Non-tidal Ocean Loading Service


Contents:
  • Abstract.
  • Change List
  • Introduction
  • Hydrology data
  • Service Information
  • Plots
  • Data for downloading
  • Method
  • Other Services
  • References
  • Abstract

    This page provides links to files relevant to the service of the non-tidal ocean loading provided by NVI Inc. and the VLBI group at NASA Goddard Space Flight Center. The service provides 12-hour time series of 3-D displacement for many VLBI sites and are updated four times a year (or upon request) when new ocean bottom pressure data is released by JPL.

    Change List

    2012.06.27 Three Sites added to the station list.
    2013.08.13 A gridded 1 x 1 degree product is currently being generated

    Introduction

    Mass loading is known to cause surface deformations of the earth. Changes in water mass, atmospheric pressure, ice mass etc. will therefore result in crustal deformations of the surface. For non-tidal ocean loading we observe peak to peak variations that can reach up to 20 mm in the vertical component, but usually the peak to peak variation of the non-tidal ocean loading is between 2-3 mm in the vertical component. The horizontal displacements are much smaller and the peak-to-peak variation is usually smaller than a millimeter.

    Non-tidal ocean loading is usually very seasonal and is most significant near the coast where the variation in ocean bottom pressure is large. Surface displacements are calculated both w.r.t. the center of the total earth (the mass of the solid earth + the mass that causes the loading), and w.r.t. the center of the solid earth.

    Ocean bottom pressure data

    The model that is being used is the ECCO model produced by JPL. It provides 12-hour ocean bottom pressure on a non-equal angular grid with 224 latitudes and 360 longitudes. The model only ranges between -80 to 80 degrees latitude but this won't have a big effect on the loading series. The model is not mass conserving which means that there is no support for the trends that can be seen in the loading series. It is recommended that the trend is taken about before analyzing the loading series.

    Service Information

    The series of the 3-D displacements with respect to the center of mass of the total Earth and the center of mass of the solid Earth due to non-tidal ocean loading with a 12-hour time resolution are computed for 170 VLBI sites.

    Plots

    These are plots of the non-tidal ocean loading series for the four stations Matera (Italy), Onsala (Sweden), Tsukuba (Japan) or Wettzell (Germany). All four sites are strong VLBI sites. Only the vertical displacement is plotted since this is the dominant part of the loading series. Even though non-tidal ocean loading generally is a much smaller signal than hydrology loading, there are displacements that can reach 20 mm.

    text describing the image

    Plots of the series for the displacements with respect to the solid earth can be found at
    Displacements wrt the center of mass of the solid earth

    Plots of the series for the displacements with respect to the total earth can be found at
    Displacements wrt the center of mass of the total earth

    Data for downloading

    Last available epoch

    Displacements with respect to the center of the total earth

    File Format Size Comment
    nontidal_oclo_cmte.zip ASCII 44.4 Mb 12-hourly non-tidal ocean series for all stations in the station list. The displacements are computed from January 1993 until the last epoch of data released by the JPL team. There is one file per station and for each station there are four columns. The first column is the date in modified julian dates. The second column is the vertical displacement, the third the East-West Horizontal displacement and the fourth the North-South horizontal displacement. All displacements are given in meters. (IMPORTANT: A linear trend must be subtracted from the loading series)
    nontidal_oclo_cmte ASCII Link to a directory with separate surface displacements for each station.
    gridded_nontidal_oclo_cmte netcdf 70 Mb/month Global 1 x 1 degree map of monthly files with the loading series given for the each lattice point (181 x 360). From this global grid, the series can be interpolated bilinearly to any point of interest. To read netcdf in matlab: Mexnc. To read netcdf in other programming languages such as C++ or Fortran: Unidata (IMPORTANT: A linear trend must be subtracted from the loading series)

    Displacements with respect to the center of the solid earth

    File Format Size Comment
    nontidal_oclo_cmse.zip ASCII 44.6 Mb 12-hourly non-tidal ocean series for all stations in the station list. The displacements are computed from January 1993 until the last epoch of data released by the JPL team. There is one file per station and for each station there are four columns. The first column is the date in modified julian dates. The second column is the vertical displacement, the third the East-West Horizontal displacement and the fourth the North-South horizontal displacement. All displacements are given in meters. (IMPORTANT: A linear trend must be subtracted from the loading series)
    nontidal_oclo_cmse ASCII Link to a directory with separate surface displacements for each station.
    gridded_nontidal_oclo_cmse netcdf 70 Mb/month Global 1 x 1 degree map of monthly files with the loading series given for the each lattice point (181 x 360). From this global grid, the series can be interpolated bilinearly to any point of interest. To read netcdf in matlab: Mexnc. To read netcdf in other programming languages such as C++ or Fortran: Unidata (IMPORTANT: A linear trend must be subtracted from the loading series)

    Additional files

    File Format Size Comment
    station_list ASCII 14 kb List with all stations for which we are providing non-tidal ocean loading series
    CMTE Green's function ASCII 0.72 Mb Values of the Green's function with the center of mass being the total earth, the solid earth + the mass that causes the loading.
    CMSE Green's function ASCII 0.72 Mb Values of the Green's function with the center of mass being the solid earth.

    Method

    We computed non-tidal ocean loading by convolving Farrell's loading Green's function derived on the basis of the preliminary Earth model (PREM) with of the ocean bottom pressure field with 12-hour time resolution provided by JPL model after subtraction of the mean from each cell. The displacements are computed both with respect to the center of mass of the total earth and the center of mass of the solid earth.

    According to Farrell (1972)
    [1]. the displacements at time t due to mass loading at a point with coordinates (λ,φ) are given by

    where
    text describing the image
    Furthermore, A is the azimuth, Δm is the change in mass, ψ is the angular distance from (λ',φ') to (λ,φ), a is the radius of the earth, G the universal constant of gravitation, g0 the mean surface gravity and Pn is the Legendre polynomial of degree n. Here l'n and hn' are the load Love numbers that are required to compute the loading. To be able to describe displacements due to loading, a wide range of Love numbers are needed. Love numbers are computed up to degree (n=9000) for a spherically, non-rotating, elastic and isotropic Earth model.

    Data citation

    "Non-tidal ocean loading displacements provided by the NASA GSFC VLBI group which are avaliable at http://lacerta.gsfc.nasa.gov/oclo (Eriksson & MacMillan). The displacement series are computed from the ECCO model provided by the JPL team"

    Other Services

    Tropospheric delays:
    http://lacerta.gsfc.nasa.gov/tropodelays
    Hydrology loading: http://lacerta.gsfc.nasa.gov/hydlo
    Temperature and Pressure http://lacerta.gsfc.nasa.gov/met

    References


    1. W.E Farrell. Deformation of the earth by surface loads. Rev. Geophys. and Spac. Phys., 10(3):751-797, 1972


    This page was created by David Eriksson (
    david.eriksson89@gmail.com )

    Last update: 2013.08.13


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