The Marine Modeling and Analysis Programs (MMAP) branch of the Coast Survey Development Laboratory has developed a nowcast/forecast model for the St. Johns River, Florida. Development of this model was sponsored through the Coastal Storms Program, a nationwide effort led by the National Oceanic and Atmospheric Administration (NOAA) to lessen the impacts to coastal communities from storms. To accomplish this goal, local, state, and federal organizations are working together on site-specific projects.
The circulation model was originally developed by the St. Johns River Water Management District (SJRWMD) as part of a program to assess water quality standards in the lower St. Johns River. An orthogonal, boundary-fitted structured grid was set up to extend from the Atlantic Ocean near Mayport to an upstream limit of Buffalo Bluff. The Environmental Fluid Dynamics Code (EFDC) was then applied to this grid to simulate water levels, currents, temperature and salinity over approximately three years, from 1995-1998.
MMAP adapted this EFDC application for use in making realtime simulations in a nowcast/forecast environment. Nowcast simulations are set up to run every hour, reproducing river conditions over the previous hour. Forecast simulations extend from the present time out to 36 hours in the future and are made four times a day. While the grid and the model are the same as those originally used in the SJRWMD hindcast simulation, the inputs to the nowcasts and forecasts require realtime information as forcings for the model. These realtime forcings are described in more detail below:
Open Ocean Boundary
The open ocean boundary in the model is forced with water levels, salinity and temperature. Water levels for the nowcast are derived from observations at a nearby National Water Level Observation Network (NWLON) tide gauge in Mayport. For the forecast, the water level is determined by adding the tide prediction at Mayport with forecasts of the subtidal water level made by the National Weather Service's Extratropical Storm Surge model. The open ocean boundary salinity is specified as 35 PSU (practical salinity units) at the surface and linearly increases to 36 PSU at the bottom. These values are held constant in time. Temperatures are forced with values based on climatology.
The upstream limit of the grid is at Buffalo Bluff, just north of Lake George. Real-time river discharge, temperature and conductivity are available from the United States Geological Survey at Buffalo Bluff. These are used as the upstream boundary condition to represent the main-stem flow of the St. Johns River.
Even at this upstream boundary, Buffalo Bluff is affected by the ocean tide. Therefore, the net discharge can potentially be negative (directed upstream) at times. To help prevent artificial reflection of the tides at Buffalo Bluff, a sponge boundary condition is used. Real-time river discharge is also specified at Dunns Creek, Rice Creek, Etonia Creek, Simms Creek, Deep Creek, South Fork Black Creek, North Fork Black Creek, and Black Creek near Doctors Inlet. For the forecast, river discharge is currently persisted from the last value in the nowcast.
For the nowcast, winds are currently derived from a station in Mayport. The forecast uses winds computed interpolated from the North American Mesoscale model at the same Mayport location.
Once the nowcast/forecast system was evaluated using NOS’ skill assessment software for operational models, it was ported to the Center for Operational Oceanographic Products and Services (CO-OPS) for operational implementation. Modeled water levels, currents, temperature and salinity are displayed in realtime and contrasted with observations where they are available.