Ocean model forecasts for the 2005 LaTTE Field Experiment

The Regional Ocean Modeling System (ROMS) results shown here are for a LaTTE model covering a portion of the mid-Atlantic Bight from the center of Long Island to south of the mouth of the Delaware River. Our preferred configuration – forecast B – is operational.

On-going: Forecast B description

Plots from Forecast B:

· Surface salinity - Sandy Hook region (Magenta vectors show the NCEP forecast wind stress)

· Surface salinity and temperature - NY Harbor to Tuckerton (Magenta vectors show the NCEP forecast wind stress)

· Surface current magnitude for comparison to HR-CODAR. (Magenta vectors show the NCEP forecast wind stress)

· Second dye injection forecast (issued Sunday 17 April, 5:30 pm EDT)

Forecast B:

10-meter winds, surface air pressure, temperature and relative humidity from the NCEP ETA-12 forecast system are used. In the hind-cast phase, these data are at most a 12-hour forecast for the preceding NCEP forecast cycle. In forecast mode, we typically obtain 60-hour forecast fields from the NCEP NOMADS Opendap data server (depending on timeliness of the NOMADS service, and occasional data drop-outs).

Forecast A:

Forecast A: Forecast A is suspended now that we have B operational, but see the A plots for a hindcast of the path of the first dye release. Forecast A used winds from the Ambrose Tower in the hind-cast phase of the simulation. For the forcast phase, wind speed and direction and air temperature from the LaTTE meteorological forecast by Louis Bowers were used (these are the summarized conditions, not the actual WRF resolved fields). The values are point data, and the conditions are assumed to hold over the entire model domain (i.e. spatial variability in the meteorological forcing is neglected). Observed daily average Hudson River flow is used in the hind-cast phase, and climatological daily flow in forecast.

Plots from Forecast A:

Model results plotted for “Level 30” are the surface-most level of the model. Vectors show ocean current at the same level.
The data plotted are 12-hour averages to reduce the effect of tides in the analysis.

· Surface salinity - Sandy Hook region

· First dye release: Concentration at 2.5 m

· First dye release: Vertically integrated concentration
Modeled release has initial peak concentration of 1.0 centered at 2.5 m depth with 1 m vertical scale and 2 km horiz-scale

· Surface salinity and temperature - NY Harbor to Tuckerton

Further details on the ROMS LaTTE configuration

The model horizontal resolution is approximately 1 km with 30 vertical levels. The domain is limited to the continental shelf. The model bathymetry was taken from the 15 arc second (~460 m) data of the NGDC Coastal Relief Model. The maximum depth is 100 m at the end of the Hudson Canyon. Most of the domain has depths shallower than 60 m.

Initial conditions are from a spin-up run than begins January 1, 2005. Initial temperature-salinity values for the spin-up are from a simple average of historic hydrographic observations computed by weighted least squares regression following isobaths. The hydro data used were a combination of stations in the NOAA/NMFS archive provided by Maureen Taylor of NMFS Woods Hole, data in the NODC archive, and other observations by the Rutgers University Long-term Ecosystem Observatory (LEO) programs.

Open boundary conditions are simple Orlanski-type radiation augmented with tidal harmonic forcing taken from an ADCIRC model of the western Atlantic (Luettich et al. 1992). Following 8-days of spin-up with meteorological and hydrologic focing, the tides were activated on April 25. The weak mean southward flow of the mid-Atlantic Bight inner shelf is not imposed in the boundary conditions.

Air-sea heat and momentum fluxes are calculated by the bulk formulae of Fairall et al. (1996,2002) using the model sea surface temperature and sea level air temperature, pressure, relative humidity, and 10-meter winds from the National Weather Service ETA model (at 12-km resolution).

In forecast (A), long-wave radiation is computed by the Berliand approximation. Short-wave radiation is a simple cloud-free value. In forecast (B), NCEP provided 3-hourly downward longwave and shortwave radiation are used.

The inflow of the Hudson River is specified using a simple estimate computed as 1.3 times the sum of the daily mean observations at the Mohawk and Fort Edward USGS gauges. The other rivers entering the domain (Delaware River, Raritan River, Wading River and Toms River) are specified as monthly climatological values.

