US GODAE : Global Ocean Prediction with the HYbrid Coordinate Ocean Model ( HYCOM )

Abstract : The main objective is to use the HYbrid Coordinate Ocean Model (HYCOM) with data assimilation in an eddy-resolving, fully global ocean prediction system with transition to the Naval Oceanographic Office (NAVOCEANO) at .08 deg equatorial (~7 km mid-latitude) resolution in 2007 and .04 deg resolution by 2011. The model will include shallow water to a minimum depth of 5 m and provide boundary conditions to finer resolution coastal and regional models that may use HYCOM or a different model. In addition, HYCOM will be coupled to atmospheric, ice and bio-chemical models, with transition to the Fleet Numerical Meteorology and Oceanography Center (FNMOC) for the coupled ocean-atmosphere prediction. Basin-scale configurations will also form the backbone of the NOAA/NCEP/MMAB Ocean Forecast System. All the systems will be transitioned with assimilation of sea surface height (SSH) from satellite altimeters, sea surface temperature (SST) and temperature (T)/salinity (S) profiles, including profiles from ARGO floats. In addition, 30-day forecasts are planned once a week. The global system will include two-way coupling to an ice model and a version with two-way coupling to an atmospheric model for transition to FNMOC. The project will ensure that an accurate and generalized ocean model nesting capability is in place to support regional and littoral applications when global HYCOM becomes operational. This will include the capability to provide boundary conditions to nested models with fixed depth z-level coordinates, terrain following coordinates, generalized coordinates (HYCOM), and unstructured grids. To facilitate this goal, HYCOM will be developed into a full-featured coastal ocean model in collaboration with a partnering project. The project will participate in the multinational Global Ocean Data Assimilation Experiment (GODAE) and international GODAE-related ocean prediction system intercomparison projects.

B y E r i c P. c h a s s i g N E t, h a r l E y E .h u r l B u r t, E .water and in handling the transition from deep to shallow water).HYCOM (Bleck, 2002) was therefore designed to extend the range of existing operational Ocean General Circulation Models (OGCMs).
The freedom to adjust the vertical spacing of the generalized (or hybrid) coordinate layers in HYCOM simplifies the numerical implementation of several processes and allows for a smooth transition from the deep ocean to coastal regimes.
HYCOM retains many of the characteristics of its predecessor, the Miami Isopycnic Coordinate Ocean Model (Bleck et al., 1992;Bleck and Chassignet, 1994), while allowing coordinates to locally deviate from isopycnals wherever the latter may fold, outcrop, or generally provide inadequate vertical resolution.
The collaboration led to the development The partnership represents a broad spectrum of the oceanographic community, bringing together academia, federal agencies, and industry/commercial entities, and spanning modeling, data assimilation, data management and serving, observational capabilities, and application of HYCOM prediction system outputs.In addition to providing real-time, eddy-resolving global-and basin-scale ocean prediction systems for the US Navy and NOAA, this project also offered an outstanding opportunity for NOAA-Navy collaboration and cooperation, ranging from research to the operational level.This paper provides an overview of the global HYCOM ocean prediction system and highlights some of its achievements.An important outcome of this effort is the capability of the global system to provide boundary conditions to even higherresolution regional and coastal models.(Hurlburt and Hogan, 2000;Smith et al., 2000;Chassignet and Garraffo, 2001;Maltrud and McClean, 2005;Hurlburt et al., 2008).The choice of the vertical coordinate system, however, remains one of the most important aspects of an ocean model's design.In practice, the representation and parameterization of processes not resolved by the model grid are often directly linked to the vertical coordinate choice (Griffies et al., 2000).(Chassignet et al., 2003(Chassignet et al., , 2006(Chassignet et al., , 2007)).A user-chosen option allows specification of the vertical coordinate separation that controls the transition among the three coordinate systems.
The assignment of additional coordinate surfaces to the oceanic mixed layer also allows the straightforward implementation of multiple vertical mixing turbulence closure schemes (Halliwell, 2004).defined (Hurlburt et al., 2008).Figure 1 shows the climatological mean derived on a 0.5° grid using surface drifters by Maximenko and Niiler (2005)  Either the Cooper and Haines (1996) technique or synthetic temperature and salinity profiles (Fox et al., 2002) can be figure 1.The top panel shows mean sea surface height (in cm) derived from surface drifters (Maximenko and Niiler, 2005), and the bottom panel shows the same from a non-dataassimilative hycoM run corrected in the gulf stream and kuroshio regions using a rubbersheeting technique.The rMs difference between the two fields is 9.2 cm.
used for downward projection of SSH and SST. Figure 2 shows an example of forecast performance.
Validation of the results is underway using independent data with a focus on the large-scale circulation features, SSH variability, eddy kinetic energy, mixedlayer depth, vertical profiles of temperature and salinity, SST, and coastal sea levels.Figures 3 and 4 show examples for the Gulf Stream region, while Figure 5 documents the performance of HYCOM in representing the mixed-layer depth.
HYCOM is also included in the international GODAE comparison of global ocean forecasting systems.
