# Atmospheric angular momentum on-line service

There are several causes of irregularities in the Earth's rotation that
are monitored by the space geodesy techniques. The major factor is a change
of the atmospheric angular momentum (AAM). Since the total angular momentum
that includes the contribution of the solid Earth, atmosphere, hydrosphere
is conserved, an increase of the AAM has to be balanced by a decrease of
the angular momentum of the solid Earth and therefore causes variation in
the vector of the Earth rotation and as a result, to variation of the Earth
orientation parameters. Knowledge of the AAM significantly improves
the accuracy of the Earth orientation forecast.
The atmospheric angular momentum can be computed by integration parameters
of the atmosphere derived from the output of 4D numerical weather models
used for weather forecast. It is convenient to express the atmospheric angular
momentum as a sum of two parts: motion part which is variable part of
the momentum of inertia and motion part which the relative angular momentum
of the atmosphere with respect to the mean rotating coordinate system.
the mass part depends on distribution of air density, the motion part depends
on distribution of air density and wind. Both parts have three components along
axis 1, 2 and 3. A linear combination of these two parts is the atmospheric
excitation function.

The 4D output of numerical weather models MERRA, GEOS-FPIT and GEOS-FP
that are run by the NASA The Global Modeling and Assimilation Office
(GMAO) is used for computation
of the AAM. The last model GEOS-FP provides forecast of the state of the
atmosphere and it is updated four times a day. The AAM computation is
initiated every hour, and if new output of the numeric weather model
became available, the data are retrieved and processed. For each epoch
the 3D output of the GMAO model at the native terrain-following grid at
72 layers is used.

**Results**:

Details of the computation procedure can be found here.
## Acknowledgment

This work is supported by NASA
Earth Surface & Interior program, grant NNX15AC10G.
Back to the Network Earth Rotation Service.

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This page was prepared by Leonid Petrov
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Last update: 2016.06.28_08:56:50