Formation

A TC has a life of several days, from the time of the first evidence of a centre of intense low pressure until its disappearance. Typically, formation takes 2-3 days while the core is still cool, then 1-2 days while the core warms and the central pressure falls, followed by a few days of maximum intensity, with extension of the cyclone's radius, and finally a dying away over one day or more. Factors which promote TC formation are the following:

(a) There must be sufficient Coriolis effect to deflect winds to the left (in the southern hemisphere), so that they rotate clockwise around a low (Figure 12.9; Note 13.F). This requires a latitude of more than about 5 degrees from the equator (Figure 13.13)

(b) TCs form over the sea where the SST is high. Comparison of Figure 11.2 and Figure 13.13 shows that at least 27°C is necessary, which occurs only at latitudes below about 20°S (or 30°N) and mainly in the summer hemisphere, i.e. January and February in the vicinity of Australia and Madagascar. They arise over the west of the oceans, where surface temperatures

Table 13.2 Classification of tropical-cyclone severity

Class

Central pressure (hPa}

Max. sustained wind* (.m/s)

Storm surgef(m)

Damage

1*

>990

18-25

< 1.0

Minor

2

990-966

25-35

1.0-2.4

Moderate

966-940

35-46

2.4-3.8

Extensive

4

940-890

46-63

3.8-7.0

Extreme

5

< 890

> 63

> 7.0

Catastrophic

* Winds are here measured over ten minutes or more at 10 m height t The raising of the sea-level above normal, taking tides into account. The figures are only indicative for a straight coastline, and may be quite different elsewhere. t Class 1 concerns tropical storms. An estimated 60 per cent of them later develop tropical-cyclone intensity. § Less than half of TCs are in Class 3 or higher

* Winds are here measured over ten minutes or more at 10 m height t The raising of the sea-level above normal, taking tides into account. The figures are only indicative for a straight coastline, and may be quite different elsewhere. t Class 1 concerns tropical storms. An estimated 60 per cent of them later develop tropical-cyclone intensity. § Less than half of TCs are in Class 3 or higher

Figure 13.13 Tracks of tropical cyclones with winds above 17.4 m/s during the period 1979-88.s

are highest (Figure 11.2), after the currents of the great gyres have traversed the equator (Figure 11.15). Less than a third of TCs develop in the southern hemisphere, where the summers are about 2 K cooler than in the northern hemisphere (Section 3.2), and where SSTs above 20° latitude tend to be lower (Figure 11.2). The most common source in the south is just east of northern Madagascar. They do not form over land, for lack of moisture to provide latent heat.

The frequency of TCs might be affected by any future rise of SSTs (Note 13.G). Years with many TCs onto Australia have been preceded by high SST around the north coast, e.g. three tropical cyclones during a year with local SSTs 0.6 K less than normal, but seventeen in a year with SSTs 1.0 K above normal.

The mid-troposphere has to be moist and conditionally unstable, without any PBL inversion to prevent warm and moist surface air from mixing with the mid-troposphere. Such conditions prevail at the western end of the equatorial segment of the gyres and quasi-permanent anticyclones (Figure 12.1). Herein lies the explanation for the absence of TCs over the South Atlantic, even though there is an area east of Brazil where the sea-surface temperature exceeds 27°C in March; there is insufficient space between Africa and South America for creating a moist, conditionally unstable troposphere, and there is the Trade-wind inversion extending from south-west Africa to Brazil (Figure 7.10).

(d) The troposphere has to be free of strong wind shear vertically, e.g. no more than 10 m/s difference between winds at 2 km and 12 km. Otherwise there is rupturing of the updraughts within any incipient TC.

(e) The stratospheric jet stream is preferably in a weak phase of the Quasi-Biennial Oscillation (Section 12.3). It has been observed that TCs are less frequent and less intense during the strong phase, presumably because the stronger easterlies shear off the top of any updraught column, preventing its full development.

(f) There has to be a tropical depression as a trigger, usually a trough in the Trade winds as part of an easterly wave (Note 13.H). Convergence occurs in the trough of this wave, leading to uplift which deepens the PBL and erodes the Trade-wind inversion (Section 7.6), so that convective updraughts can penetrate the entire troposphere. The resulting latent heating may reduce the surface pressure further, which produces even more cyclonic rotation and convergence.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment