Establishing Confidence in Marine Forecast Systems: The design and skill assessment of the New York Harbor Observation and Prediction System, version 3 (NYHOPS v3) Nickitas Georgas1 and Alan F. Blumberg2 Abstract We briefly describe the new NYHOPS v3 OFS (Operational hydrodynamic Forecast System) and quantify its performance against National Ocean Service (NOS) standard OFS evaluation metrics. Given the relatively large area of the NYHOPS v3 OFS (including the NY/NJ Harbor estuary, Long Island Sound, and their coastal ocean), and the proliferation of sensor networks, the presented skill assessment is one of the most extensive performed to date: model results are compared to in situ observations of water level, currents, temperature, salinity, and waves from over 100 locations, collected in a 2 year period. The model’s ability to describe the hydrodynamic conditions in the extensive area it is employed is remarkable. The average index of agreement for water level is 0.98, for currents is 0.87, for water temperature is 0.98, for salinity is 0.77, and for significant wave heights is 0.88. Respective, average root- mean-square errors are: 10cm for water level, 13cm/s and 9° for currents, 1.4°C for water temperatures, 2.8psu for salinities, and 32cm for significant wave heights. 1. Introduction The New York Harbor Observation and Prediction System (NYHOPS, Bruno et al 2006) was established at Stevens Institute of Technology (Stevens) in 2004, through coordinated efforts from academia, industry, local and federal US government to: • Permit an assessment of ocean, weather, and environmental conditions throughout the New York Harbor and New Jersey Coast regions, • Provide marine forecasts (general circulation and waves) for the said area up to 48 hours in advance, • Establish a continuous history of the marine conditions in and around the New York / New Jersey Harbor, and, • Provide a test bed for environmental systems integration into situation awareness scenarios, ranging from flooding alerts, to search and rescue, to chemical spills. The first version of the NYHOPS system included 48hr hydrodynamic circulation predictions based on the Princeton Ocean Model (Blumberg and Mellor 1987) and, specifically, its Estuarine and Coastal Ocean Model derivative (ECOM) as 1Senior Research Engineer, Center for Maritime Systems, Stevens Institute of Technology, 711 Hudson Street, Hoboken, NJ 07030; PH (201) 216-8218; [email protected] 2George Meade Bond Professor and Director, Center for Maritime Systems, Stevens Institute of Technology, 711 Hudson Street, Hoboken, NJ 07030; PH (201) 216-5289; [email protected] 1 implemented for the New York Harbor region (Blumberg et al 1999). A 2nd version of the NYHOPS forecast model went into effect in January 2007, with a higher resolution numerical hydrodynamic forecast grid, and included uncoupled surface wave forecasts (Georgas et al 2007). After a two-year period of continuous development, updates, testing, and complete automation, the new model has been operational and publicly available (www.stevens.edu/maritimeforecast) since June 2009; under the name NYHOPS v3, it is an integral part of the regional component of the global Integrated Ocean Observing System (IOOS). The new OFS builds upon the older NYHOPS versions, providing marine conditions in a high resolution grid, for a larger area, based on improved representations of physics and physical constraints (such as boundary conditions), and is more accessible (including Google Earth kml files, OpenDAP/THREDDS servers, etc.). The complete NYHOPS v3 environmental system of systems infrastructure is described in Georgas et al 2009. In section 2 we describe the NYHOPS v3 hydrodynamic forecast model and its implementation to the New York Bight and its estuaries, sounds, and tidal fresh waters (Figure 1). Sections 3 and 4 are the main focus of the paper: the assessment of the new model’s skill in predicting water level, η, currents, U, water temperature, T, salinity, S, and significant wave height, Ho, in the model region. The paper is designed to emulate the model evaluation process followed in the Delaware River and Bay Model Evaluation Environment (DRB-MEE) that resulted in six publications in the previous Estuarine and Coastal Modeling proceedings (ECM10, Patchen 2008 and references therein). We are going to concentrate on a 2 year hindcast period, between 02/01/2007 and 02/01/2009, for which in situ observations were available from a multitude of sensors dispersed throughout the NYHOPS area. Comparisons of the NYHOPS predicted sea surface temperatures (SST) to remote observations (satellite- derived SST) are a focus of another paper in this issue (Bhushan et al 2010). Figure 1. Map of geographic locations referenced in this paper. 