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Experimental East Coast Water Level Data Assimilation Nowcast/Forecast Model System
Introduction
The Coast Survey Development Laboratory (CSDL) of NOS's Office of Coast Survey has undertaken a project to develop an water level data assimilation nowcast/forecast model system for the East Coast of United States. The system is designed to provide to mariners and port mangers with up-to-minute information on ocean conditions for safe navigation as well as to provide water level boundary conditions for bay models at US East Coast.
The optimal adjoint variational data assimilation technique (Zhang, et. al., 2000) has been developed to assimilate subtidal water levels sampled along the U.S. East Coast into the two-dimensional Princeton Ocean Model (POM). This scheme uses the wind drag coefficient as the control variable since the subtidal water levels at the coast are assumed to be predominately determined by the wind set-up on the shelf. In the optimal data assimilation procedure, the water level misfit is defined as the cost function and it's gradient can be determined by the adjoint model. In turn, the limited memory Broyden-Fletcher-Goldfarb-Shanno (BFGS) (Liu and Nocedal, 1989) quasi-Newton optimization method is implemented to search for the optimal wind drag coefficient by minimizing the cost function for the large scale optimization.
The data assimilation system has been tested successfully by the identical experiment in which the pseudo-observations are generated by the numerical model with predefined wind drag coefficients. The experiment shows that the wind drag coefficients can be recovered from observed water levels very accurately by using this adjoint optimal data assimilation system. Boundary conditions Using NCEP's EDAS (Eta Data Analysis System) wind, the system has been running experimentally on CSDL computer to assimilate previous day 24 hours subtidal water levels for nowcast simulation (Data Assimilation). Initialized with the water level assimilated nowcast, the system also makes 48 hours subtidal water level forecasts forced with NCEP's Eta forecasted winds (Operational Model System).
(In PDF format)
Zhang, A., E. Wei, and B. Parker, 2003. "Optimal estimation of tidal open boundary conditions using predicted tides and adjoint data assimilation technique." Continental Shelf Research, 23, 1055-1070.(In PDF format)
Data Assimilation Procedure
Model System Operational Procedure
Model Grid
EXPERIMENTAL NOWCAST/FORECAST
References
Liu, D. and J. Nocedal, 1989. "On the limited memory BFGS method for large scale optimization." Mathematical Programming, B45, 503-528.
Zhang, A., B. Parker, and E. Wei, 2002. "Assimilation of water level data into a coastal hydrodynamic model by an adjoint opticmal technique." Continental Shelf Research, 22, 1909-1934.(In PDF format)
Revised Thursday January 19 2006by OCS Webmaster