along the
central line but was still, at most, 2.5øC warmer than
ature were generally large during offshore winds and
small during onshore winds. During offshorewinds,
large cross-shelfdifferencesoccurredwithin the closest
cline was never observed in moored
water
column
mea-
34km
of shore.
Offshore
winds tended
to be concur-
rent with upwelling-favorablewinds in August, during
which the air-water temperature contrast at the FRF
was as large as 10øC. The air temperature difference
surface water at buoys closer to shore.
between the FRF and d2 was a strong function of the
air-water temperature contrast during offshorewinds
4.3. Air Temperature
Field
(Figure 17). When warm air blew over coldwater, the
The air temperature wasmeasuredat FRF, d2, NDBC air temperature decreasedwith distance offshore, con44006, NDBC 44019, and NDBC 44014. Cross-shelf sistent with the coolingof air in contact with the cool
Table 5. Summary of Wind Measurement Statistics
Location
FRF
d2
44006
44019
44014
Offshore
Mean Speed,ms- •
Distance, km
Aug.
0.5
5.3
34
54
92
5.0
4.9
5.2
5.8
5.8
Oct.
5.8
6.2
6.3
6.7
6.7
Orientation
Aug.
Oct.
44
39
42
39
42
49
33
43
42
42
Major Axis,m s- •
Minor Axis,m s-1
Aug.
Oct.
Aug.
Oct.
5.0
5.4
5.6
5.9
6.3
6.4
2.2
2.2
2.8
2.9
3.1
2.8
3.0
3.2
3.6
3.9
4.9
5.0
5.7
5.8
All wind data were adjustedto 5-m height usingneutral stability assumption.
18,180
AUSTIN AND LENTZ' METEOROLOGICAL FORCING ON THE NC SHELF
+
+
'
.•-
3
Offshore wind
* Onshore wind
:
:
:
.............. .+._•
...........
+
4-•-,½'
..........................................
-3
-6
'
•...+++.............................
I
-4
-2
0
2
4
6
o
AT-WT, d2, C
Figure 17. Cross-shelfair temperature difference as a function of the air-water temperature
difference,from both August and October. Data points representmidnight values,in order to
avoid potentially inaccurate daytime FRF values.
sea surface. During onshorewinds, the air temperature enceis smallerthan 2øC.Data fromthe R/V CapeHatrarely changedmore than 1øC betweend2 and the FRF. teras suggestthat the air-water temperature difference,
averagedover time and the along-shelfdimension,de4.4. Implications for Meteorological Forcing
creaseswith distanceoffshoreduring offshorewindsand
As the turbulent heat fluxesdependon the difference remainsrelativelyconstant(and small) duringonshore
between the air and water temperature, the cross-shelf winds (Figure 19), consistentwith the buoy analysis.
adjustmentscaleduring offshorewinds
structure of this quantity is of interest. Both the air and The cross-shelf
water temperature cross-shelfgradients were functions appears to be of the order of 10 km. As the resolution of
of the wind direction, and the air-water temperature the R/V CapeHatterasair temperaturemeasurements
was 2øC, this result is qualitative.
contrast will be viewed from that perspective.
During October, the air-water temperaturedifference
During August, the greatest differencebetweenair
andwatertemperature(Figure18) occurredat the pier, stays constantacrossthe shelf, regardlessof the direcoccurduring
with differencesof up to 10øC during strongoffshore, tion of the wind. The largestdifferences
poleward(upwelling-favorable)
winds. This difference onshorewinds, with air-water temperature differences
dropped to -1 øC during strong onshore,equatorward of up to 8øC (Figure18). Buoydata fromfarthernorth
(downwellingfavorable)winds,showinga clear asym- (NDBC 44008) suggestthat both air and water are
muchcoldernorth of the site. The air temperaturedoes
temperaturedifferencesare due primarily to upwelling not appear to be approachingthe water temperature,
metry between onshore and offshore winds. The water
fronts (duringupwellingfavorablewinds). The mag- regardless
of the directionof the wind.Duringonshore
nitude of the air-water temperature differencedrops
steadily as a function of offshoredistanceas the air temperature adjusts to the surfacewater temperature until
NDBC 44019, 55 km offshore,beyondwhich the differ-
winds,this may be due to the fact that as coolair blows
over warm water, the vertical densitystructureof the
air is unstable,andthe boundarylayerquicklybecomes
so thick that sensibleheat flux into the bottom of the
AUSTIN
12
AND LENTZ:
METEOROLOGICAL
FORCING
ON THE NC SHELF
18,181
i
lO
8
6
ø¸ 4
-2
-4
-6
-8
Aug 3
i
i
Aug 8
Aug 13
Aug18
Aug 23
Aug 28
Sep 2
Sep 7
12
58ct
2
I
I
I
Oct 12
Oct 22
Nov 1
Nov 11
Nov 21
Figure 18. Low passedAT-WT time seriesfrom DP, d2, NDBC 44006, and NDBC 44019 for
August(upperpanel) and October(lowerpanel).
