CBLAST: Coupled Boundary Layers and Air-Sea Transfer

1. Modeling the Heat Budget of Southeast New England Shelf Waters for CBLAST-LowJohn WilkinH. Arango, K. Fennel, L. Lanerolle, J. LevinInstitute of Marine and Coastal SciencesRutgers, The State University of New Jersey NSF CoOP Buoyancy driven flow (LATTE)Northeast Observing System (NEOS)Northeast North American shelf (NENA)North Atlantic Basin (NATL)

2. CBLAST: Coupled Boundary Layers and Air-Sea Transfer The ONR CBLAST-Low program focuses on air-sea interaction and coupled atmosphere/ocean boundary layer dynamics at low wind speeds where processes are strongly modulated by thermal forcing. • Precise observations of air-sea fluxes and turbulent mixing from CBLAST are ideal for evaluating the suite of ocean model vertical 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 time-scales, and spatial scales of order 1 km. • Modeling complements the interpretation of the field observations by quantifying unobserved lateral transport and mixing of heat.

3. Improve Flux Parameterizations =  CD U2 Research Question: What physical processes are responsible for this enhancement? Low Winds

4. Solar, IR, rain, U, T, QHeat, mass & momentum flux, ε U, T, QHeat, mass & mom. flux, εWaves 23m WavesT, S Heat, mass mom. flux, ε Irradiance 15m Irradiance

5. Flux Aircraft IR Aircraft ASIMET mooringswith ocean T(z) and ADCP Nantucket SODAR 3-D mooring Remote Sensing CBLAST-Low Observing System 2002: ASIT MVCO K

6. The Regional Ocean Modeling System (ROMS/TOMS) has been configured for a region of the southeastern New England shelf encompassing the CBLAST observation area Purpose: Obtain model hind-cast of summertime ocean conditions that captures the essential features of the ocean heat budget on diurnal to several day time-scales, and spatial scales of order 1 km

7. The Regional Ocean Modeling System (ROMS/TOMS) has been configured for a region of the southeastern New England shelf encompassing the CBLAST observation area • Motivation (1) • Model evaluation: • Compare model heat budget to observations: • Evaluate heat budget sensitivity to vertical turbulent closures in ROMS • Evaluate heat budget sensitivity to air-sea flux bulk formulae • Evaluate contribution to hind-cast skill of meteorological model (COAMPS) compared to using observed marine boundary layer conditions

8. Motivation (2) • Observational data analysis: • Horizontal mixing and advection are largely unobserved by the CBLAST field instrumentation. This affects closure of the observed heat budget, especially: • Advection of vertically mixed waters originating on the Nantucket Shoals • Advection past the CBLAST tower of tidally generated eddies transporting Vineyard Sound water through Muskeget channel • To what extent does wind-driven upwelling maintain stratification, and contribute to local heat budgets?

9. ROMS CBLAST domain • 1 km grid resolution • 20 vertical levels (stretched) • Surface forcing: • Observed ASIT/MVCO and modeled (COAMPS) mairne boundary conditions • Initial and inflow/outflow boundary conditions from bi-monthly climatology • Tides

10. Courtesy S. Wang and Q. Wang, NRL COAMPS CBLAST, 3km, 91x91 9 km 27 km, 151x121x30 • Surface forcing: • Heat and momentum fluxes from bulk formulae • model SST • Tair, pair, rel. humidity, U10, V10, and short-wave radiation from 3km resolution nested COAMPS 6--72 hr forecast • observed downward long-wave at MVCO or net long-wave from COAMPS

11. ROMS model attributes Split-explicit, free-surface, hydrostatic, primitive equation model Generalized, terrain-following vertical coordinates Orthogonal curvilinear, horizontal coordinates, Arakawa C-grid 3rd- and 4th-order advection and time-stepping; weighted temporal averaging; reduced pressure gradient and mode-splitting error Simultaneous conservation and constancy preservation for tracer equations in combination with evolving coordinate system due to free-surface Continuous, monotonic reconstruction of vertical gradients to maintain high-order accuracy ROMS/TOMS MPI shared and distributed memory f90 code netCDF I/O Split-explicit, free-surface, hydrostatic, primitive equation model Generalized, terrain-following vertical coordinates Orthogonal curvilinear, horizontal coordinates, Arakawa C-grid 3rd- and 4th-order advection and time-stepping; weighted temporal averaging; reduced pressure gradient and mode-splitting error Simultaneous conservation and constancy preservation for tracer equations in combination with evolving coordinate system due to free-surface Continuous, monotonic reconstruction of vertical gradients to maintain high-order accuracy ROMS/TOMS MPI shared and distributed memory f90 code netCDF I/O

12. ROMS Vertical Turbulence Closure options • Mellor-Yamada level 2.5 • Non-local, k-profile parameterization (KPP) surface and bottom closure scheme • surface boundary layer (KPP; Large et al., 1994) • bottom boundary layer (inverted KPP; Durski et al., 2001) • Generalized Ocean Turbulence Modelhttp://www.gotm.net • Eddy viscosity and diffusivity product of a non-dimensional stability function, normalized TKE, and macro length scale • The stability functions are the result of various second-moment closures. TKE and the length scales are calculated by dynamic equations (as in k-epsilon or Mellor-Yamada models) or algebraic formulations. Umlauf, L. and H. Burchard. A generic length-scale equation for geophysical turbulence modelsJ. Mar. Res., 2003 Warner, J.C., Sherwood, C.R., Butman, B., Arango, H.G., and Signell, R.P., Implementation of a generic length scale turbulence closure in a 3D oceanographic model." Ocean Modeling, 2003.

