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The
Eta Model is a state-of-the-art atmospheric model used for research
and operational purposes. The model is a descendent of the earlier
HIBU (Hydrometeorological Institute and Belgrade University)
model, developed in the seventies in the former Yugoslavia (the
earliest reference being Mesinger and Janjic, 1974). In the
eighties, the code has been upgraded to the Arakawa-style horizontal
advection scheme of Janjic (1984), then rewritten to use the
eta vertical coordinate (Mesinger et al. 1988), and subsequently,
at NCEP, supplied with an advanced physics package (Janjic 1990,
Mesinger and Lobocki 1991). It became officially operational
at NCEP on 8 June 1993 (Black 1994). In its various versions,
the model has been and/or is widely used in numerous countries,
including Algeria, Argentina, Belgium, Brazil, Cameroon, China,
Costa Rica, Cyprus, Czech Republic, Denmark, Egypt, Finland,
Germany, Greece, Iceland, India, Israel, Italy, Malta, Tunisia,
Turkey, Peru, Philippines, Serbia and Montenegro, South Africa,
Spain, Sweden, and the United States.
The code is available for downloading at the NCEP site, and,
in an updated version, at this site. It is a very efficient
code which can run on small personal computers in UNIX or LINUX
systems.
The name of the model derives from the Greek letter  (eta)
which denotes the vertical coordinate (Mesinger 1984), one of
the model features, defined as
where is
the atmospheric pressure. The indices s and t refer to the surface
and the top of the model atmosphere, respectively. The index
ref refers to a prescribed reference
atmosphere, and  s
is the surface height. The model orography is formed of steps.
The steps can have slopes in the version downlodable here (Mesinger
and Jovic 2004).
The model prognostic variables are: surface pressure, horizontal
wind components, temperature, specific humidity, turbulent kinetic
energy, and cloud hydrometeors. Model variables are distributed
on the Arakawa E-grid.
Major features of the Eta dynamical core are:
- The eta vertical coordinate (Mesinger 1984), resulting in
quasi-horizontal coordinate surfaces, and thus prevention of
pressure-gradient force errors due to steep topography than
can occur with terrain-following coordinates;
- Forward-backward scheme for time differencing of the gravity-wave
terms, modified so as to suppress separation of solutions on
two C-subgrids of the model's E-grid (Mesinger 1974, Janjic,
1979);
- The Arakawa approach in space differencing, with conservation
of enstrophy and energy, as defined on the C-grid, in horizontal
advection within the nondivergent barotropic part of the flow
(Janjic 1984), thereby enforcing a strong constraint on the
false systematic cascade of energy toward smaller scales;
- Energy conservation in transformations between the potential
and the kinetic energy in space differencing (Mesinger et al.
1988);
- Option to run the model in a nonhydrostatic mode (Janjic et
al. 2001);
- Lateral boundary conditions are prescribed along a single
outer line of grid points. All variables are prescribed at the
inflow points; at the outflow points tangential velocity components
are extrapolated from inside of the model domain, while other
variables are prescribed. There is no boundary relaxation (Mesinger
1977).
Model physics package comprises:
- Convection schemes: Betts-Miller (Betts and Miller (1986),
Betts-Miller-Janjic (Janjic 1994), or Kain-Fritsch (Kain 2004);
- Cloud mycrophysics: Ferrier scheme (Ferrier et al. 2002);
- Radiation scheme: SW − Lacis and Hansen (1974); LW − Fels
and Schwarzkopf (1975);
- Land surface scheme: Noah (Chen et al. 1997) with 12 types
of vegetation and 7 types of soil texture, 4 soil layers;
- Turbulence and PBL: Mellor-Yamada 2.5, and Monin-Obukhov similarity
theory in the surface layer, with Paulson stability functions.
Upgrades included in the CPTEC ("ICTP 2005") Eta code downloadable
here, compared to the NCEP Workstation Eta code that is downloadable
at the NCEP site, are listed on page 40 of the lecture Mesinger
(2005).
While the primary use of the model has been for regional weather
prediction and NWP type applications (for a review, see Mesinger
2000), the model has been very successful also in regional climate
and seasonal prediction applications (e.g., Altshuler et al.
2002, Chou et al. 2005, Katsafados et al. 2005).
A more complete list of references can be found here. Users
and to-be-users are all invited to take part in the Eta Model
Forum.
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