ABOUT_TPJAOS is the jdata module of jLab.

 ABOUT_TPJAOS  Sea surface height anomalies from the Beckley merged dataset.
    _______________________________________________________________________
    
    
    _______________________________________________________________________
 
    TPJAOS.MAT and TPJAOS.NC contain sea surface height anomalies from the 
    integrated alongtrack altimetry dataset, available online at
 
      https://podaac.jpl.nasa.gov/Integrated_Multi-Mission_Ocean_AltimeterData
 
    This dataset is produced by Brian Beckley and collaborators on NASA's
    Ocean Surface Topography Science Team (OSTST).  It contains data from
    the TOPEX/Poseidon, Jason-1, OSTM/Jason-2, and Jason-3 satellites.
 
    Currently TPJAOS contains data through May 30, 2018.
 
    TPJAOS is a slightly reorganized matfile version of the NetCDF files
    distributed at PO.DAAC, and is redistributed with permission.
 
    TPAJOS.MAT is a Matlab version, while TPAJOS.NC is a NetCDF version.
    These both contain identical data.
 
    TPJAOS is distributed as a part of JDATA, a supplement to the 
    software toolbox JLAB, and is available at http://www.jmlilly.net.
    _______________________________________________________________________
 
    Overview
 
    Various processing steps have been carried out to remove bad data 
    points, as described below.
 
    In Matlab, LOAD TPJAOS for the Matlab version, or NCLOAD TPJAOS for the 
    NetCdf version, leads to a structure containing the following fields:
 
      tpjaos.about     Pointer to this document                   
      tpjaos.num       Cycle start in DATENUM format     [946 x 1]
      tpjaos.dnum      Days from cycle start date        [3375 x 254]
      tpjaos.lat       Latitude                          [3375 x 254]
      tpjaos.lon       Longitude                         [3375 x 254]
      tpjaos.atd       Along-track distance (km)         [3375 x 254]
      tpjaos.dfc       Distance from coast (km)          [3375 x 254]
      tpjaos.depth     Bathymetry depth (km)             [3375 x 254]
      tpjaos.mss       Mean sea surface height (cm)      [3375 x 254]
      tpjaos.ssh       Sea surface height anomaly (cm)   [3375 x 254 x 946] 
   
    The mean sea surface height is given in the PO.DAAC documentation as 
    being the DTU10 Global Gravity field of Anderson (2010). 
 
    Note that sea surface height is here given in centimeters. 
 
    The along-track distance ATD is computed by SPHEREDIST.  Distance from 
    the coast DFC is provided by the source dataset.  DEPTH is computed by 
    interpolating the one-minute Smith and Sandwell database, see READTOPO.
 
    The first dimension, of length 3375, is the number of measurements
    along each track.  The second, of dimension 254, is the number of 
    tracks split into their descending and ascending portions.  The third
    dimension is the number of cycles. 
 
    In Matlab, use the JLAB routine USE, as in 'use tpjaos', to map the 
    fields into named variables in the workspace--num, lat, lon, ssh, etc.
 
    The figure shown at the top is the standard deviation of sea surface 
    height, mapped onto a regular grid with POLYSMOOTH. It can be 
    re-created with 'polysmooth --f2'.  Note that this is computationally 
    expensive and may take a while.
 
    Regional datasets can be extracted with the JLAB function TRACKEXTRACT.
    _______________________________________________________________________
 
    Time details
  
    NUM is the time at the beginning of each cycle, in Matlab's DATENUM 
    format. DNUM is the mean difference from this time, averaged over all 
    cycles, for each location along each track. 
 
    Specifically, NUM is defined as beginning at 4:05 AM on Sept 23, 1992, 
    and proceeding uniformly with a time interval of 9.915645 days.  This 
    matches the first time for each cycle to within about two minutes. 
 
    The exact measurement time is approximating by adding the NUM value for 
    each cycle to the DNUM value appropriate for each alongtrack location. 
 
