Environmental Engineering Reference

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Telecommunication System (GTS), conversion of the accepted data to a format

required by the analysis scheme, processing of boundary values at the lateral

boundaries of the model domain and statistical procedures employed to make cor-

rections to first guess forecasts (usually forecasts from the previous analysis time), so

that the differences between the corrected first guess and the accepted observations

at the analysis time are minimised. All of these improvements are computer intensive

and will be facilitated by ever increasing availability of computer power.

One of the basic principles underlying the numerical modelling of the atmo-

sphere is that the atmosphere as a continuous fluid can be represented by the values

of the seven meteorological variables at a finite set of discrete points. These points

are arranged in a three-dimensional grid with fixed spatial coordinates. The spacing

of the grid points is also known as the spatial resolution or the grid length. Different

spatial resolution is generally used in the vertical compared to the horizontal, due to

the stratified structure of the atmosphere in the vertical. Increasing the spatial

resolution increases the accuracy of the atmospheric model, but at the cost of

additional computational effort. Thus, if the grid size is halved, the computational

effort increases by a factor of 2
4

ΒΌ
16 as the number of grid points is doubled in

each space direction and the number of time steps must also be doubled.

The choice of vertical and horizontal spatial resolutions is clearly very

important. In the atmosphere there are motions with spatial scales ranging from

many thousands of kilometres down to a millimetre. In order to capture all atmo-

spheric processes, the atmospheric model should ideally be constructed with spatial

resolutions down to a millimetre. Clearly this would impose impossibly high bur-

dens on computer resources. On the other hand, a particular spatial resolution will

exclude from consideration any physical atmospheric process whose spatial scale is

smaller than the spatial resolution. The choice of spatial resolution is therefore a

trade-off between an acceptable level of accuracy and available computer power.

For a particular grid resolution there will always be sub-grid scale physical

processes that the model cannot resolve. In order to be realistic, the model must

take account of the net effect of these processes. This is done by parameterisation

of these effects, based on statistical or empirical representations of the sub-grid size

physics. These parameterisations will also contribute to the computational effort.

NWP models can be global or limited area models. The limited area model is

nested within the global model. The high-resolution limited area model (HIRLAM)

is used widely in Europe and is a result of cooperation between the meteorological

institutes of Denmark, Finland, Iceland, Ireland, Netherlands, Norway, Spain,

Sweden and France. HIRLAM receives its lateral boundary conditions from the

European Centre for Medium Range Weather Forecasting (ECMWF) global

atmospheric model every six hours. The Danish Meteorological Institute (DMI)

HIRLAM domains are shown in Figure 6.2. The inner domains (around Denmark

and around Greenland) are of higher resolution, with 5.5 km grid length, and use

lateral boundary conditions from the outer, lower resolution domain that has a

15 km grid length. All domains have 40 layers in the vertical. The time step for the

outer domain is 300 seconds while for the inner domains it is 90 seconds. The

physical parameterisations are applied every third time step.

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