Chesapeake 3dimensional Physical Oceanographic Model

The circulation of a large estuary, like Chesapeake Bay, is driven by a combination of winds, tides, oceanic water level, river discharge and density gradients inside the bay.  The astronomical tides by themselves are the most noticeable motion in the bay.  But wind driven currents can change the water level by more than 3 feet at times.  The water level at the mouth of the bay, set by the oceanic water level, has a large influence on the water levels throughout the bay.  To accurately simulate and predict the water levels in the bay all of these forcings must be taken into account.

Water levels throughout Chesapeake Bay are simulated by a numerical model run twice daily on the SGI computer at NOAA's Coast Survey Development Laboratory in Silver Spring MD.  The numerical model brings together the observations of water level, winds and river discharges of the past 24 hours to run a simulation of the water level throughout the bay, the nowcast.  A forecast of the water levels are produced by replacing the observation data with wind field forecasts from the NCEP Eta meteorological model, and oceanic water level forecasts from the MDL Extra Tropical Storm Surge model

The results of the model synthesis of data and forecasts are plots of the water level at several selected stations throughout the bay.  These plots show the current observations of the water level, the astronomical tidal prediction the model's calculated water level for the nowcast (the past 24 hours) and the forecast (the next 24 hours).

Click here to view a water level plot of Solomons Island.

The water motions in Chesapeake Bay are quite complicated.  The strongest forcing is the tidal oscillation causing the water level to rise and fall twice a day at every point in the bay.  But the water is moving in different directions in different parts of the bay.  When the tide is high at the mouth, near the Chesapeake Bay Bridge Tunnel, it can be low mid way up the bay, near Solomons Island, while still further up the bay the water level may be high, at Baltimore Harbor.  This difference in times of high and low water level is due to a very long, but low, wave which propogates up the bay, taking roughly twelve hours to move from the mouth to the head.  The motion of this wave is displayed in this image, showing the exagerated sea surface undulating through a tidal cycle (the stationary image of the bay below shows the bathymetry.)  Notice that the amplitude of the up and down motions is greatest at the mouth of the bay and decreases as the wave moves up the bay, losing energy to friction as it travels from Norfolk to Baltimore.

Click here for a 3D Chesapeake Bay Sea Surface animation (3.7M).

The sea surface motions are quite complicated.  But the water currents are even more so.  Again the currents are driven primarily by the tidal motions.  But the complicated bathymetry of the bay bends and curves the currents around islands and through the deep central channel.  The central channel has a very great effect on the currents.  Water, once put into motion moving down the channels has great inertia and tends to continue in motion.  When the tide is reversing the water on the shallow banks on either side of the channel will quickly turn and start heading in the opposite direction while the channel current takes more time to turn.  The figure below shows the shallow currents on either side of the channel moving southward while the flow down the channel is still 0.20 m/s northward (1 Knot = 0.51 meters per second.  The inset plot is a tidal cycle of currents at the location of the CBOS buoy, marked by the black dot in the NW corner of the plot.).  Of course this sort of complication is mainly evident near high or low water when the currents are weak and reversing direction.

Click here for a Velocity Vector Plot animation (7M).

The Chesapeake Bay water level calculations are performed using a Finite Element Model ( Quoddy http://www-nml.dartmouth.edu/circmods/gom.html).  The Quoddy model uses an unstructured mesh of triangles to allows great flexibility and has advanced physics, including the full three dimensional transport of Salinity and Temperature, fresh water river sources and wind forcing.  The output of the model is the surface elevation, surface water velocity vectors and the salinity, temperature and density fields throughout the bay.  The main forcing of the model is the imposed oceanic water level and the wind blowing across the bay.  The oceanic water level and wind are obtained from measurements taken by instrumented stations in the bay in nearly real time.  The data are accessed via satellite and ground lines and then processed and made available on the internet by NOAA/NOS/CO-OPS. For forecasts the model relys on the CBRAMS model of the wind field and NOAA/Techniques Developement Laboratory model of coastal sea level.

C3PO uses an unstructured grid of triangular elements to accurately portray the geometry of the bay. The triangles vary is size from very large 10 kilometers off shore to small 200m elements used to define the channel and harbors of the complicated inlets throughout Chesapeake Bay.  By varying the size of elements great resolution may be had where it is needed while limiting the total number of elements to make the most of our limited computer resources.  The grid has 6919 nodes and 10000 triangles and uses 9 levels to resolve the depth varying structure of the water column.