mately 0.16 in August and 0.23 in October. Literature
atmosphericboundarylayer makeslittle differenceto
the surfacetemperature. In contrast, in August, the
water was colder than the air, which produced a sta-
valuesrangefrom 0.1 (low latitudes)to 0.45 (high latitudes)[Perry and Walker,1977].Althoughthereis no
ble air column and a much thinner boundary layer, so
reason to believe that the Bowen ratio is constant
across
that the differencein temperaturechangedwith offshore the shelf, it allowsan order of magnitudeestimate of the
distance. Without vertical profiles of air temperature,
however,this hypothesiscannotbe verified.
The spatial structure of the air-water temperature
differenceallowsus to speculateas to the structureand
sizeof the cross-shelf
gradientin the turbulentheatflux.
spatial variation of the latent flux to be made in the absenceof humidity measurements. The validity of this
assumptioncannot be tested without offshorehumidity
measurements. The upward longwaveradiation term is
a function of the surfacewater temperaturealone (see
Enoughinformationis presentto estimatethe sensible (4)) and can be estimatedat thesesites. Downward
flux at the FRF, d2, NDBC 44006, and NDBC 44019. longwave and shortwave radiation was measured only
The Bowenratio [Lewis,1995],definedas the ratio of
the sensibleto latent flux, can be usedto estimate the
at d2 and will be assumed to be constant
over the shelf.
The cross-shelfvariation in the net heat flux, given
(Figure20), showsa markeddifgradientin the latent heat flux acrossthe shelf. The the aboveassumptions
Bowen ratio at the d2 site was observedto be approxi-
ferencebetween the August and October time periods.
18,182
AUSTIN AND LENTZ: METEOROLOGICAL
7
FORCING ON THE NC SHELF
....................
Offshore
Onshore
-
40
45
5
? 4
• 3ii
2
0
-1
-2
0
5
10
15
20
25
30
35
cross-shore distance, km
Figure 19. Air-water temperaturedifferenceas a functionof cross-shelf
distancefor onshore
andoffshore
winds,fromtheR/V CapeHatteras
underway
data. Dataareaveraged
overtime
and the along-shelfdimension,from sectionswithin 30 km north or southof the central line.
During August, the total instantaneouscross-shelfdifference is often large comparedto the temporal varia-
during offshore(poleward,upwellingfavorable)winds,
the temperature contrast between the air and the wa-
tionsobserved
at d2, up to 400Wm-2 duringoffshoreter was greatestonshoreand decreasedoffshore,since
(upwelling) winds, but much smaller during onshore the air was warmer onshore and the water colder on(downwelling)winds. In the Octobertime period, the shore. This difference
wasdue to watertemperature
meteorological fields exhibit less cross-shelfvariation, gradientscausedby upwellingand to the adjustment
and this is reflected in the relatively small differences of the air temperature
to the surfacewatertemperain the estimated
fluxes across the shelf. Table 6 is a
ture. In this case, spatial variation acrossthe shelf in
summary of the mean flux values in the 2 months.
the surfaceheatfluxcanbe at leastaslargeasthe subTo summarize, in August, there were weak cross- diurnaltemporalvariation. During the Octobertime
shelf gradients in the turbulent fluxes during onshore period, the cross-shelf
gradientin the fluxeswas small.
(equatorward,downwellingfavorable)winds,whereas Thisspatialvariationin the surface
heatfluxmayhave
Table
6. Estimated
Mean
Turbulent
Fluxes
(QSEN
andQLAT)
and
Upward
Longwave
Radiation
QLWu
Pat
SitesAlong Central Array.
Location
FRF
d2
NDBC 44006
NDBC 44019
QSEN
QLAT
• QSEN/Ba
Aug.
Oct.
Aug.
12
5
-2
-5
-16
-15
-15
-11
75
-33c
-12
-31
Oct.