2 2. NYHOPS v3 Model Implementation The three-dimensional hydrodynamic model ECOM (Blumberg et al 1999), a derivative of the Princeton Ocean Model (POM, Blumberg and Mellor 1987), is used to forecast the ocean processes across the large coastal, estuarine, and riverine NYHOPS domain. As used in NYHOPS v3, the hydrodynamic code includes significant developments not included in the original ECOM/POM, such as wetting- and-drying (W&D) and thin-dam (obstruction grid) formulations, a new dynamically coupled wave module, a new one-way-coupled atmospheric module, and complete Climate and Forecasting Conventions (CF 1.4) compliance of the NetCDF outputs (Georgas 2010). The hydrodynamic NYHOPS v3 model provides forecasts of water level, 3D circulation fields (currents, T, S), significant wave height, and wave period. The model incorporates the Mellor-Yamada 2.5 level turbulent closure model (Mellor and Yamada 1982).The Smagorinsky constant, HORCON, is set to 0.01, the bottom roughness length Z0 to 0.001m, the minimum bottom drag coefficient CDmin to 0.003, and the molecular diffusivity UMOL to 10-6 m2/s everywhere. No local calibration of the bottom drag coefficient has been performed. However, CDmin is allowed to dynamically adjust based on the presence of the local wave boundary layer computed from the dynamically coupled wave model and Grant-Madsen theory (Georgas 2010). The wave module is based on the GLERL wind-wave momentum model (Donelan 1977, Schwab et al 1984). The GLERL code has been modified with the NYHOPS coastal region in mind to add bottom frictional dissipation (wave friction factor set to 4x10-3), tidally-adjusting depth-induced breaking, unresolved obstructions (thin dams), and open boundary forcing through specification of significant wave height and direction at the oceanic boundary. The empirical fraction of the wind stress that is retained by the waves is set to 2.8% in NYHOPS v3. Added skin friction at the surface uses a coefficient set to 0.7x10-3. More details are found in Georgas 2010. In OFS forecasting mode, NYHOPS v3 is run daily, to provide a hindcast (-24hrs) and forecast (+48hrs) of the hydrodynamic circulation and wave conditions in the coastal (<200m deep), estuarine, and freshwater zones from coastal Maryland to Cape Cod, Massachusetts (Figure 1). The hydrodynamic model is initiated at 0 hrs local every day, and completes a 24hr hindcast cycle based on observed forcing followed by a 48hr forecast cycle based on forecast forcing. The 72hr NYHOPS v3 daily run code (W&D 3D hydrodynamics with coupled waves and 2D atmospherics) has been compiled with Portland Group’s auto-parallelizable pgf77®. It runs on a Dell Nehalem computer with eight 2.93GHz cores (2 quads with hyper-threading) in about 1.5hrs with a 1sec barotropic (2D) and a 10sec baroclinic (3D) timestep. Coupled chemical kinetics (in particular, fate and transport of chromophoric dissolved organic matter), acoustic transmission loss, and offline data assimilative nowcasting are included in the NYHOPS v3 system of systems, but will not be elaborated here. The NYHOPS v3 computational domain is discretized on an Arakawa “C” finite- difference grid (147x452 cells, 15,068 of which are designated as water). A high- resolution curvilinear model grid is used to encompass the entire Hudson-Raritan 3 (New York/New Jersey Harbor) Estuary, the Long Island Sound, and the New Jersey and Long Island coastal ocean (Figure 2). The resolution of the grid ranges from approximately 7.5km at the open ocean boundary to less than 50m in several parts of the NY/NJ Harbor Estuary. In order to resolve coastline features that could not be resolved on a grid cell scale, most notably the NJ Atlantic coast barrier islands, 96 cell interfaces across which transport or mixing is disallowed (“thin dams”) have been defined. In the vertical, the model uses a sigma-coordinate system with bathymetrically-stretched sigma layers to permit better representation of bottom topography. The vertical resolution of the grid is 10 sigma layers. NYHOPS has also been tested with 40 sigma layers, but runtimes are currently operationally prohibitive. A B TD TD C TD Figure 2. High-resolution NYHOPS v3 finite difference grid created with Delft3d RGFGRID®: A) Complete grid, B) NY/NJ Harbor zoom, C) Long Island / Block Island sounds zoom. Contoured bathymetry is in meters [max of 200m offshore]. Some thin dams (explained in the text) are visible (pointed out with TD). Regardless of the effort that went into designing the new higher-resolution grid for NYHOPS v3, it is obvious than any descritization of a continuous field will retain errors that may, in places, be significant. Tidal waves are surface gravity shallow 4 water waves that propagate based on shallow water physics with celerity highly dependent on depth (e.g. Blumberg and Georgas 2008). Figure 3 depicts a metric for the resolution of the new, greatly improved grid, based on the variation (here, standard deviation) of actual sounding depths found within a grid cell described numerically by a single average depth value. Based on the resolution metric shown in Figure 3, the relative sub-grid variation in tidal propagation, σTP, may be approximated as: ± σ − g(H H ) gH σ H 1 σ H σ TP = = 1 ± −1 ≈ ± (1) gH H 2 H where, H is the (mean) cell depth, σH is the standard deviation of actual soundings taken from within that cell, and the Maclaurin expansion is used for illustration only.