These tools and their data distribution methods are described below.In the current setup, the OPeNDAP component provides the middleware necessary to access distributed data, while LAS functions as a user interface and a product server.The abstraction offered by the OPeNDAP server also makes it possible to define a virtual data set that LAS will act upon, rather than physical files.An OPeNDAP "aggregation server" uses this approach to append model time steps from many separate files into virtual data sets.The HYCOM Data Service has been in operation for the last four years and has seen a steady increase in the user base.In the last year, the service received approximately 20,000 hits per month.In addition to the numerous requests from educational institutions and researchers, this service has been providing nearreal-time data products to several private companies in France, the Netherlands, Portugal, and the United States.

BouNdary coNditioNs for rEgioNal aNd coastal ModEls NEstEd iN hycoM
An important attribute of the data assimilative HYCOM system is its capability to provide boundary conditions to even higher-resolution regional and J o s E P h M E t z g E r , o l E M a r t i N s M E d s ta d , J a M E s a .c u M M i N g s , g E o r g E r .h a l l i w E l l , r a i N E r B l E c k , r E M y B a r a i l l E , a l a N J .wa l l c r a f t, c a r l o s l o z a N o , h E N d r i k l .t o l M a N , a s h wa N t h s r i N i Va s a N , s t E V E h a N k i N , P E t E r c o r N i l l o N , r o B E r t w E i s B E r g , a l E x a N d E r B a r t h , r u o y i N g h E , f r a N c i s c o w E r N E r , a N d J o h N w i l k i N Oceanography Vol.22,No.2 us godaE us godaE global ocean Prediction with the hybrid coordinate ocean Model (hycoM) N o P P s P E c i a l i s s u E » E x c E l l E N c E i N Pa r t N E r i N g awa r d w i N N E r s global ocean Prediction with the hybrid coordinate ocean Model (hycoM) academia, federal agencies, and industry/ commercial entities in activities that span modeling, data assimilation, data management and serving, observational capabilities, and application of HYCOM prediction system outputs.All participating institutions were committed and the collaborative partnership provided an opportunity to leverage and accelerate the efforts of existing and planned projects, consequently producing a highquality product that should collectively serve a wider range of users than would the individual projects.The collaboration was initiated in the late 1990s by ocean modelers at the Naval Research Laboratory, Stennis, Mississippi, who approached colleagues at the University of Miami's Rosenstiel School of Marine and Atmospheric Science regarding an extension of the range of applicability of the US Navy operational ocean prediction system to coastal regions (e.g., the US Navy systems at the time were seriously limited in shallow of a consortium for hybrid-coordinate data assimilative ocean modeling supported by NOPP to make HYCOM a state-of-the-art community ocean model with data assimilation capability that could: (1) be used in a wide range of ocean-related research, (2) become the next-generation eddy-resolving global ocean prediction system, and (3) be coupled to a variety of other models, including littoral, atmospheric, ice, and biochemical models.One outcome of this collaboration was the establishment of a near-real-time North Atlantic prediction system based on HYCOM. ." These efforts were intended to be pilot projects under Ocean.US, the National Office for Integrated and Sustained Ocean Observations, and to iNtroductioN A broad partnership of institutions has collaborated over the past five to ten years to develop and demonstrate the performance and application of eddyresolving, real-time global-and basinscale ocean prediction systems using the HYbrid Coordinate Ocean Model (HYCOM).These systems are in the process of being transitioned to operational use by the US Navy at the Naval Oceanographic Office (NAVOCEANO), Stennis Space Center, Mississippi, and by the National Oceanic and Atmospheric Administration (NOAA) at the National Centers for Environmental Prediction (NCEP), Washington, DC.The systems run efficiently on a variety of massively parallel computers and include sophisticated, but relatively inexpensive, data assimilation techniques for satellite altimeter sea surface height (SSH) and sea surface temperature (SST) as well as in situ temperature, salinity, and float displacement.The partnership represents a broad spectrum of the oceanographic community, bringing together aBstr act.During the past five to ten years, a broad partnership of institutions under NOPP sponsorship has collaborated in developing and demonstrating the performance and application of eddy-resolving, real-time global-and basin-scale ocean prediction systems using the HYbrid Coordinate Ocean Model (HYCOM).