    This approximate measurement time is given by
 
    FULLNUM=VREP(PERMUTE(NUM,[3 2 1]),[SIZE(DNUM,2) SIZE(DNUM,3)],[2 3])...
        + VREP(DNUM,LENGTH(NUM),3);
 
    which approximates the exact time with an average deviation of less 
    than one minute, and a maximum deviation of less that five minutes.
 
    In order to save space, the exact times for each measurement are not 
    re-distributed here.  
    _______________________________________________________________________
    
    Organization
 
    The data has been re-organized somewhat from its original format.  Each
    track has been split into a descending and ascending portion. 
 
    All descending passes are presented first, but flipped north/south to 
    be sorted by the longitude of their southernmost point, beginning with 
    the first track having a southernmost point east of longitude -180.  
 
    Then all ascending passes are presented, also sorted from longitude 
    -180 based on their initial or southernmost point.
 
    This reorganization makes it easier to locate tracks that fall through
    particular latitude / longitude points.  Also, each row of SSH occurs
    at essentially the same latitude, so one can directly average the 
    alongtrack data to get meaningful statistics across latitudes.
 
    The time offset from the cycle start time are therefore scrambled among
    the different tracks.  These time offsets are recorded in DNUM.
 
    The field ATD is the cumulative along-track distance from the initial 
    point of each track, as computed by SPHEREDIST.
 
    A figure illustrating the TPJAOS format is shown below.  This is 
    the standard deviation of sea surface height, as in the figure at the 
    top but without any mapping.  Each column is a track with ascending and
    descending tracks split.  The distorted view reflects the way the 
    satellite tracks actually sample the ocean. 
    _______________________________________________________________________
    
    
    _______________________________________________________________________
 
    Processing
 
    Several processing steps have been applied to remove bad data points.  
    Firstly, all data over land has been set to NaNs, affecting about 0.3% 
    of all valid data points.
 
    Secondly, the data has been despiked, with spikes set to NaNs.  This
    is done by looking at the statistics of the first central difference. 
 
    A representative standard deviation of this quantity is 4 cm. To
    despike the data, all points having a first central difference with a 
    magnitude exceeding three times this standard deviation, or 12 cm, are 
    set to NaNs.  This affects about 3% of the valid data points.  
    _______________________________________________________________________
 
    Dataset creation
 
    The NetCDF files on which TPJAOS is based are available for download
    from PO.DAAC as follows 
 
        ftp ftp://podaac.jpl.nasa.gov
        cd  allData/merged_alt/L2/TP_J1_OSTM/cycles
        mget *
   
    Note that PO.DAAC distributes two versions, one with each cycle in its 
    own file, and one (at TP_J1_OSTM/all) that has all data in one file.  
    We use the former because updates are available more frequently. 
 
    For completeness, the m-file TPJAOS also contains the processing 
    steps used in the creation of the corresponding matfile.  This 
    requires that the current version of JLAB is on your search path.
 
    Then 'tpjaos --create' will recreate TPJAOS.MAT and TPJAOS.NC by 
    reading in the files downloaded from PO.DAAC. This will take a while.
 
    You'll need to make sure to put the directory containing the files on
    your Matlab search path, e.g. 'addpath ~/Data/tpjaos/cycles'.
    _______________________________________________________________________
 
    Further details
 
    From the handbook, cycles 1--355 are TOPEX/Poseidon, 356--582 are 
    Jason-1, and 583--865 are Jason-2 / OSTM, and cycles 866 and onward are 
    Jason-3.  
    _______________________________________________________________________
 
    See also JDATA, ABOUT_FLOATS, ABOUT_DRIFTERS.
 
    'about_tpjaos --f' generates the two figures shown above.
 
    Usage: about_tpjaos
           about_tpjaos --f 
           about_tpjaos --create     
    __________________________________________________________________
    This is part of JLAB --- type 'help jlab' for more information
    (C) 2014--2018 J.M. Lilly --- type 'help jlab_license' for details

contents | allhelp | index