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.