The choice of the vertical mixing parameterization is also of importance in areas of strong entrainment, such as overflows.Data assimilation is essential forocean prediction because: (a) many ocean phenomena are due to nonlinear processes (i.e., flow instabilities) and thus are not a deterministic response to atmospheric forcing, (b) errors exist in the atmospheric forcing, and (c) ocean models are imperfect, including limitations in numerical algorithms and in resolution.Most of the information about the ocean surface's space-time variability is obtained remotely from instruments aboard satellites (SSH and SST), but these observations are insufficient for specifying the subsurface variability.Vertical profiles from expendable bathythermographs (XBT), conductivitytemperature-depth (CTD) profilers, and profiling floats (e.g., Argo, which measures temperature and salinity in the upper 2000 m of the ocean) provide another substantial source of data.Even together, these data sets are insufficient to determine the ocean's state completely, so it is necessary to exploit prior statistical knowledge based on past observations as well as our present understanding of ocean dynamics.By combining all of these observations through data assimilation into an ocean model, it is possible, in principle, to produce a dynamically consistent depiction of the ocean.However, in order to have any predictive capabilities, it is extremely important that the freely evolving ocean model (i.e., non-data-assimilative model) is skilled in representing ocean features of interest.To properly assimilate the SSH anomalies determined from satellite altimeter data, the oceanic mean SSH over the altimeter observation period must be provided.In this mean, it is essential that the mean current systems and associated SSH fronts be accurately represented in terms of position, amplitude, and sharpness.Unfortunately, Earth's geoid is not presently known with sufficient accuracy for this purpose, and coarse hydrographic climatologies (~ 0.5°-1° horizontal resolution) cannot provide the spatial resolution necessary when assimilating SSH in an eddy-resolving model (horizontal grid spacing of 1/10° or finer).At these scales of interest, it is essential to have the observed means of boundary currents and associated fronts sharply as well as the mean currently used in the Navy global HYCOM prediction system (see following section for details).The HYCOM mean was constructed as follows: a five-year mean SSH field from a non-data-assimilative 1/12° global HYCOM run was compared to available climatologies, and a rubber-sheeting technique(Carnes et al., 1996) was used to modify the model mean in two regions (the Gulf Stream and the Kuroshio) where the western boundary current extensions were not well represented and where an accurate frontal location is crucial for ocean prediction.Rubber sheeting involves a suite of computer programs that operate on SSH fields, overlaying contours from a reference field and moving masses.The first system is the NOAA Real Time Ocean Forecast System for the Atlantic (RTOFS-Atlantic), which has been running in real time since 2005.The Atlantic domain spans 25°S to 76°N with a horizontal resolution varying from 4 km near the US coastline to 20 km near the African coast.The system is run daily with one-day nowcasts and five-day forecasts.Prior to June 2007, only the SST was assimilated.In June 2007, NOAA implemented the threedimensional variational data assimilation of: (1) SST and SSH (Jason-1, Geosat Follow-On [GFO], and soon Envisat), (2) temperature and salinity profile assimilation (e.g., Argo, CTDs, moorings), and (3) GOES data.Plans are to expand this system globally using the US Navy configuration described in the following paragraph.The NCEP RTOFS-Atlantic model data is distributed in real time through NCEP's operational ftp server (ftp://ftpprd.ncep.noaa.gov) and the NOAA Operational Model Archive and Distribution System (NOMADS; http://nomads6.ncdc.noaa.gov/ncep_data/index.html) server.The latter server is also using Open Project for a Network Data Access Protocol (OPeNDAP) middleware as a data-access method.NCEP's RTOFS-Atlantic model data is also archived at the National Oceanographic Data Center (NODC, http://data.nodc.noaa.gov/ncep/rtofs).The second system is the global US Navy nowcast/forecast system using the 1/12° global HYCOM (6.5-km grid spacing on average, 3.5-km grid spacing at the North Pole, and 32 hybrid layers in the vertical), which has been running in near real time since December 2006 and in real time since February 2007.The current ice model is thermodynamic, but it will soon include more physics as it is upgraded to the Polar Ice Prediction System (PIPS, based on the Los Alamos ice model known as CICE).The model is currently running daily on 379 processors on an IBM Power 5+ at NAVOCEANO using a part of the operational allocation on the machine.The daily run consists of a five-day hindcast and a five-day forecast and takes about 15 wall clock hours.The system assimilates (1) SSH (Envisat, GFO, and Jason-1), (2) SST (all available satellite and in situ sources),(3) all available in situ temperature and salinity profiles (e.g., Argo, CTDs, moor- figure 2. Verification of 30-day ocean forecasts: median ssh anomaly correlation vs. forecast length in comparison with the verifying analysis for the global us Navy hycoM over the world ocean and five subregions.The red curves verify forecasts using operational atmospheric forcing, which reverts toward climatology after five days.The green curves verify "forecasts" with analysis quality forcing for the duration, and the blue curves verify forecasts of persistence (i.e., no change from the initial state).The plots show median statistics over twenty 30-day hycoM forecasts initialized during January 2004-december 2005, a period when data from three nadir-beam altimeters, Envisat, gEosat follow-on, and Jason-1, were assimilated.The reader is referred to hurlburt et al. (2008) and an article scheduled for the september 2009 issue of Oceanography for a more detailed discussion of these results.
figure 5. Median bias error (in m) of mixed layer depth (Mld) calculated from simulated and approximately 66,000 unassimilated observed profiles over the period June 2007-May 2008.Blue (red) indicates a simulated Mld shallower (deeper) than observed; 53% of the simulated Mlds are within 10 m of the observations, and these are represented as gray.The basinwide median bias error is -6.6 m and the rMs error is 40 m.

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figure 9. Modeled phytoplankton distribution (mmol m -3 ) on august 27, 2005.The pink dot is the location of hurricane katrina.