Wind power forecasting - Wind Power Integration: Connection and System Operational Aspects - page 219

differ widely. In the horizontal the spatial scale is very large, which can readily be
appreciated from photographs of the earth from geo-stationary satellites. This
horizontal scale is quite large, of the order of the earth's size (the earth's mean
radius is 6,370 km). In the vertical the spatial scale is very much smaller. Although
the atmosphere does not have any definite upper surface, 90 per cent of its weight is
concentrated in the lower 16 km, which corresponds to only about 0.3 per cent of
the earth's radius (McIlveen, 1992). The lower 16 km, although small in scale, is
very significant in its effect in meteorology, since it is here that most of the cloud
activity and the weather is produced. This part of the atmosphere is called the
troposphere.
The atmospheric structure is also quite different in the horizontal and the
vertical, with higher gradients in the vertical than in the horizontal, giving rise to a
markedly stratified appearance. An example of this difference is how temperature
decreases with height in the vertical at about 6 C/km, whereas the strongest
horizontal gradients associated with fronts in mid-latitudes would rarely exceed
0.05 C/km (McIlveen, 1992).
The region of the atmosphere of most interest from a wind power perspective is
the planetary boundary layer, which is the layer directly above the earth's surface.
It is variable in depth but of the order of 500-2,000 m. This region is dominated by
turbulent interactions with the surface, with turbulent eddies in the spatial range
from 500 to 5 m. The atmosphere above the planetary boundary layer is called the
free atmosphere and is much less affected by friction with the earth's surface. The
free atmosphere is dominated by large-scale disturbances.
Air motion is a very complicated phenomenon and covers a wide range of
scales, both spatial and temporal. Small-scale features are visible in the movements
of smoke from a burning candle, medium-scale features in the plumes from fossil
fuel burning power stations and the large-scale circulations of the atmosphere are
visible from geo-stationary satellites. In the atmosphere there is a range of eight
orders of magnitude in spatial scale and an almost equally large range of temporal
scales corresponding to the lifetime of the disturbance phenomena. This can be
seen in Figure 6.1, which is adapted from McIlveen (1992). The disturbance phe-
nomena range from short-lived and small spatial scale turbulence through con-
vection processes in the formation of clouds, to meso-scale systems such as sea
breezes, through to the synoptic scale growth and decay of extra-tropical cyclones
which dominate the weather patterns of north-western Europe. Over most of the
range of these phenomena the ratio of spatial to time scales is approximately 1 m/s,
and is shown as the straight dashed line in Figure 6.1. It is a measure of the intensity
of the activity of the atmosphere.

6.3 Numerical weather prediction

The state of the atmosphere can be described by seven meteorological variables:
pressure, temperature, amount of moisture, air density and wind velocity (two
horizontal components and a vertical component). The behaviour of these variables