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Modeling the Summertime Heat Budget of Southeast New England Shelf Waters
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John Wilkin and Lyon Lanerolle
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Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, N.J.
CBLAST: Coastal Boundary
Layers and Air-Sea Transfer
The ONR CBLASTCBLAST-Low program focuses on airair-sea interaction and coupled
atmosphere/ocean boundary layer dynamics at low wind speeds where
where
processes are strongly modulated by thermal forcing. (There is a companion
CBLASTCBLAST-Hurricane program.)
Turbulence and mean flow observations
are being used to quantify the turbulent
kinetic energy, momentum, mass, and
heat budgets in the oceanic mixed-layer
and atmospheric boundary layer.
The field program is centers on the
Martha’s Vineyard Coastal Observatory
(MVCO) and Air-Sea Interaction Tower.
U, T, Q
Heat, mass &
mom. flux, ε
Waves
Irradiance
15m
Irradiance
Regional Ocean Modeling System Mean circulation and heat budget
(ROMS) numerical features
The open boundary climatology imposes Circulation around the
a south and westward flow from the Gulf Nantucket Shoals is
• SplitSplit-explicit, freefree-surface, hydrostatic, primitive equation model [1,2]
• Generalized, terrainterrain-following vertical coordinates
• Orthogonal curvilinear, horizontal coordinates, Arakawa CC-grid
• 3rd- and 4th-order advection and timestepping;
weighted
temporal averaging; reduced
time
pressure gradient and modesplitting
error
mode
• Simultaneous conservation and constancy preservation for tracer equations in combination
with evolving coordinate system due to freefree-surface [2]
• Highorder
accurate
continuous,
monotonic
reconstruction of vertical gradients
High
• Adjoint and tangentlinear
implemented;
4D
tangent
4 variational assimilation under test
• MPI and OpenMP shared and distributed memory parallel FF-90 code
• All input/output via NetCDF
• NPZD biology; EcoSim biobio-optics; Community sediment transport model, Lagrangian floats
a south and westward flow from the Gulf
of Maine, through Great South Channel
and around Nantucket Shoals.
Southwest of Martha’s Vineyard, and
within Vineyard Sound, winds drive
eastward depth averaged flow.
augmented by strong tidal
rectified cyclonic flow that
carries water northward into
Vineyard Sound through
Muskegat Channel (between
Nantucket and the Vineyard).
Vertical turbulence closure options
Solar, IR, rain, U, T, Q
Heat, mass &
momentum flux, ε
23m
wilkin@marine.
rutgers..edu
[email protected]
http://marine.rutgers
wilkin//wip/
http://marine.rutgers..edu/~
edu/~wilkin
wip/cblast
http://oceanhttp://ocean-modeling.org
Waves
T, S
Heat, mass
mom. flux, ε
Coherent
structures
(Fanbeam)
Fanbeam)
Heat & mom. flux
U(z), Waves
(ACDP)
Observational assets deployed in
July/August of 2002 and 2003
include in situ observations of
vertical fluxes and mixing rate
profiles from fixed towers and
moorings, satellite and aircraft
remote sensing, and measurements
of small-scale and breaking waves.
• MellorMellor-Yamada level 2.5
• K-profile parameterization (KPP) surface and bottom boundary layers
layers [3]
• Generalized Length Scale scheme [4,5]: Eddy viscosity and diffusivity
diffusivity are the product of a
nonnon-dimensional stability function, TKE, and length scale. Stability functions are the result of
various 2nd-moment closures. TKE and length scales are calculated by dynamic (as in kk-ε or
M-Y) or algebraic formulations. GLS encompasses k-ε, k-ω and MM-Y in a single code.
ROMS CBLAST configuration
1 km horizontal resolution
20 ss-levels (stretched toward surface)
Surface forcing:
CBLASTCBLAST-Low Observing System:
MVCO
Aircraft
ASIT
K
Nantucket SODAR
ASIMET moorings with
ocean T(z) and ADCP
3-D
Mooring
Remote
Sensing
CBLAST Modeling using ROMS
Heat and momentum fluxes from bulk
formulae [6] with model SST,
observed downward longlong-wave at
MVCO, and Tair, pair, rel.
rel. humidity,
U10, V10, and shortshort-wave radiation
from 3 km resolution nested
COAMPS 6-36 hr forecast
6--36
Open boundary conditions:
9 km
Tidal stirring
MVCO
Ocean temperature increase (storage)
is largest south of The Islands, primarily
due to surface heating. Horizontal
divergence is small in the region of the
B-C ASIMET moorings - indicating a
region of approximate 11-D vertical heat
balance suited to evaluating ROMS
vertical turbulence closures.
