Sea Breezes

These coastal winds are due to sea-surface temperatures (SST) varying each day by only a degree or so, whilst surface air temperatures onshore change by around ten times as much (Section 3.4). The result is that daytime temperatures inland are appreciably warmer than the SST, and the warming spreads throughout the planetary boundary layer. Also, the onshore warmth leads to thermals which ascend to the top of the PBL and gradually extend it upwards (Figure 7.1). The warming of a column of 1 km by 5 K, for instance, whole feather = 10 knots flag = 50 knots \y o—' 7 d4"

Beddington Windrose

Figure 14.5 Wind rose of measurements each three hours during the period October 1976—March 1977 at Silverwater in Sydney. The length of each segment is proportional to the frequency of winds of the indicated direction and speed. There is a calm during 2.7 per cent of the time. Also, easterly and southeasterly winds occur about half the time, with 60 per cent of them over 4 m/s.

Figure 14.5 Wind rose of measurements each three hours during the period October 1976—March 1977 at Silverwater in Sydney. The length of each segment is proportional to the frequency of winds of the indicated direction and speed. There is a calm during 2.7 per cent of the time. Also, easterly and southeasterly winds occur about half the time, with 60 per cent of them over 4 m/s.

easterly wind (m/s)

July

15

18<^

mean ^^

wind

"712

i u i

westerly wind 21

/

9

Figure 14.6 Hodographs of surface winds at Sydney. Each month's diagram shows the average wind direction and strength each three hours, by the positions of the eight corners. For example, the wind at 6 p.m. in January (shown by the corner labelled '18') is on average easterly, with a slight southerly component. The monthly mean wind in July is westerly at nearly 2.5 m/s.

Figure 14.6 Hodographs of surface winds at Sydney. Each month's diagram shows the average wind direction and strength each three hours, by the positions of the eight corners. For example, the wind at 6 p.m. in January (shown by the corner labelled '18') is on average easterly, with a slight southerly component. The monthly mean wind in July is westerly at nearly 2.5 m/s.

Table 14.2 The maximum gust experienced at 10 m height at places in Australia once each R years (i.e. R is the return period)

Place

R = 5 years

25

100

Adelaide

33 m/s

39

45

Darwin

30

44

55

Brisbane

34

45

54

Canberra

32

37

39

Giles

29

34

38

Hobart

34

39

44

Melbourne

32

37

41

Perth

31

40

45

Sydney

34

41

46

reduces the MSLP by 2 hPa (Section 13.4, Note 1.G). This reduction does not occur offshore, so a surface-pressure difference develops, which drives marine air onshore—the sea breeze (Figure 14.8). This slides inland under the warmer PBL there.

The cool breeze is shallow at first, perhaps 100 m, and becomes deeper during the day, typically to 200-500 m in temperate climates and 1,000-1,400 m in the tropics. The wind causes atmospheric divergence over the water, and hence a subsidence of air offshore, which in turn creates a return flow from the air ascending over the land, completing a circulation. The return flow is perhaps twice as deep and half as fast as the sea breeze, but is often masked by the quasi-gradient wind. Figure 14.9 is the result of pioneering measurements; it now appears that the return flow is less prominent and less common than once thought. There is an inversion between the cool sea breeze and any warmed return flow above (Section 7.6).

Prerequisites for a sea breeze include a clear sky, to allow sufficient radiation to heat the land surface (Note 14.C). The greater radiation of summer facilitates sea breezes, so that they are more common, involve stronger winds and start earlier in the day in that season (Figure 14.10). Sea breezes are less common in autumn, and if they do occur they are generally weaker, start later, and penetrate less far inland, because the SST has continued to rise (Section 11.2) while daytime warming of the land declines. The breezes are rare in winter, when the land is usually cooler than the ocean. For example, nine sea breezes, on average, reach Renmark (290 km from the West Australian coast) in February, but only one in July.

The fundamental mechanism driving a sea breeze is the same as that for a monsoonal circulation or a Hadley cell (Section 12.3), being a 'thermally direct circulation' (Figure 12.14). The difference is that a sea-breeze circulation is confined to the PBL and lasts only 6-12 hours.

Orientation Wind Victoria Breeze
Figure 14.7 Variation of annual-mean daily wind run across Australia and the adjacent seas.

The same process operates around large lakes, where there are miniature sea breezes (i.e. lake breezes) on land, within a kilometre or two of the shoreline. Such a wind is often observed in the afternoon along the shores of Lake Victoria, at about 2°S in Africa. Similarly, there may be a cold breeze along the edge of a large area of melting snow or ice, or a cool wind blowing towards the sunny region outside the shadow of stationary stratus clouds or fog. Likewise, there might be a country breeze of up to 3 m/s blowing in the afternoon towards the centre of a large city, when there is appreciable urban heating (Section 3.7). In each case, the wind is driven by a temperature difference. But a 'direct circulation' is not a thermal wind (Note 12.F). The latter blows at right-angles to the difference and at a level above it, whereas a sea breeze, for example, blows at a low level and directly towards the area of higher temperature.

An established sea-breeze circulation has a certain momentum which maintains its orientation whilst the Earth turns, so that a midday easterly sea breeze at Sydney (on a roughly north-south coast) becomes a northeasterly by the evening. This Coriolis effect was noticed in the seventeenth century by William Dampier, who observed that shore winds turn with the Sun each day, i.e. counterclockwise in the southern hemisphere. A similar backing of the sea breeze is observed on the desert coast of Namibia.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment