# vgeo

##

Last updated Fri Nov 19 12:59:54 1999

Calculate geostrophic current

## 1. USAGE

vgeo control_file

## 2. DESCRIPTION

This utility calculates the geostrophic current from CTD profiles located in a CODAS database. Several methods are implemented to determine the reference level. However, be aware that this utility is still experimental.

## 3. PARAMETERS

The control file must contain the following parameters:

year_base YYYY

  Year base to use for converting time into decimal days.

ctd_db: name

  Name (with path) of CTD database to visit.

output name

  Base name (with path) for output files.

## 4. OPTIONS

There are many options for controlling the behaviour of vgeo. These are:

OPTIONS:

  This key word can be used to improve the clarity of the control file and specify the beginning of options. It has no effect.

reference_method: keyword

  Specify the method for determining the reference level. Default is ctd_max_depth. Possible keywords are:

  adcp_min_H_shear

    This method uses ADCP data from a separate CODAS database or external file. It calculates the horizontal current shear in the section direction and select the reference level at the depth where this shear is minimal. The mean velocity at this depth is taken as the reference velocity. Further options for this method are:

    range:

      Specify the range of profiles to consider from the external ADCP data. Default is a time range with all profile times. The following syntax are allowed:

      TIME_RANGE: YYYY/MM/DD hh:mm:ss to YYYY/MM/DD hh:mm:ss to specify a time range.

      DAY_RANGE: yd to yd to specify a time range given with decimal days.

      BLOCK_RANGE: blk to blk to specify a range of blocks. Be aware that blocks are numbered from zero.

      BLKPRF_RANGE: blk prf to blk prf to specify a range by block and profile numbers. Be aware that blocks and profiles are numbered from zero.

    data_profiles: keyword

      Specify which ADCP profiles to choose for calculating the minimal horizontal shear at profiles of geostrophic current. Available possibilities are:

      constant

        Always choose a constant number of profiles. These are chosen so that they are centered on the geostrophic profile. In this case, the following options are implemented:

        nprf= n

          Specifies the number of profiles to choose. There is no default value for this option, i.e. it has to be defined if option constant is chosen.

        end

          This keyword is mandatory and terminates the list of selected sub-options.

      between_ctd

        All ADCP profiles between the two CTD profiles used to calculate the geostrophic current are used. This is the default.

    profile_tolerance= float

      Because ADCP bins can contain bad data which are not taken into account when calculating the horizontal shear, one has to avoid the reference level to be chosen at depths where too few points where available. This parameter specifies the minimal value of good profiles relative to the total number of profiles considered. This value is used at each depth to validate the computed current shear. Default value is 0.75.

    data_good_bins= n

      Minimal number of good bins within an ADCP profile. This parameter is used to validate those profiles that might be considered healthy. Default is 15.

    data_file: file

      Name (with path) of database or external ASCII file containing the ADCP data to use for determining the horizontal current shear.

    use_adcp_pos

      Specifies that geostrophic profile positions should be taken from the ADCP data instead from the CTD ones. This can be useful when CTD data have been collected simultaneously with a CTD SeaSoar, since ADCP positions undergo usually a more consistent calibration procedure.

    flag_data

      When extracting current data from an ADCP CODAS database, data can be flagged according to the profile flags. It is a good idea to specify this option if the database has been edited and updated.

    data_format: keyword

      This option specifies the format of the specified file containing ADCP data to use for calculating the current shear. Implemented formats are:

      ascii

        Read current data from a plain ASCII file. This method is not completely implemented yet. The idea was to read data from a contour file generated by adcpsect and con2con(1). If only a plain file is available, then one should use con2db(1) to create a CODAS database from it and select keyword codas_db instead.

      codas_db

        Read data from an ADCP CODAS database. This is the default.

    data_interp_axis: keyword

      This option specifies an axis common to the CTD and ADCP data. Default is time. Implemented axes are:

      time or none

      longitude

      latitude

    adcp_reference: keyword

      This option specifies the reference for calculating absolute currents from data stored in an ADCP CODAS database. Possible references are:

      none

        Use current values as is. This is useful if the database already contains absolute currents, such as one created by con2db(1).

      ship

        Use the final ship velocities stored in the ACCESS_VARIABLE structure of the database. This is the default.

      navigation

        Use the navigation data stored in the NAVIGATION structure of the database.

