Modeling the Summertime Heat Budget of Southeast New England Shelf Waters John Wilkin and Lyon Lanerolle Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, N.J. CBLAST: Coastal Boundary Layers and Air-Sea Transfer CBLAST Modeling using ROMS 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. (There is a companion CBLAST-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. Split-explicit, free-surface, hydrostatic, primitive equation model [1,2] Split-explicit, free-surface, hydrostatic, primitive equation model [1,2] Generalized, terrain-following vertical coordinates Generalized, terrain-following vertical coordinates Orthogonal curvilinear, horizontal coordinates, Arakawa C-grid Orthogonal curvilinear, horizontal coordinates, Arakawa C-grid 3 rd - and 4 th -order advection and time-stepping; weighted temporal averaging; reduced pressure gradient and mode-splitting error 3 rd - and 4 th -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 [2] Simultaneous conservation and constancy preservation for tracer equations in combination with evolving coordinate system due to free-surface [2] High-order accurate continuous, monotonic reconstruction of vertical gradients High-order accurate continuous, monotonic reconstruction of vertical gradients Adjoint and tangent-linear implemented; 4-D variational assimilation under test Adjoint and tangent-linear implemented; 4-D variational assimilation under test MPI and OpenMP shared and distributed memory parallel F-90 code MPI and OpenMP shared and distributed memory parallel F-90 code All input/output via NetCDF All input/output via NetCDF NPZD biology; EcoSim bio-optics; Community sediment transport model, Lagrangian floats NPZD biology; EcoSim bio-optics; Community sediment transport model, Lagrangian floats Vertical turbulence closure options Mellor-Yamada level 2.5 Mellor-Yamada level 2.5 K-profile parameterization (KPP) surface and bottom boundary layers [3] K-profile parameterization (KPP) surface and bottom boundary layers [3] Generalized Length Scale scheme [4,5]: Eddy viscosity and diffusivity are the product of a non-dimensional stability function, TKE, and length scale. Stability functions are the result of various 2 nd -moment closures. TKE and length scales are calculated by dynamic (as in k-  or M-Y) or algebraic formulations. GLS encompasses k- , k-  and M-Y in a single code. Generalized Length Scale scheme [4,5]: Eddy viscosity and diffusivity are the product of a non-dimensional stability function, TKE, and length scale. Stability functions are the result of various 2 nd -moment closures. TKE and length scales are calculated by dynamic (as in k-  or M-Y) or algebraic formulations. GLS encompasses k- , k-  and M-Y in a single code. 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. 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). 1 km horizontal resolution 20 s-levels (stretched toward surface) References [1] Haidvogel, D.B., H. Arango, K. Hedstrom, A. Beckmann, P. Rizzoli and A. Shchepetkin, 2000: Dyn. Atm. Oceans, 32, [2] Shchepetkin, A., and J.C. McWilliams, 1998: Monthly Weather Review, 126, [3] Large, W., J. McWilliams, and S. Doney, 1994: Rev. Geophys., 32, [4] Umlauf, L. and H. Burchard. A generic length-scale equation for geophysical turbulence models, J. Mar. Res., accepted [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, [7] Bi-monthly regional climatology provided by C. Naimie, Dartmouth University [8] Marchesiello, P., J.C. McWilliams, and A. Shchepetkin, 2001: Ocean Modelling, 3, [9] Luettich, R. A., Westerink, J. J., and Scheffner, N. W., 1992: ADCIRC: An advanced three- dimensional circulation model for shelves, coasts, and estuaries, Tech. Report DRP-92-6, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. Regional Ocean Modeling System (ROMS) numerical features Regional Ocean Modeling System (ROMS) numerical features Mean circulation and heat budget 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. CBLAST-Low Observing System: 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. Irradiance 23m 15m U, T, Q Heat, mass & mom. flux, ε Waves Waves T, S Heat, mass mom. flux, ε Solar, IR, rain, U, T, Q Heat, mass & momentum flux, ε MVCO Nantucket SODAR K ASIT ASIMET moorings with ocean T(z) and ADCP RemoteSensingAircraft3-DMooring Coherent structures (Fanbeam) Heat & mom. flux U(z), Waves (ACDP ) ROMS CBLAST configuration Open boundary conditions: Inflow climatology [7] + outflow radiation [8] on T,S, u, v Climatology, tides [9], radiation (  gh) on  and depth average u,v 160 x 380 x 20 grid requires approximately 2 CPU mins per model day on 16-processor HP/Compaq Tidal stirring COAMPS 72-hour forecast is generated every 12 hours at ARL.HPC.mil and transferred to IMCS where ROMS runs for the same forecast cycle. Real-time validation is available using CODAR on Nantucket (operational after July 7, 2003). ROMS forecasts will be factored into the deployment strategy for drifting instrument strings providing Lagrangian observations of evolving mixed-layer. Operational forecasts commence mid-July, 2003 M 2 displacement ellipses from ADCIRC 3-day composite SST for 30-Aug-2002 Vigorous tidal mixing generates a region of perpetually cold SST on the eastern flank of the Nantucket Shoals CTD temperature section between ASIT and mooring-A, late July ObservedModeled Tidal phase eddies transport cold tidally-mixed Nantucket Shoals water into Vineyard Sound, and warmed VS water toward MVCO. COAMPS CBLAST, 3km, 91x91 9 km 27 km, 151x121x30 The open boundary climatology imposes 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. Air-sea flux (Q net ) 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. July 2002 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. Wind-driven upwelling circulation contributes to the heat budget southwest of Martha’s Vineyard. A 1-D heat balance occurs near the B-A-C ASIMET mooring sites, and these data will be used for evaluation of model turbulent closures. July 2002 mean Summary MVCO Time series of the heat budget (below) in a box near MVCO shows half the air-sea flux goes to warming the water column, and half is removed by lateral divergence. The time mean advection cools the box at, on average, 200 W/m 2. The net “eddy” divergence (u’T’) warms the MVCO region at about 150 W/m 2. Episodic positive divergence (cooling) events briefly arrest the warming trend. Surface forcing: Heat and momentum fluxes from bulk formulae [6] with model SST, observed downward long-wave at MVCO, and T air, p air, rel. humidity, U 10, V 10, and short-wave radiation from 3 km resolution nested COAMPS hr forecast