To provide HYCOM (Hybrid-coordinate ocean model) with data-assimilating capability.
Simulations of oceanic circulation, whether for forecasting evolving temperature distributions and their impact on climate or for analyzing the how the ocean has been evolving in the recent past in order to improve our understanding of its dynamics, can be improved by assimilating observations. Errors in the simulations due to inadequate knowledge of the ocean's initial conditions and its forcing by the atmosphere as well as those that are due to deficiencies in the model's dynamics can be corrected by available observations.
This project is a component of a multi-institutional collaboration with the objective of developing HYCOM into an operation tool for oceanic forecasting. HYCOM is characterized by its treatment of the vertical coordinate. while conventional models employ as so-called z-coordinate with oceanic fields computed on a grid of points separated by fixed distances, HYCOM describes the ocean in terms of a stack of layers whose thicknesses and characteristics change dynamically as the flow evolves. For the bulk of the ocean the layers should correspond to water of specified densities in order to capture the well-defined stratification, while near the surface where the water is mixed, the layers are specified by their distances from the surface rather than density, and in shallow regions their thicknesses are proportional to the local water depth. such a modeling strategy allows HYCOM to use the optimal coordinate system for each region and to transform naturally from one to the other. However, such a model design complicates the task of assimilating observations.
The method for assimilating data into HYCOM is optimal interpolation, where the corrections to the model state are based on the differences between the simulation and the observations with the size and influence of the corrections determined by the error covariances of both model and data. Implementing this method requires taking into account the models evolving layers, so that where they are isobaric density rather than pressure is corrected, where they are isopycnic pressure rather than density is corrected, and in between both are corrected to some extent.
Current efforts toward at making HYCOM operational are centering on a data-assimilation package the Navy has developed for use with more conventional models. This package already includes the important modules for handling both archived and real-time observations and has efficient algorithms for implementing optimal interpolation. Efforts are underway to interface this package with HYCOM. A first attempt has focused on the interpolating back and forth between the native vertical coordinates of the two systems, but plans exist for modifying the data-assimilation package so that such interpolations will not be needed.
Implementation of optimal interpolation for HYCOM's hybrid coordinate system is described in the papers by Thacker and Esenkov (2002) and Thacker, Lee, and Halliwell (2004). The issue of the compatibility of the data and their error statistics, which are the basis for optimal interpolation is discussed by Thacker (2003).
Thacker, W. C. and O. E. Esenkov (2002) Assimilating XBT data into HYCOM, Journal of Atmospheric and Oceanic Technology, 19, 709--724.
Thacker, W. C. (2003) Data-model-error compatibility, Ocean Modelling, 5, 233-247.
Thacker, W. C., S.-K. Lee and G. R. Halliwell, Jr. (2004) Assimilating 20 years of Atlantic XBT data into HYCOM: a first look, Ocean Modelling, 7, 183-210.