Ground or Radiation Inversion

The ground begins to cool from about the midafternoon (Figure 3.12), increasingly lowering the temperature of the air above and producing a shallow inversion. For instance, one set of soundings in Sydney showed a surface inversion 10 m deep at 5.30 p.m., but 100 m by 8.45 p.m. and 150 m by dawn. Thereafter the ground is warmed by the Sun, and air rising from it establishes a dry adiabatic lapse rate from the surface. Figure 7.1 shows such lifting of the base of the inversion to 100 m by 8.40 a.m., and a total erasing of it by 9.55 a.m.

Ground inversions create problems. They may lead to radiation frosts (Section 3.6) on winter nights, and fans may be required to protect crops (Note 7.K). An inversion prevents

Table 7.4 The characteristics of layers of air with various lapse rates

Condition

Cause

Lapse rate

Height range

Location

Effects

Environmental Superadiabatic

Dry adiabatic

Dry neutral

Saturated adiabatic* (i.e. moist neutral) Isothermal

Inversion

Past events Solar heating of ground Theoretical vertical motion of parcel Turbulence

Condensation plus cooling of parcel Uncertain

See Section 7.6

Various Over 1OK/km

1OK/km

1OK/km

About 6K/kmt

Same temp, at all levels

Warmer above cool Various

Lowest few tens of metres Not applicable

PBL) Cloud depth

Small

Measured anywhere Governs stability Close to the ground Absolute instability

Not applicable Not applicable

The PBL (and other well-mixed layers) Within convective clouds Rare

Various

Uninhibited vertical motion Increased instability

Stability

Great stability

* Or 'moist adiabatic lapse rate'

t Values may be between 4-9K/km, depending on the water vapour content of the air, i.e. on the amount of condensation occurring with cooling (Note 7.C)

the dispersion upwards of air pollutants, which is why fire bans are often imposed on calm, clear winter days in large cities such as Sydney. (On the other hand, air pollution from a high chimney is prevented by a ground inversion from affecting people beneath, see Note 7.L.) Also, any sound radiating upwards from the ground is bent back down by the higher speed through the warmer air in the upper part of a ground inversion, which augments the sound at the surface (Note 7.M). Therefore, aircraft noise during take-off or landing is more troublesome on calm nights.

Radiation inversions are prevented by even moderate winds, which stir the air together, producing an adiabatic lapse rate. But an established inversion is difficult to erode by wind, because stable air is reluctant to mix. As a result, weak wind occurs below the inversion and stronger wind above (Figure 7.11). In other words, a ground inversion disconnects the surface air from the energy of the upper winds, so that calm conditions prevail at the surface at night. This has several consequences. For instance: (i) the intensity of bush-fires is reduced at night; (ii) the surface calmness due to a ground inversion makes sounds more clear by removing the background noise of wind and turbulence and by focusing the sound (Note 7.M); and (iii) an onshore wind encountering a nocturnal ground inversion inland is forced to rise over it, and the uplift may induce rain (Chapter 9), which explains why 14 per cent more rain falls in Perth at night (midnight to 6 a.m.) than between midday and 6 p.m. (Chapters 10 and 16).

The inversions are weak when the sky at night is cloudy because of the reduced cooling caused by increased sky radiation (Section 2.7). Also, the warmth of cities (Section 3.7) and the stirring of surface winds induced by a city's roughness (Chapter 14) inhibit urban ground inversions.

Several of these kinds of inversion may occur simultaneously or leave traces for days after

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n j I

j j

f /

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Temperature Wind Speed

Figure 7. 11 Effect of an inversion layer on the wind profile, shown either with an inversion present A (dashed line), or without B (solid line). The upper wind is decoupled from the surface air by the inversion, so speeds become higher above the layer, free of the restraint of surface friction. Speeds are reduced below the layer, for lack of momentum from the upper winds.

Temperature Wind Speed

Figure 7. 11 Effect of an inversion layer on the wind profile, shown either with an inversion present A (dashed line), or without B (solid line). The upper wind is decoupled from the surface air by the inversion, so speeds become higher above the layer, free of the restraint of surface friction. Speeds are reduced below the layer, for lack of momentum from the upper winds.

their original formation. This explains the irregularity of temperature profiles within the atmosphere. The variety of conditions is summarised in Table 7.4.

Thus we have considered the properties of a stable atmosphere, as well as an unstable one. Stable layers cause the thin flat shape of stratus clouds, whilst instability leads to tall clouds, which are all discussed in the next chapter.

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