Data Description:
Data are NCEP values of effective atmospheric angular momentum
functions as calculated from NCEP/NCAR reanalyses archived on pressure
surfaces. The Transfer functions and Principal moments in Eubanks (1993)
are employed to derive the atmospheric angular momentum functions (see Table 1
for several other choices).
Table 1. Constants for Earth rotation/polar motion transfer.
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Authors chi1_P,Chi2_P chi1_W,Chi2_W chi3_P chi3_W moments omega
Barnes et al.,1983 1.00/[omega(Cm-Am)] 1.43/[omega(Cm-Am)] 0.70/[omega*Cm] 1.00/[omega*Cm] Cm=7.04*10^37kg.m^2,Cm-Am=2.344*10^35kg.m^2 7.29*10^(-5)s^(-1)
Eubanks,1993 1.098/[omega(C-A)] 1.5913/[omega(C-A)] 0.753/[omega*Cm] 0.998/[omega*Cm] Cm=7.1236*10^37kg.m^2,C-A=2.610*10^35kg.m^2 7.292115*10^(-5)s^(-1)
Wahr,1982,1983 1.12/[omega(C-A)] 1.61/[omega(C-A)] 0.756/[omega*Cm] 1.00/[omega*Cm]
Wahr,2004 1.11/[omega(C-A)] 1.57/[omega(C-A)]
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See also Dickman (2003) for four more formulations depending on fullness of
core-mantle coupling and compressibility (Love numbers).
Data are given four-times daily, and each epoch consists
of a header record giving the date and time (year,month, day, hour),
and data records.
The line after the header contains 9 values of inverted barometer (IB) pressure
(or, mass) terms, Non-IB pressure terms, wind (or, motion) terms of X1, X2 and X3,
respectively. The wind terms are computed by integrating winds from the Earth
surface to 10 hPa, the top of atmospheric model. Note that the inverted barometer
correction involves applying the mean atmospheric surface pressure
over the whole world ocean to every point over the world ocean.
Each days data may be loaded into MATLAB directly, or may be read with the following
FORTRAN code:
read(lunit,100) iyear, imonth, iday, ihour,
+ AAMF_mass_IB_X, AAMF_mass_IB_Y, AAMF_mass_IB_Z,
+ AAMF_mass_NonIB_X, AAMF_mass_NonIB_Y, AAMF_mass_NonIB_Z,
+ AAMF_motion_X, AAMF_motion_Y, AAMF_motion_Z
100 format(13E16.7)
To convert AAMF_X & Y into milliarcsecond (mas) and AAMF_Z into millisecond (ms),
one needs to apply the following scale factors:
AAMF_X & Y (in mas) = AAMF_X & Y *360*60*60*1000/(2*pi);
AAMF_Z (in ms) = AAMF_Z * 24*60*60*1000;
For more information about this data set, consult the following
references. We ask that any use of this data in a published work refer
to these articles.
Salstein, D.A., D.M. Kann, A.J. Miller, R.D. Rosen, 1993: The
Sub-bureau for Atmospheric Angular Momentum of the International Earth
Rotation Service: A Meteorological Data Center with Geodetic
Applications. Bull. Amer. Meteor. Soc., 74, 67-80.
Salstein, D.A., and R.D. Rosen, 1997: Global momentum and energy
signals from reanalysis systems. Preprints, 7th Conf. on Climate
Variations, American Meteorological Society, Boston, MA, 344-348.
Salstein, D.A., Y.H. Zhou, and J.L. Chen, 2005: Revised angular momentum
datasets for atmospheric angular momentum studies, European Geophysical Union
(EGU) Spring Meeting, Vienna, Austria.
Zhou YH, Salstein DA, Chen JL, 2006: Revised atmospheric excitation function series
related to Earth variable rotation under consideration of surface topography,
J. Geophys. Res., 111, D12108, doi:10.1029/2005JD006608.
For more information abot the NCEP/NCAR reanalysis project, consult
the following:
Kalnay, E., et al., 1996: The NCEP/NCAR 40-year reanalysis
project. Bull. Amer. Meteor. Soc., 77, 437-471.
For more information about the calculations of atmospheric angular momentum
functions, consult the following:
Barnes, R. T. H., R. Hide, A. A. White, and C. A. Wilson, 1983:
Atmospheric angular momentum fluctuations, length-of-day changes and
polar motion. Proc. R. Soc. London, Ser. A, 387, 31-73.