Convection In The Atmosphere

We have seen that the atmosphere is normally stable in the absence of condensation. Hence most convection in the atmosphere is moist convection, accompanied by saturation and hence cloud formation. Downwelling air parcels do not become saturated because descending air

FIGURE 4.20. Schematic of convective clouds: Cu = cumulus; Cb = cumulonimbus. The condensation level is the level above which q = q„. Cb clouds have a characteristic ''anvil,'' where the cloud top spreads and is sheared out by strong upper level winds.

warms adiabatically. They would therefore become positively buoyant, if it were not for radiative cooling, a process that is much slower than latent heat release in the updrafts; so the descent must be slow. Thus moist convection comprises narrow, cloudy, vigorous updrafts, with larger areas of clear, dry, air slowly descending between.

4.6.1. Types of convection

Convective clouds have two main forms: cumulus (Cu) clouds (usually small, ''fair-weather,'' and nonprecipitating) and cumulonimbus (Cb) clouds (usually associated with thunderstorms and heavy rain, and perhaps hail).7

Modest convection is common and usually shallow (up to just a few km) and capped by Cu clouds (see Figs. 4.20 and 4.21). They are typically 1-2 km tall and towers can be seen to grow this far in about 15 min, implying a vertical velocity of around 2 ms-1. Typical temperature fluctuations are of order 0.1 K (see the application of parcel theory below). The sensible heat flux is pCpw'T', where the primes denote differences from the time mean, represented by an overbar. Using the above estimates and Table 1.4, we obtain a heat flux of 200 W m-2, an impressive number and comparable to the radiative fluxes shown, for example, in Fig. 5.5.

Deep convection is common in the tropics (see Section 4.6.2) and occasionally elsewhere. It is manifested by huge Cb clouds, illustrated in Figs. 4.20, 4.22, and 4.23, the tops of which may reach the tropopause, and become so cold that the cloud top is sheared off by wind to form an anvil made of ice crystals. Vertical motions can reach tens of m s-1 with temperature fluctuations around 1 K. The vertical heat flux associated with individual cumulonimbus clouds is many kWm-2. However, they are intermittent both in space and time. They are the primary mechanism of vertical heat transport in the tropics.

Luke Howard (1772-1864). An English manufacturing chemist and pharmacist, Howard was also an amateur meteorologist. He wrote one of the first textbooks on weather and developed the basis for our cloud classification system; he is responsible for the cloud nomenclature now in standard use.

FIGURE 4.22. A mature cumulonimbus (thunderstorm) cloud producing rain and hail on the Great Plains. The hail core is evident in the bright white streaks (center). As the updrafts rise through the cloud and into noticeably warmer air, the top of the cloud spreads out and flattens (top). From the University Corporation for Atmospheric Research.

FIGURE 4.22. A mature cumulonimbus (thunderstorm) cloud producing rain and hail on the Great Plains. The hail core is evident in the bright white streaks (center). As the updrafts rise through the cloud and into noticeably warmer air, the top of the cloud spreads out and flattens (top). From the University Corporation for Atmospheric Research.

Dynamics of cumulus convection

We have seen that at a certain height, rising moist air parcels become saturated and a cloud forms. What happens above the cloud base depends on the type of cloud. Cumulus clouds are observed to mix with their surroundings, entraining ambient air. The result is that they rapidly lose their buoyancy. This is illustrated in Fig. 4.24, where we sketch air parcels ascending a little along wet adiabats, followed by complete mixing (the short horizontal lines).

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