Synoptic-scale winds are those blowing at least 1 km from the ground, above the PBL (Section 12.2) and beyond the influence of the ground's irregularities. They equal the local gradient winds if there is no acceleration or slowing up, so we will call them 'quasi-gradient winds'. They drag surface air along by transferring energy downwards through turbulent eddies, which are circulations in all directions including the vertical, lasting for seconds or minutes. They are smaller in scale closer to the ground. Strong winds and atmospheric instability increase their intensity. The descending part of an eddy carries brisk quasi-gradient air downwards, to replace low-momentum surface air in the rising part, previously slowed by friction with the ground. In this way, eddies transfer momentum downwards into the PBL, creating average speeds at each level which vary logarithmically with height from the ground (Note 14.A).
The fact of lower wind speeds close to the ground's friction is illustrated by birds flying over a wide beach or the sea; those flying against the wind skim the surface, whilst birds going with the wind fly many metres above. A practical consequence is that comparing winds at different places requires measurements at a standard height, e.g. 10 m, else it is necessary to adjust measurements to the 10-metre equivalent. Also, the strong vertical wind shear near the ground explains why shouted messages carry further downwind than upwind; the wind profile bends sound waves from downwind down to the listener on the ground, focusing them, whereas upwind sound is deflected overhead (Note 7.M).
The downward sharing of a gradient-wind's energy is prevented by any inversion layer, since an inversion inhibits vertical movement. Therefore, a ground inversion (Section 7.6) decouples the gradient wind from the surface air, which releases the upper wind from the braking effect of the ground's friction. Hence, for instance, the average 1,000 m wind at Sydney in winter is 7.5 m/s at 3 p.m., but 9 m/ s at 3 a.m. On the other hand, detachment from the driving force of the gradient wind is associated with the calm conditions at the surface at night (Figure 14.1).
A rough surface creates eddying within the atmosphere, on a scale proportional to the roughness. It increases the frictional retardation and spreads it over a greater depth (Figure 14.2), according to the wind speed. The increase of wind with height extends to 200 m if the wind is light, but 1,500 m if it is strong. Speeds measured near the relatively smooth surface at sea are about 65 per cent of the quasi-gradient wind's speed, while surface winds over rough ground are less than 50 per cent. These percentages are greater when either the atmosphere is unstable, the gradient wind light or the latitude high. In each case, the surface friction makes the surface wind flow more directly towards the low-pressure region than the gradient wind does (Figure 12.9c).
Of course, it makes little sense to think of average wind profiles between the tall buildings of a city, where the eddies are not transient or mobile but locked to particular buildings, so that some places are sheltered and others subject to funnelled winds. The same would apply to any extremely rough surface.
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