Condensation Levell

SAT '' L \

MIXING ^

Vdalr

RATIO / J \ *

Temperature ■

Figure 5.4 Schematic adiabatic chart used to determine the convective condensation level (see p. 91). T0 represents the early morning temperature: Tp T2 and T3 illustrate daytime heating of the surface air.

downward it will warm at the dry adiabatic rate; the parcel will always be warmer and less dense than the surrounding air, and tend to return to its former position (unless prevented from doing so). However, if local surface heating causes the environmental lapse rate near the surface to exceed the dry adiabatic lapse rate (b), then the adiabatic cooling of a convective air parcel allows it to remain warmer and less dense than the surrounding air, so it continues to rise through buoyancy. The characteristic of unstable air is a tendency to continue moving away from its original level when set in motion. The transition between the stable and unstable states is termed neutral.

We can summarize the five basic states of static stability which determine the ability of air at rest to remain laminar or become turbulent through buoyancy. The key is the temperature of a displaced air parcel relative to that in the surrounding air.

• Conditionally unstable: SALR < ELR < DALR

• Absolutely unstable: ELR > DALR

Air that is colder than its surroundings tends to sink. Cooling in the atmosphere usually results from radiative processes, but subsidence also results from horizontal convergence of upper tropospheric air (see Chapter 6B.2). Subsiding air has a typical vertical velocity of only 1-10 cm s-1, unless convective downdraft conditions prevail (see below). Subsidence can produce substantial changes in the atmosphere; for instance, if a typical airmass sinks about 300 m, all average-size cloud droplets will usually be evaporated through the adiabatic warming.

Figure 5.5 illustrates a common situation where the air is stable in the lower layers. If the air is forced upward by a mountain range, or through local surface heating, the path curve may eventually cross to the right of the environment curve (the level of free convection). The air, now warmer than its surroundings, is buoyant

Mixing Condensation Level
Figure 5.5 Schematic tephigram illustrating the conditions associated with the conditional instability of an airmass that is forced to rise. The saturation mixing ratio is a broken line and the lifting condensation level (cloud base) is below the level of free convection.

and free to rise. This is termed conditional instability; the development of instability is dependent on the airmass becoming saturated. Since the environmental lapse rate is frequently between the dry and saturated adiabatic rates, a state of conditional instability is common. The path curve intersects the environment curve at 650 mb. Above this level the atmosphere is stable, but the buoyant energy gained by the rising parcel enables it to move some distance into this region. The theoretical upper limit of cloud development can be estimated from the tephigram by determining an area (B) above the intersection of the environment and path curves equal to that between the two curves from the level of free convection to the intersection (A) in Figure 5.5. The tephigram is so constructed that equal areas represent equal energy.

These examples assume that a small air parcel is being displaced without any compensating air motion or mixing of the parcel with its surroundings. These assumptions are rather unrealistic. Dilution of an ascending air parcel by mixing of the surrounding air with it through entrainment will reduce its buoyant energy. However, the parcel method is generally satisfactory for routine forecasting because the assumptions approximate conditions in the updraft of cumulonimbus clouds.

In some situations a deep layer of air may be displaced over an extensive topographic barrier. Figure 5.6 shows a case where the air in the upper levels is less moist than that below. If the whole layer is forced upward, the drier air at B cools at the dry adiabatic rate, and so initially will the air about A. Eventually the lower air reaches condensation level, after which this layer cools at the saturated adiabatic rate. This results in an increase in the actual lapse rate of the total thickness of the raised layer, and, if this new rate exceeds the saturated adiabatic, the air layer becomes unstable and may overturn. This is termed convective (or potential) instability.

Vertical mixing of air was identified earlier as a possible cause of condensation. This is best illustrated by use of a tephigram. Figure 5.7 shows an initial distribution of temperature and dew-point. Vertical mixing leads to averaging these conditions through the layer affected. Thus, the mixing condensation level is determined from the intersection of the average values of saturation humidity mixing ratio and potential temperature. The areas above and below the points where these x u at D

FINAL\ V LAPSE RATE \ ADIABAT (UNSTABLE)

Amount of lifting

SATURATED A\ ADIABAT V

Condensation \

Amount of lifting

INITIAL LAPSE RATE (STABLE)

Amount of lifting

SATURATED A\ ADIABAT V

Condensation \

INITIAL LAPSE RATE (STABLE)

TEMPERATURE ■

Figure 5.6 Convective instability. AB represents the initial state of an air column; moist at A, dry at B. After uplift of the whole air column the temperature gradient A' B' exceeds the saturated adiabatic lapse rate, so the air column is unstable.

Figure 5.7 Graph illustrating the effects of vertical mixing in an airmass. The horizontal lines are pressure surfaces (P2, P^. The initial temperature (7!) and dew-point temperature (Td|) gradients are modified by turbulent mixing to T2 and Td2. The condensation level occurs where the dry adiabat (0) through T| intersects the saturation humidity mixing ratio line (Xs) through Td2.

Figure 5.7 Graph illustrating the effects of vertical mixing in an airmass. The horizontal lines are pressure surfaces (P2, P^. The initial temperature (7!) and dew-point temperature (Td|) gradients are modified by turbulent mixing to T2 and Td2. The condensation level occurs where the dry adiabat (0) through T| intersects the saturation humidity mixing ratio line (Xs) through Td2.

average-value lines cross the initial environment curves are equal.

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