COAMPS 7272-hour forecast is generated every 12
hours at ARL.HPC.mil and transferred to IMCS where
ROMS runs for the same forecast cycle.
RealReal-time validation is available using CODAR on
Nantucket (operational after July 7, 2003).
Qualitative comparison to subsurface validation data (below) shows
shows
realistic vertical stratification and mixed layer depths. In 2003,
2003, an array
of 5 subsurface moorings between ASIT and ASIMET mooringmooring-A will
enable validation of the modeled evolution of the diurnal mixed layer.
CTD temperature section between ASIT and mooring-A, late July 2001.
Precise observations of airair-sea fluxes and turbulent mixing from CBLAST are
ideal for evaluating the suite of ocean model vertical turbulence
turbulence closure
schemes implemented in ROMS.
This comparison will be possible provided the model captures the essential
features of the ocean heat budget on diurnal to several day timetime-scales, and
spatial scales of order 1 km.
Modeling complements the interpretation of the field observations
observations by
quantifying unobserved lateral transport and mixing of heat.
Vigorous tidal mixing
generates a region of
perpetually cold SST
on the eastern flank
of the Nantucket
Shoals
The time mean advection cools
the box at, on average, 200
W/m2. The net “eddy”
divergence (u’T’
(u’T’)) warms the
MVCO region at about 50 W/m2.
Operational forecasts commence mid-July, 2003
July 2002
3-day composite SST for 30-Aug-2002
A 11-D heat balance occurs near the B-A-C ASIMET mooring
sites, and these data will be used for evaluation of model
turbulent closures.
Episodic positive divergence
(cooling) events briefly arrest the
warming trend.
M2 displacement ellipses from ADCIRC
160 x 380 x 20 grid requires
approximately 2 CPU mins per model
day on 1616-processor HP/Compaq
WindWind-driven upwelling circulation contributes to the heat
budget southwest of Martha’s Vineyard.
Time series of the heat budget (below) in a box near MVCO
shows half the airair-sea flux goes to warming the water column,
and half is removed by lateral divergence.
AirAir-sea flux (Q
(Qnet) is greatest east of
Vineyard Sound where SST is cold, but
is largely balanced by divergence due
to tidal mixing.
27 km, 151x121x30
Inflow climatology [7] + outflow
radiation [8] on T,S, u, v
Climatology, tides [9], radiation (√
(√gh)
gh)
on ζ and depth average u,v
Tides significantly affect the mean circulation and heat
budget. Lateral heat transport is large in much of the region,
including near MVCO, and will need to be considered in the
analysis of ASIT heat budgets.
Tidal phase eddies transport cold tidallytidally-mixed
Nantucket Shoals water into Vineyard Sound,
and warmed VS water toward MVCO.
July 2002 mean
COAMPS CBLAST, 3km, 91x91
Summary
Observed
Modeled
ROMS forecasts will be factored into the deployment
strategy for drifting instrument strings providing
Lagrangian observations of evolving mixedmixed-layer.
References
[1] Haidvogel, D.B., H. Arango, K. Hedstrom, A. Beckmann, P. Rizzoli and A. Shchepetkin, 2000:
Dyn. Atm. Oceans, 32, 239-281.
[2] Shchepetkin, A., and J.C. McWilliams, 1998: Monthly Weather Review, 126, 1541-1580.
[3] Large, W., J. McWilliams, and S. Doney, 1994: Rev. Geophys., 32, 363-403.
[4] Umlauf, L. and H. Burchard. A generic length-scale equation for geophysical turbulence
models, J. Mar. Res., accepted 2003.
[5] Warner, J., Sherwood, C., Butman, B., Arango, H., Signell, R., Implementation of a generic
length scale turbulence closure in a 3D oceanographic model." Ocean Modelling, submitted.
[6] Fairall, C., E. Bradley, D. Rogers, J. Edson, and G. Young, 1996: JGR, 3747-3764.
[7] Bi-monthly regional climatology provided by C. Naimie, Dartmouth University
[8] Marchesiello, P., J.C. McWilliams, and A. Shchepetkin, 2001: Ocean Modelling, 3, 1-20.
[9] Luettich, R. A., Westerink, J. J., and Scheffner, N. W., 1992: ADCIRC: An advanced threedimensional circulation model for shelves, coasts, and estuaries, Tech. Report DRP-92-6, U.S.
Army Engineer Waterways Experiment Station, Vicksburg, MS.