Ocean modeling at NCEP

1. Ocean modeling at NCEP Hendrik L. Tolman NOAA / NWS / NCEP / EMC Marine Modeling and Analysis Branch Hendrik.Tolman@NOAA.gov

2. 1 RTOFS-Atlantic • HYCOM application: • Primitive equation with free surface. • Sub-grid scale parameterizations. Vertical and horizontal eddy viscosity and mixing. Diapycnal mixing. • Tides & river outflow (USGS, RIVDIS). • Atmospheric fluxes (GFS). • North Atlantic grid with 4-7km US coastal resolution (ETOPO2, NGDC) • Open boundaries: T,S from climatology • SSH and barotropic velocity from tidal model (TPX06) and climatology.

3. 2 RTOFS-Atlantic

4. 3 RTOFS-Atlantic • 2007 model updates: • Revised vertical grid to 26 layers : • Higher resolution in the shallow waters. • Better resolution on the shelf break. • Better representation of Denmark & Iceland overflows. • Resolving 4 vertical dynamical modes in major sub-basins • Improved barotropic / baroclinic inputs at open boundaries • Updated Climatology (NCEP – version 6) • Mean dynamic topography (Rio 5) • Historical transports

5. 4 RTOFS-Atlantic • 2007 model updates cont’ed: • Revise river inflow data (location and strength) from USGS • Remove noise in net heat flux • Update of model algorithms • Surface initialized Montgomery Potential • Employ two invariant external mode boundary conditions. • Stabilization of sigma_star. • Enforced salinity minimum by refreshing the water column

6. 5 RTOFS-Atlantic • 2007 model updates cont’ed: • Major upgrades to data assimilation • SST: spatially varying bias removal algorithm • SSH: absolute sea surface height ; • 2D + 1D approach, conserving momentum. • T&S: assimilation of vertical profiles of temperature and salinity • 2D density, temperature and layer thickness anomaly, preserves volume, momentum; updates mass and heat. • New model output: • GRIB files on native grids. • GRIB files sets for selected sub-regions.

7. 6 RTOFS-Atlantic • SSH without (upper-left panel) and with SSH assimilation (lower-left panel) from RTOFS-Atlantic • Data: JASON, GFO and RIO5 mean dynamic topography. • U. of Colorado analyses. Uses also ENVISAT data.

8. 7 RTOFS-Atlantic Mean Gulf Stream path from old model (left) tends to overshoot the annual mean path derived from altimetry data near 72◦W as compared to the Gulf Stream location in the new implementation (right)

9. 8 RTOFS-Atlantic • Product distribution • FTP site (ftpprd) : • Nowcast and 5-day forecasts available for 3 days. • NOMADS server: • Nowcasts and 5-day forecasts available for 30 days. Tools to extract sub-regions available. • Targeted to become operational in 2008. • NODC Archives: • Long term archival. Under construction.

10. 1 Coupled HWRF-HYCOM IC/BC HWRF: O(9 km) GFS: O(25 km) interaction RTOFS (HYCOM–Basin): O(4–17 km) Hurricane (HYCOM–Regional): O(8–14 km) IC/BC

11. 2 Coupled HWRF-HYCOM

12. Examples from Rita (2005). Model forced with GFS only. Fully coupled system is in testing mode. 3 Coupled HWRF-HYCOM

13. Where to go • Original plan called for regional West Coast and Hawaii models. • We need a global model first: • Much time lost with boundary conditions of regional model. • More essential element of ‘national backbone’ • High resolution global model can be backbone for all other NCEP requirement (such as HWRF). • NCEP focus on US coast, eventually higher resolution around the US coast. • Needs to fit in the larger picture. • Will be HYCOM based for several years,

14. 1 NCEP considerations • For short-term ocean forecasts to have value for the NWS, the model needs to be eddy resolving (1/12° spatial resolution or better, Gulf Stream and eddy prediction). In this context, SSH assimilation is crucial. • For ocean modeling in the context of HWRF hurricane modeling, the same holds true for the limited ocean domain used in such models. Nesting in similar large scale ocean models would appear beneficial. • For coupled atmosphere-ocean modeling in the context of weather forecasting, lower resolution ocean models may be acceptable initially. • Lower resolution ocean models have been used in climate modeling, the necessity of high resolution models for climate now is a topic of research.

15. 2 NCEP considerations • NCEP’s operational Atlantic HYCOM system (the Real Time Ocean Forecast System, RTOFS) : • 1200x1684x26 grid points (1/12° spatial resolution). • 8 nodes with 128 processor cores (IBM power 5 mist/dew). • 20min data assimilation, 23min for 24h nowcast and 107min for 5 day forecast. • Running daily on the 00z model cycle only. • This fits `comfortably’ on our system, but we lack the appropriate computational power to provide new initial conditions for a model or model update in a reasonable time. This hampers development and makes model recovery tricky.

16. 3 NCEP considerations • A candidate for global model would be the Navy 1/12° global model. • 4500x3928x32 global grid with Arctic bipolar patch Base resolution is 1/12°. • 24 nodes with 379 HYCOM processes. • 0.9 wall hours for NCODA analysis step, 0.9 wall hours per HYCOM forecast day (IBM power 5+ Kraken) • NRL runs this system on a weekly basis. Courtesy of Alan Wallcraft (NRL)

17. 4 NCEP considerations • NCEP could tentatively run an analysis cycle, a hindcast day and a single forecast day of this Navy model running in a time slot similar to RTOFS-Atlantic using the other three model cycles on mist/dew (8 nodes, 3h per cycle). • NCEP cannot support initialization of this model or serious research using this model on the presently available computer resources at NCEP. • Eddy resolving global ocean modeling at NCEP’s present computer system is only feasible in a close partnership with other entities such as NRL, FSU, etc .....