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The Eta Model, defined and developed by Fedor Mesinger and Zavisa Janjic, is a typical example of a regional model.

The development of the Eta Model was initiated back in 1972 by the Federal Hydrometeorological Institute (FHMI) and the Institute of Meteorology of the University of Belgrade (IMUB), and that is why the model was named HIBU (Hydrometeorological Institute and Belgrade University). Since January 1979 the model has been in operational use at the FHMI.

The new version of the model based on the HIBU model is the Eta Model developed in cooperation among IMUB, GFDL Princeton, and NMC Washington. The Eta Model was named after its unique vertical coordinate (eta). The computer programme of the model was written in the standard FORTRAN (ANSI FORTRAN), so that it is easy to transfer and install on different computers. The model has been used so far on mainframes produced by all major western manufacturers (e.g. IBM, CDC, DEC, Honywell-Bull, Cray, Convex) as well as on widely used workstations (e.g. Sun, Indigo, Indy, Power Challenge, Indigo2, PC, Cluster parallel computer).

The model is being upgraded continuously. In particular, special attention is paid to further improvement of the parameterizations of physical processes.

Today, the model is used for research purposes in more than 20 meteorological institutions throughout the world. In five countries it is used for operational weather forecasting (Italy, South Africa, Turkey, U.S.A. and Yugoslavia), while in several other countries its operational use is imminent or planned (Brazil, Greece, Korea). At the NCEP (formerly NMC) Washington, the model has been operational (it is run twice daily) since mid-1993.

General characteristics of the Eta Model
  • Limited area integration domain;
  • Forecasts up to two days (extension to 2.5 days is presently contemplated at the NCEP, Washington);
  • Primitive equations, hydrostatic;
  • Basic prognostic variables are temperature, specific humidity, wind components, turbulent kinetic energy and
        surface pressure. Some recent model versions include cloud water as the sixth three-dimensional prognostic
        variable. Additional surface prognostic variables are soil temperature and moisture, total and convective rainfall,
        and snow depth;
  • Finite difference grid-point method is used;
  • The vertical coordinate (eta) is a generalization of the sigma coordinate allowing step-like mountain representation
        (Figure 3). The model can be used in both modes (eta or sigma) without any change in the code other than setting
        a logical switch;
  • In the horizontal advection the model has a built-in control of the non-linear energy cascade;
  • Split-explicit time differencing;
  • Second order turbulence closure scheme following the Mellor-Yamada approach;
  • Land surface processes;
  • Large-scale rainfall;
  • Betts-Miller-Janjic deep and shallow moist convection scheme;
  • Goddard Laboratory for Atmospheres (GLA) radiation package.

    Applications of the Eta Model
    The model is used operationally for general purpose forecasting. An example of initial situation and the 48h-120h forecast are shown in Figures 4 and 5, respectively. Direct model forecast products are used for preparation of various specific products that may be required by the users.

    The model-produced data in the digital form can be used as input in automated management systems such as those used in electric power generation systems, industry, transportation, ecology, etc.

    The model is also used for predicting severe storms, including tropical cyclones. The development and movement of a number of tropical cyclones, e.g. Hugo, Allison, hantal, etc., were highly successfully predicted using the Eta Model at the NCEP (formerly NMC) Washington.

    Along with the prediction of a trajectory where a particle of air is followed during its movement, the Eta Model makes it possible to predict transport of pollutants through atmosphere. Figure 6 shows forward and backward trajectories of a particle of air calculated for the same locations in Europe. The model products are also the basis for calculation of chemical transformations of pollutants in the atmosphere and their deposition.