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1.0E+008

1 m/s

Earth's mean radius

Climate

change

1.0E+006

Season

1.0E+004

1.0E+002

1.0E+000

1.0E-002

Week

Hour

Year

Minute

Second

Day

1.0E-004

1E+000

1E+002

1E+004

1E+006

1E+008

1E+010

Time scale (s)

Figure 6.1

Spatial and temporal scales of disturbance phenomena in the

atmosphere

is governed by seven physical equations, three arising from thermodynamic con-

siderations and four arising from hydrodynamic considerations (Atkinson, 1981).

The thermodynamic equations are the gas law, also known as the equation of state,

the first law of thermodynamics and an equation representing the conservation of

moisture. The hydrodynamic equations are the equation of continuity and the three

equations of motion corresponding to the components of Newton's second law in

three directions. The seven governing equations involve the atmospheric variables

and their spatial and time derivatives. In order for the atmospheric model to provide

a usable description of the behaviour of the atmosphere, a solution must be found to

the equations. In general, no analytical solution is possible and numerical methods

must be adopted. The task involved is to calculate how each of the meteorological

variables will change as the simulation runs forward in time.

NWP is an objective forecast in which the future state of the atmosphere is

determined by the numerical solution of a set of equations describing the evolution

of meteorological variables which together define the state of the atmosphere

(Meteorological Office, 1991). In the excellent review of the developments in NWP

to be found in Kalnay
et al.
(1998) it is pointed out that, while there has been huge

progress in the last two decades, with a doubling of forecast skill, there are three

major requirements for improved NWP: (1) better atmospheric models, (2) better

observational data and (3) better methods for data assimilation. Data assimilation

involves the quality check run on the weather observations received via the Global

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