14. Circulation around Nantucket Shoals is augmented by tidal rectified anti-cyclonic flow that carries water into Vineyard Sound through Muskeget Channel

15. July 2002 mean Mean circulation and heat budget The open boundary climatology imposes a south and westward flow from the Gulf of Maine, through Great South Channel and around Nantucket Shoals. Circulation around the Nantucket Shoals is augmented by strong tidal rectified cyclonic flow that carries water northward into Vineyard Sound through Muskegat Channel (between Nantucket and the Vineyard). Southwest of Martha’s Vineyard, and within Vineyard Sound, winds drive eastward depth averaged flow.

16. 3-day composite SST for 30-Aug-2002 Tidal mixing generates a region of perpetually cold SST on the eastern flank of the Nantucket Shoals

17. July 2002 Air-sea flux (Qnet) is greatest east of Vineyard Sound where SST is cold, but is largely balanced by divergence due to tidal mixing. 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 1-D vertical heat balance suited to evaluating ROMS vertical turbulence closures.

18. MVCO The time mean advection cools the box at, on average, 200 W/m2. The net “eddy” divergence (u’T’) warms the MVCO region at about 50 W/m2. Episodic positive divergence (cooling) events briefly arrest the warming trend. Time series of the heat budget in a box near MVCO shows half the air-sea flux goes to warming the water column, and half is removed by lateral divergence.

19. CTD temperature section between ASIT and mooring-A, late July 2001. Observed Modeled Qualitative comparison to subsurface validation data (below) shows realistic vertical stratification and mixed layer depths. In 2003, an array of 5 subsurface moorings between ASIT and ASIMET mooring-A will enable validation of the modeled evolution of the diurnal mixed layer.

20. 2003

21. Operational forecasts were generated forJuly 21 through September 3, 2003 COAMPS 72-hour forecast was generated every 12 hours at ARL.HPC.mil and transferred to IMCS where ROMS ran for the same forecast cycle.

30. Raw CTD data acquired via Iridium from a Slocum Glider transiting between ASIT and ASIMET mooring-A.

31. Long-range CODAR at ’Siasconset Nantucket

32. CBLAST: Lessons for ocean modeling: • Differing vertical turbulence parameterizations lead to different3-dimensional coastal mesoscale flows • CBLAST data suited to turbulence closure evaluation: • Combination of direct air-sea flux and vertical turbulence observations, and in situ oceanic conditions for validation • Spatially variable atmospheric forcing (COAMPS) is important • Heat budget requires further analysis of horizontal/vertical circulations: overturning/upwelling vs. depth-average flow contributions, especially at moorings and ASIT

33. CBLAST: Lessons for data analysis: • Tides affect the circulation and heat budget through residual mean currents and vertical mixing • Wind-driven upwelling circulation contributes to the heat budget southwest of Martha’s Vineyard • 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 • Vineyard Sound, Nantucket Shoals, MVCO, shows differing heat balances in July mean • Modeling shows a 1-D heat balance occurs near the B-A-C ASIMET mooring sites, which suggests vertical turbulence closures can be evaluated locally there

34. 1 2 1 2 4 4 3 3 5 5 6 Nested Grids 1) NENA 2) NEOS 3) CBLAST4) LATTE 5) NY/NJ Bight 6) Caribbean

35. North Atlantic Climatological heat/freshwaterfluxes 3-day averageNCEP winds

36. Northeast North Atlantic (NENA) embedded in NATL 3-day average open boundary values from NATL10-component NPZD ecosystem: NO3,NH4, chl-a, phytoplankton, zooplankton, small/large detritus, TIC, alkalinity, oxygen Temperature Chlorophyll

37. CO2 flux (mol m-2 yr-1) Day of year • Air-sea CO2-flux: • simulated (above and top right) • observed (bottom right; Boehme et al., Mar. Chem. 1998)

38. NEOS NENA Northeast Observing System (NEOS) Salinity at 20 m Initialized 01-Jan-1993

39. Northeast Observing System (NEOS) • Assimilate regional CODAR with 4D-variational method • Develop AUV deployment strategies; tangent linear and adjoint give singular vectors of model showing regions of most rapid perturbation growth • Multiple-scale nesting in support of sub-region studies (LaTTE, CBLAST)

40. LaTTE: Lagrangian Transport and Transformation Experiment • Dye release in Hudson River plume • 4D-var assimilation with ROMS • Coupled bio-optical modeling with EcoSim

41. Summary • Hierarchy of integrated observational/modeling studies from basin to coastal using ROMS/TOMS tools • Processes: coastal bio-optics, sediment transport, CO2 cycling, buoyancy-driven flow, wind-driven upwelling, air-sea interaction • Adjoint, tangent linear, variational assimilation codes feature in projects • Objectives (2004-2005) for coastal prediction system development: • turbulence closure: evaluate with CBLAST-Low data • air-sea fluxes, coupling to atmospheric models (COAMPS, WRF) • 4D-variational assimilation • new observing system technologies (CODAR, gliders) • use adjoint for sampling design and predictability studies