- ADCP
- (Acoustic Doppler Current Profiler)
An instrument which generates a 3-dimensional current profile
by analyzing the doppler-shift of fixed-frequency acoustic echos.
See
Jeffrey Gartner's Home Page for a diagram and explanation.
The instrument measures the Doppler-shifted, fixed-frequency echos backscattered
from scatterers (plankton and sediment) in the water and converts the echos
to north/south, east/west, and vertical velocity
components. Velocity profiles are determined by range gating echos so
that velocities are determined at pre-set intervals along the acoustic path
(called bins). Velocity measurements with bin size as small as 5cm are
possible with the Broad Band version of the ADCP operating in certain
high resolution modes.
- Bathymetry
-
The underwater equivalent of topography. Bathymetry describes the spatial variations of water depth (ie locations and depths of undewater hills, plains, valleys etc).
- Datum
For marine applications, a base elevation used as a reference from
which to reckon heights or depths. It is called a tidal datum when defined in
terms of a certain phase of the tide. Tidal datums are local datums and
should not be extended into areas which have differing hydrographic features
without substantiating measurements. In order that they may be recovered when
needed, such datums are referenced to fixed points known as bench marks.
A chart datum is the datum to which soundings on a chart are referred.
It is usually taken to correspond to a low-water elevation (ie Mean Lower-Low
Water).
The datum used on this site is:
- MLLW -- Mean Lower Low Water
There are two low tides in each tidal cycle (so usually two low tides in each day). These two low tides are not quite the same height because one tide is generated by the gravitational interaction with the sun (which is small), and the other is generated by the gravitational interaction with the moon (which is not so small). Since the two low tides (or water levels) are different levels of low, one is naturally the higher low water (higher low tide) and the other is the lower low water (lower low tide). So Mean Lower Low Water is the average of the lower low water height of each tidal day (ie average of the lowest low tide from each day). The averages are taken over a period called the National Tidal Datum Epoch (a 19-year epoch).
For a more technical definition of MLLW and other terms related to
tides and currents consult the NOAA publication
"
Tide and Current Glossary", NOS [4]
- Knot
A speed of 1 nautical mile per hour (abbreviated kt). A speed of 1 nautical mph (1 knot) is equal
to 1.15 mph or 1.85 kph. This is commonly used in navigation and meteorology.
- Mean Lower Low Water (MLLW)
See Datum:MLLW above.
- Now-cast
Using computer modeling and simulations to extrapolate a complete,
current set of data from a limited set of real-time data.
The SF PORTS project combines real-time data from 5 current sensors
in the bay with the TRIM model developed by
Cheng and Casulli to produce a complete
map of the current vector field in the SF Bay.
The
San Francisco Bay Area Wind Page is another shining example of nowcasting.
- PORTS
"The Physical Oceanographic Real-Time
System is a program of the National Ocean Service that
supports safe and cost-efficient navigation by providing ship masters and
pilots with accurate real-time information required to avoid groundings and
collisions." (taken from
NOS PORTS
page).
- TRIM
- Tidal, Residual, and Intertidal Mudflat Model.
"Trim also means simple and elegant, a goal that we are striving
for."
The simulation code, or the "engine", was the contribultion of Vincenzo Casulli
and his research team at the
University of Trento, Italy. The hydrodynamic
model solves the nonlinear, depth averaged shallow-water equations using
a semi-implicit finite-difference scheme in conjunction with an Eulerian-
Lagrangian mass-conservative treatment of the convective terms (See Casulli [1] for details).
Further enhancement of the model includes the treatment of Tidal,
Residual, and Intertidal Mudflats. Thus the numerical model system is known
as TRIM2D.
The present version of the code has been improved by Casulli and Cattani
using a Crank-Nicolson type of time integration which gives better numerical
accuracy and computational efficiency [3].
TRIM2D has been applied to San Francisco Bay and it has been
calibrated and validated against an extensive field data set by Cheng,
Casulli and Gartner, ([2]).
Citations and References
Users of the TRIM modeling system are encouraged to consult the following
references:
[1] Casulli, V., 1990, Semi-implicit Finite Difference Methods for the Two-
Dimensional Shallow Water Equations, J. Computational Physics,
Vol. 86, No. 1, p. 56-74.
[3] Casulli, V., and E.Cattani, 1994, Stability, Accuracy and Efficiency of
a Semi-implicit Method for Three-Dimensional Shallow Water Flow,
Computers Mathematics Applications, Vol. 27, No. 4, p.
99-112.
[4] Hicks, Steacy D.,
"Tide and Current Glossary"
National Ocean Service, NOAA, US Dept. of Commerce
January 1984, Rockville, MD
