Ole Martin Smedstad

Variational Data Assimilation for the Global Ocean

A fully three dimensional, multivariate, variational ocean data assimilation system has been developed that produces simultaneous analyses of temperature, salinity, geopotential and vector velocity. The analysis is run in real-time and is being evaluated as the data assimilation component of the Hybrid Coordinate Ocean Model (HYCOM) forecast system at the U.S. Naval Oceanographic Office. Global prediction of the ocean weather requires that the ocean model is run at very high resolution. Currently, global HYCOM is executed at 1/12 degree resolution ( ∼ 7 km mid-latitude grid mesh), with plans to move to a 1/25 degree resolution grid in the near future ( ∼ 3 km mid-latitude grid mesh). These high resolution global grids present challenges for the analysis given the huge model state vector and the ever increasing number of satellite and in situ ocean observations available for the assimilation. In this paper the development and evaluation of the new oceanographic three-dimensional variational (3DVAR) data assimilation is described. Special emphasis is placed on documenting the capabilities built into the 3DVAR to make the system efficient for use in global HYCOM.

Global Ocean Prediction Using HYCOM

One important aspect of ocean model design is the choice of the vertical coordinate system. Traditional ocean models use a single coordinate type to represent the vertical, but model comparison exercises performed in Europe (DYnamics of North Atlantic MOdels - DYNAMO) (Willebrand et al., 2001) and in the United States (Data Assimilation and Model Evaluation Experiment - DAMEE) (Chassignet et al., 2000) have shown that none of the three main vertical coordinates presently in use (depth [z-levels], density [isopycnal layers], or terrain-following [sigma-levels]) can by itself, be optimal everywhere in the ocean. The HYbrid Coordinate Ocean Model (HYCOM) (Bleck, 2002) is configured to combine all three of these vertical coordinate types. It is isopycnal in the open, stratified ocean, but uses the layered continuity equation to make a dynamically smooth transition to a terrain-following coordinate in shallow coastal regions, and to z-level coordinates in the mixed layer and/or unstratified seas. The hybrid coordinate extends the geographic range of applicability of traditional isopycnic coordinate circulation models toward shallow coastal seas and unstratified parts of the world ocean. It maintains the significant advantages of an isopycnal model in stratified regions while allowing more vertical resolution near the surface and in shallow coastal areas, hence providing a better representation of the upper ocean physics

Eddy Resolving Global Ocean Prediction

This is the first year of a three-year Challenge Project with the principal goal of performing the necessary research and development to prepare to provide real time depiction of the three-dimensional global ocean state at fine resolution (1/25° on the equator, 3.5 km at mid-latitudes, and 2 km in the Arctic). The prediction system won’t run in real time until FY12, since this is when the first computer large enough to run it in real time is expected to be available at NAVOCEANO. A major sub-goal of this effort is to test new capabilities in the existing 1/12° global HYbrid Coordinate Ocean Model (HYCOM) nowcast/forecast system and to transition some of these capabilities to NAVOCEANO in the existing 1/12° global system, and others in the 1/25° system. The new capabilities support (1) increased nowcast and forecast skill, the latter out to 30 days in many deep water regions, including regions of high Navy interest, such as the Western Pacific and the Arabian Sea/ Gulf of Oman, (2) boundary conditions for coastal models in very shallow water (to zero depth with wetting and drying), and (3) external and internal tides, the latter with initial testing at 1/12° but transition to NAVOCEANO only in the 1/25° system (all these will greatly benefit from the increase to 1/25° resolution). At 1/25°, the entire first year will be spent on initial climatologically forced non-assimilative simulations that are necessary before we can start data assimilation hindcasts. At 1/12°, we have started exploring improved model configurations with climatologically forced runs and testing improved data assimilation with hindcast cases.

Real-time Data Assimilation of satellite derived ice concentration into the Arctic Cap Nowcast/Forecast System (ACNFS)

Over the last decade, ice conditions in the Arctic have changed dramatically resulting in the Arctic having a minimum in ice extent during the summers of 2007, 2008 and 2010. With this rapidly changing polar environment, the need for accurate ice forecasts is essential. The Naval Research Laboratory (NRL) has developed the Arctic Cap Nowcast/Forecast System (ACNFS), a two-way coupled ice/ocean system, to forecast ice conditions in the polar regions. This system applies the Los Alamos Community Ice CodE (CICE) coupled via the Earth System Modeling Framework (ESMF) to the HYbrid Coordinate Ocean Model (HYCOM). The Navy Coupled Ocean Data Assimilation (NCODA), a 3-Dimensional VARiational analysis (3DVAR) scheme, is used to assimilate ice and ocean observations into the forecast system. Ice concentration data from two sources: the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager (SSM/I) and the Advanced Microwave Scanning Radiometer for Earth Observation System (AMSR-E) are used as observations for the ice analysis. Results from the coupled system using both concentration input datasets will be discussed.