An atmospheric prediction model is a system of simulating the
atmospheric physical processes. The atmosphere is simulated with a discrete
number of grid points in specific levels, that begin from the ground, and reach
the upper atmospheric layers. By this way, a three dimensional grid is
produced, which is used for all the necessary mathematical calculations (Figure
1).
1. INTRODUCTION TO WEATHER PREDICTION MODELS


An atmospheric prediction model is a system of simulating the
atmospheric physical processes. The atmosphere is simulated with a discrete
number of grid points in specific levels, that begin from the ground, and reach
the upper atmospheric layers. By this way, a three dimensional grid is
produced, which is used for all the necessary mathematical calculations (Figure
1).

Figure 1: A three dimensional grid of a numerical
weather prediction model

The time is procceding with very small time steps of the
order of few seconds. The continuously repetition of estimations in each time
step leads to the weather prediction for next day or next week. The denser the
grid the more realistic the atmospheric simulation by the models. The weather
prediction models are divided in two categories:



2. Global models


In those models the grid points extended to the whole earth
and the equations are integrated in the three dimentional atmosphere (north and
south hemisphere).



3. Limited Area Models (LAM)


The limited area models are used for the prediction of the
small scale disturbances.

These models are based in almost the same equations as the
global models . The main differences is that LAM are executed in a limited
area, their grid is denser, and they are cabable of simulating accurately the
small scale disturbances. Whereas in global models the equations are integrated
in the whole earth, with a sparse grid. In limited area models the attention is
focused in a small, specific area (eg Europe or Greece) and the grid is much
denser.

The limited area models are separated in two main categories:
the hydrostatic and the non hydrostatic models. The nonhydrostatic models are
used for the simulation of atmospheric procedures with a smaller scale.

The execution of numerical weather models requires
internationally the use of supercomputers for two main reasons:

Weather prediction models are generally large code and
requires an execution of a large amount of data.

The results of operational prediction model has to be
available in an operational time.

The HNMS (Hellenic Meteorological Service) is a member of
ECMWF (European Center for Range Weather Forecasts), so we use ECMWF`s products
operationally. As well as the products of GM (Global Model) of the German
Meteorological Service. The ECMWF and GM products are used as boundary
conditions for our Limited Area Models.