-70
-94c
-65
-48
QLWi•
P
Q%OT
Oct.
Aug.
Oct.
Aug.
-423
-431
-439
-444
-408
-408
-411
-408
261
147
144
117
-25
-19
-22
2
Allvalues
areinWm-•'. Subscripts
denote
SEN,sensible
heatflux;LAT,latent
heatflux;LWvpupward
longwave
radiation; TOT, total.
aB --0.16 in August, 0.23 in October.
bTotalwascomputed
usingdownward
longwave
andshortwave
fromd2.
CLatentvaluesfor d2 are frombulk formula,not Bowenratio.
AUSTIN
AND LENTZ:
METEOROLOGICAL
FORCING
ON THE NC SHELF
18,185
6OO
400 ....
200
-'
I
0
-200
-
-400
....
-6RuOg
3 Aug 8
Aug13
Aug18
600
Aug23
•
Aug28
•
Se2
Sep 2
7
i
ß
FRFJ
........
......
400
ß
d2
NDBC 44006'
.
ß
2OO
I
...., ..............
I'•...............
I
__•
: ••
I
,,•
:.-'•',,,
.......................
.
,'•,•,:,,-•,,
-
NDBC
44019J
,.......... t• ........ i..................
•:1
,,'./
:
.•..,?• ..,,'..'•! :
,.4
. .'.:"'
0
-200
i..1 !?
-400
.................:":-......•.-•
..................V.
-6•0ct
2
Oct 12
Oct 22
' '!.............'""•i.;i.........
Nov
1
Nov 11
Nov 21
Figure 20. Time seriesof net surfaceheat flux computed at the FRF, d2, NDBC 44006 and
NDBC 44019. Assumesa Bowenratio of 0.16 in Augustand 0.23 in October(from d2 data) and
no gradient in the downward shortwaveor longwaveradiation.
implications for the heat balance of the region, neces- led to a strong correlation betweenwind direction and
sitating more intensivemeasurementsof meteorological the estimated surface heat flux. When the water was
conditionsto better understandthe heat budget of the vertically thermally stratified, the correlation between
inner shelf.
the wind direction and the air-water temperature difference was enhancedby upwelling-favorablewinds bringing coldwater to the surface,further increasingthe heat
5. Conclusions
flux into the ocean. A strong cross-shelfgradient in the
Data collectedin Augustand October-November1994 surfaceheat flux is postulated during offshore,poleward
were usedto estimate atmosphericforcingon the North windsin August, owingto observedcross-shelf
gradients
Carolina inner shelf north of Cape Hatteras. Most of in air and water temperature. Cross-shelfgradients at
the variation at the synoptic scale can be attributed other times, including all of the October time period,
to one of three types of events: cold fronts, which oc- appear to be much smaller.
curred a total of nine times during the two time periods;
Acknowledgments.
The moored meteorological
tropical storms, which occurred twice; and one warm
front.
All of these events were associated with lowand oceanographicmeasurementsutilized in this papressuresynoptic weather systems. The structure and per were funded by National ScienceFoundationgrant
orientation of the cold fronts, the most common event, OCE-92-21615. Other data were kindly provided by
18,184
AUSTIN AND LENTZ: METEOROLOGICAL
FORCING ON THE NC SHELF
eppley precision infrared radiometers part I: theory and
the Army Corps of Engineer's Field ResearchFacility
application, J. Atmos. Oceanic Tech., 16, 739-751, 1999.
at Duck, North Carolina, and the National Data Buoy
Friehe, C. A., W. J. Shaw, D. P. Rogers,K. L. Davidson,W.
Center. Support for the analysis of the data was proG. Large, S. A. Stage, G. H. Crescenti, S. J. S. Khalsa,
vided by National ScienceFoundation grant OCE-96G. K. Greenhut, and F. Li, Air-sea fluxes and surface
33025. J.A.A. was supported by an Office of Naval
layer turbulence around a sea surfacetemperature front,
Research AASERT grant N00014-93-1-1154. ConverJ. Geophys.Res., 96, 8593-8609, 1991.
sations held with R. Beardsley, L. Illari, and R. Weller, Fung, I. Y., D. E. Harrison, and A. A. Lacis, On the variability of the net longwaveradiation at the oceansurface,
and commentsfrom two anonymousreviewersprovided
Rev. of Geophys.,22, 177-193, 1984.
useful input. This is WHOI contribution 9417.
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