Download ODV4 - User's Guide - Ocean Data View

ODV User’s Guide
14 Tools
14.1 Geostrophic Flows
You may calculate and visualize geostrophic flows perpendicular to the current section
using the Tools > Geostrophic Flows option. Specify the name of the .odv output collection receiving the results and define a reference layer by choosing a depth horizon, isopycnal or any other iso-surface. By default the output collection is written to subdirectory GeoVel of the parent collection directory. This subdirectory is created automatically if not already present. ODV will start the calculations and export the station-pair
results and geostrophic velocities to the .odv collection specified above.
Once completed, you should view the log file of the calculations and then open the
newly created collection containing the station pair and geostrophic velocity data. You
should define a suitable map domain by zooming, and switch to a SECTION layout by
pressing F11. Then define Geostr. Vel. [cm/s], Geostr. Vel. (north comp.) [cm/s] and/or
Geostr. Vel. (east comp.) [cm/s] as Z-variable of one or more plot windows. The station
label of a given station-pair A->B:alpha consists of the original station numbers A and B
of the stations used in the pair and the direction alpha of positive Geostr. Vel. flow values (alpha=0: eastward; alpha=90: northward; etc.). The meta-variable Distance contains the distance between the two stations in km.
Geostrophic velocities are derived from dynamic height differences between two hydrographic stations. Calculations proceed in two steps: (1) First the data of the two stations
are mapped to a set of standard depths by piecewise linear least squares. (2) Then dynamic heights at the standard depths are calculated for both stations, and the geostrophic velocities for the station pair at the standard depths are obtained from the dynamic height differences. ODV also calculates average values for all variables in the collection (pair-averages). Both, the pair-averages and the geostrophic velocities are representative for the mid-point between the two stations involved, and ODV writes those
values to the output .odv collection for a virtual pair station located at the mid-point between the two stations in the pair. The station label of the pair station contains the station labels of the original stations involved.
Note that geostrophic velocities are not calculated close to the Equator or for stations
with sparse hydrographic observations. Because results are very sensitive to data errors, it is essential to quality control the temperature and salinity data of the section
carefully prior to invoking the Tools > Geostrophic Flows option. One way to proceed is
to flag suspicious data as Questionable or Bad using the Edit Data option and to accept
only Good and Unknown data by applying appropriate sample selection criteria. Also, by
applying appropriate station selection criteria, you should make sure that only stations
for a given expedition are included in the calculations and that stations from other
cruises crossing the section are excluded. You should eliminate stations covering only
part of the water column and only use full, top-to-bottom profiles.
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