System Description

HYCOM is designed as a generalized (hybrid isopycnal/σ/z) coordinate ocean model. It is isopycnal in the open stratified ocean, but reverts to a terrain-following coordinate in shallow coastal regions, and to z-level coordinates near the surface in the mixed layer. This generalized vertical coordinate approach is dynamic in space and time via the layered continuity equation, which allows a dynamical transition between the coordinate types. Like MICOM, HYCOM allows isopycnals intersecting sloping topography by allowing zero thickness layers. HYCOM was developed from MICOM using the theoretical foundation for implementing a hybrid coordinate system set forth in Bleck and Boudra (1981; J. Phys. Oceanogr.), Bleck and Benjamin (1993; Mon. Wea. Rev.), Bleck (2002; Ocean Modelling), Chassignet et al. (2003, J. Phys. Oceanogr.) and Halliwell (2003; Ocean Modelling).

HYCOM is maintained as a single scalable/portable source code. The Gulf of Mexico model has 1/25° equatorial resolution and latitudinal resolution of 1/25° cos(lat) or ~3.5 km for each variable at mid-latitudes. It has 27 coordinate surfaces in the vertical. The version of the HYCOM used is 2.2.77. This experiment includes tidal forcing.

Data Assimilation

The data assimilation is performed using the Navy Coupled Ocean Data Assimilation (NCODA) (Cummings, 2005; , Cummings and Smedstad, 2013) system with a model forecast as the first guess. NCODA assimilates available satellite altimeter observations (along track obtained via the NAVOCEANO Altimeter Data Fusion Center), satellite and in situ sea surface temperature (SST) as well as available in situ vertical temperature and salinity profiles from XBTs, ARGO floats and moored buoys. FGAT (first guess at appropriate time) is used for the SST and profile assimilation. The HYCOM Consortium is also in the process of evaluating other assimilation techniques, the reduced order information filter (ROIF, Chin et al., 1999; J. Geophys. Res.), the ensemble Kalman filter (Evensen, 1994; J. Geophys. Res.) and the singular evolutive extended Kalman filter SEEK filter (Pham et al., 1998; J. Mar. Syst.).

Operational Details

The system is run in real time at the Naval Oceanographic Office (NAVOCEANO) Major Shared Resource Center (the 1/25° GOM run does not use operational computer time). The model performs one daily analysis using all observations received since the previous analysis and then runs a 7 day forecast. The data are made available typically within two days after the model run via servers located at the Center For Ocean-Atmospheric Prediction Studies (COAPS), Florida State University (FSU). The HYCOM format archive files are converted to CF 1.0 compliant NetCDF files using standard HYCOM post-processing package. These files contain diagnostic output at standard Levitus depth levels for several variables including: currents, temperature and salinity. The archive files are snapshots. This experiment started in September 2013 (data is available from April 2013 on) and has hourly 3d outputs available.

Detailed Information

Run Configuration (blkdat.input)

Contains model run configuration information such as time steps, advection scheme, mixing, vertical structure, etc.

Model Bathymetry (depth.[ab])

Files containing the model bathymetry.

Computational Grids (regional.grid.[ab])

Files containing the location of model grid point.

References

NRL Website

Contains snapshots, animations forecast verification statistics and model data comparisons.

(Cummings and Smedstad, 2013)

J. A. Cummings and O. M. Smedstad. 2013: Variational Data Assimilation for the Global Ocean. Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications vol II, chapter 13, 303-343.

(Cummings, 2005)

Cummings, J.A., 2005: Operational multivariate ocean data assimilation. Quart. J. Royal Met. Soc., Part C, 131(613), 3583-3604.