Real Time Numerical Modeling of Salinity in the
Chesapeake Bay

The real-time distribution of surface salinity in Chesapeake Bay is generated by running a near-real time version of a three-dimensional hydrodynamic model.  The hydrodynamic model, called CH3D (Curvilinear Hydrodynamics in 3 Dimensions), solves a set of mathematical equations using the existing conditions such as observed freshwater inflows, winds and water levels to predict currents, salinity and temperature distributions in estuaries and coastal environments.  CH3D is a boundary-fitted, or curvilinear, finite difference  Z-coordinate model developed at the U.S. Army Corps of Engineers Waterways Experiment Station (Johnson et al. 1991).  The near-real time version of CH3D used in this project (Chesapeake Bay model grid) has 10,657 computational cells with horizontal grid cell sizes ranging from 1 kilometer in tributaries to 5 kilometers at the outer boundary of the continental shelf region (Wang and Johnson, 1999).  Each  vertical layer is 5 feet (1.52 meters).  To estimate salinity, we "spin up" our version of the model for at least four simulated months prior to the desired date.

The real-time nowcast/forecast system of surface salinity in the Chesapeake Bay consists of three parts:  pre-processing, running CH3D and post-processing (flowchart of the system).   Pre-processing is performed on a workstation at the National Ocean Service (NOS) where real-time observated data is gathered, processed, and transferred to a remote workstation at the Horn Point Laboratory, University of Maryland where the model is run.  Real-time wind data at Patuxent, Norfolk and BWI airport are obtained through the real-time access of meteorology data at NOS.  Real-time water level data at Wachpreaque, VA and Duck, NC are obtained from the NOS Center for Operational Oceanographic Products and Services' web site.  Real-time river discharge information are obtained from the USGS web site.  After the model is run, the output is transferred to NOS where the image of surface salinity is generated.  The image is then transferred to NOAA National Environmental Satellite, Data, and Information Service (NESDIS) where it is fed into a Geographic Information System, along with sea surface temperatures derived from satellite observations, to estimate the probable location of sea nettles.  The system runs on a weekly basis every Sunday night.

References

Johnson, B.H., R.E. Heath, B.B. Hsieh, K. W. Kim, and Butler, H. L. 1991. "User's Guide for a Three-dimensional Numerical Hydrodynamic, salinity, temperature model of Chesapeake Bay" Technical Report HL-91-20, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.

Wang, H.V. and Johnson, B. H. 1999. "Validation and Application of the Second Generation Three Dimensional Hydrodynamic Model of Chesapeake Bay". Submitted to Journal of Water Quality and Ecosystem Modeling.

Revised Thursday May 08 2003by OCS Webmaster