Hydrology Loading Service


Contents:
  • Abstract
  • Change List
  • Introduction
  • Hydrology data
  • Service Information
  • Plots
  • Movies of the loading components
  • Data for downloading
  • Method
  • Other Services
  • References
  • Abstract

    This page provides links to files relevant to the service of the hydrology loading provided by NVI Inc. and the VLBI group at NASA GSFC. The service provides monthly time series of 3-D displacement for many VLBI sites and are updated every month when new new hydrology data is released by the NASA GSFC GLDAS team. The data products on this site are IERS Global Geophysical Fluids Center Provisional Products.

    Change List

    2012.03.30 Correction of invalid series for the sites DSS13 and YEBES40M due to invalid coordinates.
    2012.06.11 Series updated and gridded series are now in netcdf-format.
    2012.06.25 Three Sites added to the station list.

    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 hydrology we observe peak to peak variations that can reach up to 2-3 cm in the vertical component, but usually the peak to peak variation of the hydrology loading is between 3-10 mm in the vertical component. The horizontal displacements are much smaller and the peak-to-peak variation is usually not more than a few millimeters.

    Hydrology loading is usually very seasonal and is most significant in South America, Southern Africa and Asia where the variation in water mass is large. Surface displacements are calculated both with respect to the center of mass of the total earth (the mass of the solid earth + the hydrology mass), with respect to the center of mass of the solid earth.

    Hydrology data

    The model that is being used is the GLDAS NOAH Hydrology model, Rodell et al. (2004)
    [1]. Since the GLDAS models doesn't account for ice sheet processes, areas with permanent frost are recommended to be masked out and this is being done in our loading calculations. The model has three parameters that are of interest to us and that is soil moisture, snow water equivalent and canopy water. No data is given over oceans and the model has no parameter for groundwater.

    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 hydrology loading with a monthly time resolution are computed for 170 VLBI sites.

    Loading time series are provided as a compressed directory (zip) containing the loading time series for a total of 170 VLBI sites or as separate text files for each station. Stations can be added to the station list upon request by sending an email to
    david.eriksson89@gmail.com

    A global angular grid of size 1x1 degrees is also provided so that users can interpolate to any point of interest. The resolution of the grid is fine enough that interpolated series are in general close to the exact series generated at the interpolation point. Plots are provided under the Plot section where we compare the interpolated loading series to the series from our model.

    Plots

    Th are plots of the hydrology loading series for the two stations Hartrao, South Africa and Wettzell, Germany. Both sites are strong VLBI sites that are characteristic midlatitude and inland sites. This means that the loading series are not sensitive to errors in the land-sea mask. It is clear that all three components are very seasonal for Wettzell while Hartrao shows sign of a little more interannual variation, especially in the East-West component. These series show the displacement with respect to the center of the solid earth.



    A global amplitude and phase plot of the hydrology loading. The loading series are computed for each lattice point on a 1 x 1 degree global grid. For every lattice point, the amplitude and phase are estimated by fitting the function to the the vertical loading series using least squares. The amplitude is given by the parameter c (mm) and the phase is the parameter d (days). The displacements are with respect to the center of mass of the solid earth.



    Plots of the series for the displacements with respect to the center of the solid earth can be found at
    Displacements with respect to the center of mass of the solid earth
    These plots are continuously updated along with the data.

    Plots of the series for the displacements with respect to the center of the total earth can be found at
    Displacements with respect to the center of mass of the total earth
    These plots are continuously updated along with the data.

    Plots of the series for the displacements with respect to the center of mass of the solid earth for comparing the exact series to the interpolated series from the gridded model.
    Comparison between interpolated loading series and the series generated using the exact coordinates
    These plots are only made for showing how well the gridded series are working and are therefore not continuously updated.

    Movies of the loading components

    These movies shows the monthly loading displacements from the mean level for each loading component computed from the 1x1 degree gridded loading series. For each cell, both the mean and a linear trend is subtracted from the data to show the yearly variation. The seasonal character in hydrology loading is clear from the movies.

    Displacements with respect to the center of the total earth:
    FPS=1:            
    Movie of the vertical component             Movie of the east-west horizontal component             Movie of the north-south horizontal component
    FPS=2:             Movie of the vertical component             Movie of the east-west horizontal component             Movie of the north-south horizontal component
    FPS=4:             Movie of the vertical component             Movie of the east-west horizontal component             Movie of the north-south horizontal component

    Displacements with respect to the center of the solid earth:
    FPS=1:             Movie of the vertical component             Movie of the east-west horizontal component             Movie of the north-south horizontal component
    FPS=2:             Movie of the vertical component             Movie of the east-west horizontal component             Movie of the north-south horizontal component
    FPS=4:             Movie of the vertical component             Movie of the east-west horizontal component             Movie of the north-south horizontal component

    Data for downloading

    Last available epoch

    Displacements with respect to the center of the total earth

    File Format Size Comment
    hydroloading_cmte.zip ASCII 0.77 Mb Monthly hydrology series for all stations in the station list. The displacements are computed from January 1979 until the last month of data released by the GLDAS team. There is one file per station and for each station there are four columns. The first column is the date in modified julian days. 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
    hydroloading_cmte ASCII Link to a directory with separate surface displacement files for each station.
    gridded_hydroloading_cmte netcdf 305 Mb Global 1 x 1 degree map 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

    Displacements with respect to the center of the solid earth

    File Format Size Comment
    hydroloading_cmse.zip ASCII 0.73 Mb Monthly hydrology series for all stations in the station list. The displacements are computed from January 1979 until the last month of data released by the GLDAS team. There is one file per station and for each station there are four columns. The first column is the date in modified julian days. 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
    hydroloading_cmse ASCII Link to a directory with separate surface displacement files for each station.
    gridded_hydroloading_cmse netcdf 305 Mb Global 1 x 1 degree map 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

    Additional files

    File Format Size Comment
    station_list ASCII 14 kb List with all stations for which we are providing hydrology loading series
    land-sea mask ASCII 2.1 Mb A 0.25 x 0.25 degree land-sea mask that masks out oceans and permanent frost
    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 hydrology mass.
    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 hydrology loading by convolving Farrell's loading Green's function derived on the basis of the preliminary Earth model (PREM) with the GLDAS hydrology field, 1 x 1 degree resolution with monthly time resolution. The mean mass was subtracted 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. Water covered cells are being masked out using a land-sea mask with resolution 0.25 x 0.25 degrees. Cells with permanent frost are also masked out due to the low quality of the data there due to insufficient ice modeling.

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

    where

    Where, A is the azimuth, Δm is the change in mass, ψ is the angular distance from to , 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.

    Data citation

    "Hydrology loading displacements provided by the NASA GSFC VLBI group which are avaliable at http://lacerta.gsfc.nasa.gov/hydlo (Eriksson & MacMillan). The displacement series are computed from the GLDAS NOAH model provided by the NASA GSFC GLDAS team"

    Other Services

    Tropospheric delays:
    http://lacerta.gsfc.nasa.gov/tropodelays
    Non-tidal ocean loading: http://lacerta.gsfc.nasa.gov/oclo
    Temperature and Pressure: http://lacerta.gsfc.nasa.gov/met

    References

    1. Rodell et al. Global land data assimilation system (gldas) products readme. 2004.

    2. 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: 2012.06.25


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