FIGURE 4.23. A supercell is a giant cumulonimbus storm with a deep rotating updraft. Supercells can produce large amounts of hail, torrential rainfall, strong winds, and sometimes tornadoes. The close-up view of the supercell thunderstorm in the picture shows a bulging dome of clouds extending above the flat, anvil top. This is caused by a very intense updraft that is strong enough to punch through the tropopause and into the stratosphere. At the time of this photograph, baseball-sized hail was falling and a tornado was causing havoc in southern Maryland. Photograph by Steven Maciejewski (April 28,2002): reported by Kevin Ambrose.
One consequence evident from the figure is that the cumulus top is expected where the wet adiabat first runs parallel to the environmental profile, more or less observed in practice.
A simple model of heat transport in cumulus convection can be constructed as follows, in the spirit of the parcel theory developed in Section 4.2.4 to describe convection in our tank of water in GFD Lab II. From Eq. 4-20 we see that the buoyant acceleration of an air parcel is gA, where AT = TP - Te. If the parcel eventually rises by a height Az, the PE of the system will have decreased by an amount per unit mass of gT Az. Equating this to acquired KE, equally distributed between horizontal and vertical components, we find that 2w2 ~ g, which is Eq. 4-9 with a replaced by T-1. The vertical heat transport by a population of cumulus clouds is then given by Eq. 4-10 with a replaced by
T-1. This fairly crude model of convective vertical heat transport is a useful representation and yields realistic values. If convection carries heat at a rate of 200 Wm-2 from the surface up to 1 km, Eq. 4-10 predicts AT ~ 0.1K and w ~ 1.5 ms-1, roughly in accord with what is observed.
Suppose we had imagined that the Cu cloud discussed in the previous section had a vertical scale of 10 km rather than the 1 km assumed. We will see in Chapter 5 that the wind at a height of 10 km is some 20-30 m s-1 faster than at cloud base (see Fig. 5.20), and so the cumulus cloud would be ripped apart. However, Cb storms move at the same speed as some middle-level wind. Fig. 4.25 shows the flow relative to a cumulonimbus storm moving along with the wind at midlevels. It overtakes the potentially warm air near the surface, and so, relative to the storm, this air
FIGURE 4.24. In cumulus convection, buoyant air parcels ascend along a moist adiabat but repeatedly mix with ambient fluid, reducing their buoyancy. Cumulus top is expected to occur at the level at which the wet adiabat first runs parallel to the environmental curve, as shown in the inset at top right of figure.
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