Background:

ROMS: The Regional Ocean Modeling System

ROMS is a free surface primitive equations ocean model being used by a rapidly growing user community for applications from the basin to coastal and estuarine scales (e.g. Haidvogel et al. 2000, Marchesiello et al. 2003, MacCready and Geyer 2001). Model features are summarized briefly in Table 1 and the computational algorithms are described in detail by Shchepetkin and McWilliams (1998; 2003a, 2003b). Careful formulation of the time-stepping algorithm allows both exact conservation and constancy preservation for tracers, while achieving enhanced stability and accuracy in coastal applications where the free surface displacement is a significant fraction of the total water depth. Conservative parabolic-spline discretization in the vertical has greatly reduced the pressure-gradient truncation error that has plagued previous terrain-following coordinates models.

Table 1: ROMS model features

- free surface, hydrostatic primitive equations model in terrain-following coordinates - 3rd-order upstream-biased advection (Shchepetkin and McWilliams, 1998) - pressure gradient and equation of state give reduced s-coordinate truncation error (Shchepetkin and McWilliams, 2003a) - split-explicit time-stepping of barotropic and baroclinic modes constrained for conservation of volume and tracer constancy (Shchepetkin and McWilliams 2003b) - radiation open boundary conditions and 1-way embedding in exterior model domains (Marchesiello et al. 2001)

- synchronous Lagrangian particle tracking - vertical turbulence closures: KPP (Large et al. 1994) and the Generalized Length Scale scheme of Umlauf, and Burchard, 2003 (http://www.gotm.net) encompassing k-e, k-w and Mellor-Yamada (1982) - intermittent sub-optimal melding assimilation - tangent linear and adjoint codes written; 4DVar assimilation in development (Moore et al. 2003) - atmospheric, oceanic, and benthic (wave and current) boundary layers (Styles and Glenn) - coupled ecosystem (7-component NPZD and EcoSim bio-optics) and sediment transport (USGS Community Model) modules

References

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Galperin, B., L.H. Kantha, S. Hassid, and A.Rosati., 1988. A quasi-equilibrium turbulent energy model for geophysical flows, J. Atmospheric Science, 45, 55-62.

Haidvogel, D.B., H.G. Arango, K. Hedstrom, A. Beckmann, P. Malanotte-Rizzoli, and A.F. Shchepetkin, 2000: Model Evaluation Experiments in the North Atlantic Basin: Simulations in Nonlinear Terrain-Following Coordinates, Dynamics of Atmospheres and Oceans, 32, 239-281.

Kantha, L.H., and Clayson, C.A., 1994. An improved mixed layer model for geophysical applications, J. Geophys. Res., 99, 25235-25266.

Large, W.G., J.C. McWilliams, and S.C. Doney, 1994: Oceanic vertical mixing: A review and a model with a nonlocal k-profile boundary layer parameterization, Rev. Geophys., 32, 363-403.

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.

MacCready, P. and W.R. Geyer, 2001: Estuarine salt flux through an isohaline surface, Journal of Geophysical Research, 106, 11629-11637.

Marchesiello, P., J.C. McWilliams, and A. Shchepetkin, 2001: Open boundary conditions for long-term integration of regional ocean models. Ocean Modelling, 3, 1-20.

Marchesiello, P., J. C. McWilliams, and A. Shchepetkin, 2003: Equilibrium Structure and Dynamics of the California Current System, J. Phys. Ocean.

Moore, A.M., H.G. Arango, A.J. Miller, B.D. Cornuelle, E. Di Lorenzo and D.J. Neilson, 2003: A Comprehensive Ocean Prediction and Analysis System Based on the Tangent Linear and Adjoint Components of a Regional Ocean Model, Ocean Modelling, in press.

Shchepetkin, A.F. and J.C. McWilliams, 1998: Quasi-monotone advection schemes based on explicit locally adaptive dissipation, Mon. Wea. Rev., 126, 1541-1580.

Shchepetkin, A.F. and J.C. McWilliams, 2003a: A Method for Computing Horizontal Pressure-Gradient Force in an Oceanic Model with a Non-Aligned Vertical Coordinate, Journal of Geophysical Research, 108, 3090-.

Shchepetkin, A.F. and J.C. McWilliams, 2003b: The Regional Ocean Modeling System: A split-explicit, free-surface, topography-following coordinates ocean model, unpublished manuscript.

Umlauf, L. and H. Burchard, 2003: A generic length-scale equation for geophysical turbulence models, J. Mar. Res, accepted.

Warner, J.C., C.R. Sherwood, H.G. Arango, R. P. Signell and B. Butman, 2003: Performance of four turbulence closure methods implemented with a generic length scale method, submitted to Ocean Modelling.