wind relative to storm sclual1 front
FIGURE 4.25. The pattern of flow relative to a cumulonimbus storm moving along with a middle-level wind. Ahead of the storm, air is sucked in (p-), ascends, and is expelled in the anvil. Upper-middle level air approaches the storm from behind and is brought down to the ground (p+). The ''squall front'' is a stagnation point relative to the storm which moves roughly at the speed of the storm. Heavy rain is represented by the vertical dotted lines in the updraught. Modified from Green (1999).
flows strongly toward the storm, ascends in the cloud and is eventually expelled as an anvil in a shallow, fast-moving sheet containing ice crystals. Release of latent heat in condensation (and associated heavy rain)
in the updraft creates positive buoyancy and vertical acceleration, powering the motion. A region of low pressure is created at the surface just ahead of the storm, ''sucking'' low-level flow into it. At the same time, upper-middle level air approaches the storm from behind, is cooled by evaporation from the rain falling into it, and brought down to the ground where it creates a region of high pressure as it decelerates. The ''squall front'' is a stagnation point relative to the storm, which moves roughly at the speed of the storm.
In contrast to a cumulus cloud, there is hardly any mixing in a cumulonimbus cloud, because the flow is so streamlined. Consequently nearly all the potential energy released goes into kinetic energy of the updraft. Hence 2 w2 ~ gATT Az, which yields w ~ 25 ms-1 if AT ~ 1K and Az ~ 10 km, roughly in accord with observations. Updrafts of this magnitude are strong enough, for example, to suspend hailstones until they grow to a large size.
4.6.2. Where does convection occur?
The short answer to this question is, in fact, almost everywhere. However, deep convection is common in some places and rare in others. In general, tropical rainfall is associated with deep convection, which is most common in the three equatorial regions where rainfall is most intense (Indonesia and the western equatorial Pacific Ocean, Amazonia, and equatorial Africa). Over the desert regions of the subtropics, it is uncommon. The contrast between these two areas is shown in the distribution of outgoing long-wave radiation (OLR) in Fig. 4.26.
OLR is the total radiative flux in the terrestrial wavelengths, measured by downward-looking satellite instruments. As discussed in Chapter 2, if we can think of this flux as emanating from a single layer in the atmosphere, then we can deduce the temperature of that layer (assuming blackbody radiation, Eq. 2.2). So OLR is a measure of the temperature of the emitting region. Note that the polar regions in Fig. 4.26 have low OLR: this is not very surprising, since these regions are cold. The OLR is also low, however, over the three equatorial regions mentioned previously. The radiation cannot be coming from the surface there, since the surface is warm; it must be (and is) coming from high altitudes (10-15 km), where the temperature is low, even in the tropics. As shown in Fig. 4.27, this happens because the radiation is coming from the tops of deep convective clouds; the low OLR is
Outgoing Longwave Radialion (Wnrr*)
Outgoing Longwave Radialion (Wnrr*)
FIGURE 4.26. Outgoing longwave radiation (OLR: contour interval 20 Wm-2) averaged over the year. Note the high values over the subtropics and low values over the three wet regions on the equator: Indonesia, Amazonia, and equatorial Africa.
FIGURE 4.27. Schematic of IR radiation from the ground (at temperature Ts ~ 300 K in the tropics) and from the tops of deep convective clouds (at temperature Tc ~ 220 K).
indicative of deep convection. Note that in the subtropical regions, especially over the deserts and cooler parts of the ocean, OLR is high; these regions are relatively dry and cloud-free, and the radiation is coming from the warm surface.
Convection requires a warm surface relative to the environmental air aloft. This can be achieved by warming the surface by, for example, solar heating, leading to afternoon convection, especially in summer and, most of all, in the tropics where deep convection is very common. However, convection can also be achieved by cooling the air aloft. The latter occurs when winds bring cold air across a warm surface; for example, thunderstorms in middle latitudes are frequently associated with the passage of cold fronts. In middle latitudes, shallow convection is frequent (and is usually visible as cumulus clouds). Deep convection (always associated with heavy rain and often with thunderstorms) is intermittent.
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