    end

      This keyword is mandatory and terminates the list of selected sub-options.

  ctd_max_depth

    This method defines the reference level to be at the lowest available cell depth of the calculated relative geostrophic current. Further options for this method are:

    v= float

      Specify the reference velocity (in m/s) to be used at the reference level. Default value is zero.

    end

      This keyword is mandatory and terminates the list of selected sub-options.

  const_depth

    The reference level is chosen at a constant depth through the whole section. Further options for this method are:

    v= float

      Specify the reference velocity (in m/s) to be used at the reference level. Default value is zero.

    z= float

      Specify the reference level depth (in m). No default is supplied for this option.

    end

      This keyword is mandatory and terminates the list of selected sub-options.

ctd_sort_axis: keyword

  Before calculating the geostrophic current, CTD profiles are sorted along the specified axis. Implemented axes are:

  time or none

    This is the default.

  longitude

  latitude

vgeo_direction: keyword

  The geostrophic current is calculated so that positive values correspond to a direction starboard to the chosen direction. Possible direction keywords are:

  northward

    This is the default.

  southward

  eastward

  westward

grid: keyword grid

  Specifies grid points for a separate gridded output. This is useful if one is only interested to have the value of the geostrophic current at specified depths. The calculated values are linearly interpolated to the given points. Currently, only one method for specifying a grid is implemented. Hence, the value of keyword must be grid_list:. It must be followed by the grid specification corresponding to this method. This is

  > number= n z1 z2 ... zn

  where n is the number of grid points and z1-zn are the n depths where data should be regridded.
time_tolerance= n

  Time tolerance in seconds for positioning inside CTD and ADCP data. Default is 30 seconds.

ctd_good_bins= n

  Minimal number of good bins allowed in a CTD profile for considering this profile as valid. Default is 20.

velocity_scale= float

  Scaling factor for geostrophic current in output files. A value of 0.001 will output data in cm/s. Default is 1.

ctd_time_offset= n

  Time offset (in seconds) to add to CTD profile times if these are wrong. This is especially important when CTD and ADCP are combined using time as a synchronization parameter between both datasets. Default is 0.

ctd_steps= n

  Number of CTD profiles to move within the CTD database. Default is 1.

longitude_180

  Specify this for converting longitude values between -180 and +180 degree before using them.

stat_output

  This option generates an output file about statistical data.

log_output

  This option generates a log file.

contour

  This option generates a contour file.

end

  This keyword is mandatory and terminates the list of selected options.

## 5. TIME RANGES

Finally, the control file is terminated by a list of time ranges. See time_rng(5) for the syntax of these.

## 6. OUTPUT FILES

The generated files have all the name specified by parameter output but differ according to the following list of suffixes:

con

  Contour data of geostrophic current. It is turned on with option contour. This file has the following columns:

  1. Time in decimal days
  2. Longitude in decimal degrees
  3. Latitude in decimal degrees
  4. Depth
  5. Geostrophic current
  The unit for the depth is the same as in the CTD database. However, during the whole process it is assumed to be in meter, and to be the same as in ADCP external datasets if these are used during the process. The units for the geostrophic current depends on the value of option velocity_scale.
sta

  Statistics about reference level. It is turned on with option stat_output. This file has the following columns:

  1. Time in decimal days
  2. Longitude in decimal degrees
  3. Latitude in decimal degrees
  4. Depth of reference level
  5. Value of reference velocity
  6. Uncertainty of reference velocity at chose reference level
  7. Number of points used to determine the reference level
  These data are especially useful if the method used to determine the reference level uses the minimal horizontal current shear from external ADCP data. It allows to control if the calculated values are consistent. The Matlab script vgeo.m can be used to visualize these data. Units are the same as those discussed for the contour output.
log

  Additional log information. This is useful to control how the geostrophic current has been determined. It is turned on with option log_output.

grd

  Regridded values of the geostrophic current. It is turned on if a grid is specified (see option grid). This file has the following columns:

  1. Time in decimal days
  2. Longitude in decimal degrees
  3. Latitude in decimal degrees
  4. Selected depth
  5. Geostrophic current
  Each line is repeated for each specified grid depth before data for a new profile is encountered. The unit of the geostrophic current here is not scaled. Hence, it is assumed to be in m/s.

## 7. AUTHOR

Pierre Jaccard, Geophysical Institute, University of Bergen, 1999

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