Up to this point in the book we have been considering winds averaged over five minutes or so. In reality, both the wind speed and direction may vary widely during those five minutes. The irregularities constitute turbulence, defined as 'the complex spectrum of fluctuating motion imposed on the average flow'. It is due to the eddies mentioned in Section 14.1. The fluctuations lead to bumpy aircraft flights, ripples across wheat crops and isolated patches of ruffled surface on a lake, called 'cats' paws'. More importantly, they are the means by which water vapour, sensible heat and friction are transferred from the ground to the free atmosphere. Also, turbulence is associated with gusts of high speed and lulls of low.
A 'gust' is defined as the 'wind-speed deviation from the mean which, on average, is exceeded once during the reference period'. Such extreme winds are important in the design of buildings, bridges and windmills, for instance, even if they are only short-lived. Very strong winds have been recorded at places such as these:
(a) An hourly average of 43 m/s at Cape Denison at 68°S in Antarctica.
(b) A gust of 75 m/s lasting three seconds has been measured on a 450 m hill at Wellington (NZ).
(c) There was a gust of 68 m/s at Onslow in Western Australia during a tropical cyclone in 1975.
(d) A speed of 89 m/s was measured in a tornado near Melbourne in 1918 (Section 7.5).
These high winds cannot be measured easily: anemometers would be blown away. Instead, extreme speeds are estimated from the resulting damage (Table 14.3).
Winds above 30 m/s occur about once each two years around the south coast of Australia, and above 23 m/s along the north coast. At Sydney, the frequency of strong winds during the period 1938-62 is indicated in Figure
14.14. For instance, there was a gust faster than 28 m/s once a year on average, over 31 m/s once each two years, and hence, it is estimated, more than 46 m/s once a century (Note 14.E). In other words, the return period (Section 10.2) for winds exceeding 46 m/s in Sydney is a hundred years. Winds exceeded on average once in fifty years (which is assumed to be the life of a building) are shown in Figure
14.15. indicating the particular hazard along coasts exposed to tropical cyclones, which are most violent off the north-west coast. The
Table 14.3 The F scale for extreme winds, devised by Theodore Fujita in the late 1960s to classify tornadoes
F5 1 16-142 Incredible: brick homes levelled, cars and trees moved over 100 m
* Only a handful out of about 1,000 tornadoes reported in Australia and a similar number of recorded tropical cyclones have had winds exceeding those of an F2 tornado
0 10 20 30 40 50 60 70 annual extreme wind V: m/s
Figure 14.14 The frequency of high winds at Sydney.
frequencies of extreme gusts at some other places are shown in Table 14.2.
It is the extra strength of gusts during a storm that causes most damage. It was found after the tropical cyclone Tracy that demolished Darwin in 1974 that the cost of repairing houses, as a fraction of the initial cost of building them, was zero where gusts were below 30 m/s (i.e. F0 in Table 14.3), but 0.2 where they reached 42 m/ s (i.e. F1), and 0.6 where 56 m/s (F2). These were houses built on stilts to catch cooling sea breezes; the fractions were about halved for houses built on the ground.
Numerical measures of gustiness compare the mean wind speed during a short-period gust with the average over an hour or ten minutes, say. The Gust Factor is the ratio of gust and average speeds, and the value depends on the selected gust period. For example, data from hurricanes yield factors of 1.2 for two-minute gusts and 1.5 for three-second gusts, compared with hourly averages. In other words, there are three-second pulses of wind which are 50 per cent faster than the hour-long average. Alternatively, gustiness may be described by the swing of wind direction, which is larger in turbulent conditions (Table 7.1; Figure 14.16). In either case, values are enhanced by the extra stirring created by the irregularity of a surface such as that of a city.
Gusts are due to volumes of air being mixed by eddies from parts of the atmosphere where the winds have quite different speeds and directions. Some of the agitation may be due to atmospheric instability (Section 7.4). The fluctuations are greater if nearby winds differ considerably, i.e. if there is strong wind shear. This is most likely in the vicinity of discontinuities such as inversions or fronts, and near obstacles such as hills.
There is also strong wind shear causing turbulence in association with thunderstorms and near low-level jets. The latter are like upper-troposphere jet streams (Section 12.5), but occur at only about 2 km above the ground. They are
fairly common in Australia on clear nights in winter or spring, and arise in the course of forming the nocturnal radiation inversion which detaches the gradient wind from the ground's friction. Also, there is often a low-level jet ahead of a cold front, blowing parallel to it from the north or north-west, at a speed of up to 30 m/s.
Thunderstorms (Section 9.5) may cause strong wind gusts near the surface in three ways:
1 Severe thunderstorms occasionally spawn vortices of extreme wind, such as tornadoes (Section 7.5).
2 Strong downdraughts within thunderstorms may carry parcels of air down from the jet stream to the surface within no more than fifteen minutes, so that the air still contains its original momentum.
3 A subsiding parcel of air may accelerate downwards under the weight of its water content, and as a result of the cooling caused by evaporation of drops within the parcel. In extreme cases, this leads to a downburst of cold air, which spreads in all directions near the surface up to a few 100 km away. Downbursts have caused several airplane crashes. The smaller downbursts, called microbursts, with a radius of a few 100 m, are a special danger to aircraft because of the powerful downdraught within the microburst, and because of the strong wind shear at each end. A plane flying into a microburst near the surface feels a sudden change from headwind to tailwind, causing a disconcertingly abrupt loss of lift. Microbursts can be either dry or wet; the wet kind are due to a downpour, while the dry kind may emerge from apparently harmless thunderstorms with a high cloud base.
The leading edge of the cold outflow from a thunderstorm is known as a gust front (Section 9.5) which spreads like a density current (Note 14.D). One in the Port Moresby area of Papua New Guinea is known as a 'Guba'. It occurs occasionally during the early morning as a sudden wind of up to 30 m/s, and lasts for about half an hour. It is a gust front associated with nocturnal thunderstorms over the Gulf of Papua, often begun by convergent land breezes from the surrounding land.
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