Data Description:

Appended below are U. S. National Centers for Environmental Prediction
(NCEP, formerly NMC) values of effective atmospheric angular momentum
functions.  Data are given up to four times daily (there are gaps in
the record), and each epoch consists of a header record giving the
date and time, and three data records.

Prior to April, 1997 (filename: aam.ncep), the three lines after the
header contain 6 values of X1, X2 and X3, respectively.  The 6 values
are those for wind-northern hemisphere to top of model (50 mb for the
NCEP), pressure-northern hemisphere, and pressure + inverted
barometer-northern hemisphere and the same three terms for the
southern hemisphere.  All data values are true values multiplied by
10**7.  Missing data are either given the special value of -99.0, or
are absent from the record.

Each days data may be read with the following FORTRAN code:

      read(lunit,101) iyear, imonth, iday, ihour
      read(lunit,102)  x1w50nh,    x1pnh,  x1pibnh,
     +                 x1w50sh,    x1psh,  x1pibsh
      read(lunit,102)  x2w50nh,    x2pnh,  x2pibnh,
     +                 x2w50sh,    x2psh,  x2pibsh
      read(lunit,102)  x3w50nh,    x3pnh,  x3pibnh,
     +                 x3w50sh,    x3psh,  x3pibsh
 101  format(1x,4i4)
 102  format(6f10.5)


Beginning in April, 1997 (filename: aam.ncep.2), the three lines after
the header contain 8 values of X1, X2 and X3, respectively.  The 8
values are those for wind-northern hemisphere to ~100 mb,
wind-northern hemisphere to top of model (10 mb), pressure-northern
hemisphere, and pressure + inverted barometer-northern hemisphere, and
the same four terms for the southern hemisphere.  All data values are
true values multiplied by 10**7.  Missing data are either given the
special value of -99.0, or are absent from the record.


Each days data may be read with the following FORTRAN code:

      read(lunit,101) iyear, imonth, iday, ihour
      read(lunit,102) x1w100nh, x1w10nh,    x1pnh,  x1pibnh,
     +                x1w100sh, x1w10sh,    x1psh,  x1pibsh
      read(lunit,102) x2w100nh, x2w10nh,    x2pnh,  x2pibnh,
     +                x2w100sh, x2w10sh,    x2psh,  x2pibsh
      read(lunit,102) x3w100nh, x3w10nh,    x3pnh,  x3pibnh,
     +                x3w100sh, x3w10sh,    x3psh,  x3pibsh
 101  format(1x,4i4)
 102  format(8f10.5)


Information about atmospheric excitation for Earth rotation/polar
motion and related fields are supplied by Salstein, et al., (1993):

Salstein, D.A., D.M. Kann, A.J. Miller, R.D. Rosen, 1986: 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.


NCEP model changes:

NCEP ocassionally introduces changes in their model which can affect
the continuity of these time series.  In particular, changes in model
orography can change the surface pressure field, and cause step jumps
in pressure related terms such as X(I)P and X(I)PIB.  Known dates of
NCEP model changes which affect surface pressure are:

   22 May 1980, 28 May 1986, 12 August 1987, 6 March 1991

We have corrected all pressure related terms in this data set by
normalizing them to the most recent NCEP model.  Users should be aware
that the values of the pressure terms may be different from past data
sets, and that they are subject to future changes.


Additional notes:

- 12Z analyses begin on 1 Jan. 1981
- 12Z winds from 1 Jan. 1983 - 31 Aug. 1983 are missing