Geography Reference
In-Depth Information
temperature T
0
at the top of the storm. Because T
s
≈
300 K and T
0
≈
200 K, the
efficiency of the heat engine is very high.
9.7.2
Hurricane Development
The origin of tropical cyclones is still a matter of uncertainty. It is not clear under
what conditions a weak tropical disturbance can be transformed into a hurricane.
Although there are many tropical disturbances each year, only rarely does one
develop into a hurricane. Thus, the development of a hurricane must require rather
special conditions. Many theoretical investigations of this problem have assumed
the initial existence of a small-amplitude cylindrically symmetric disturbance and
examined the conditions under which unstable amplification of the disturbance can
occur. As shown in Chapter 8, this sort of linear stability theory is quite successful
in accounting for the development of extratropical baroclinic disturbances.
In the tropics, however, the only well-documented linear instability is condi-
tional instability. This instability has its maximum growth rate on the scale of an
individual cumulus cloud. Therefore, it cannot explain the synoptic-scale orga-
nization of the motion. Observations indicate, moreover, that the mean tropical
atmosphere is not saturated, even in the planetary boundary layer. Thus, a parcel
must undergo a considerable amount of forced ascent before it reaches its LFC
and becomes positively buoyant. Such forced parcel ascent can only be caused
by small-scale motions, such as turbulent plumes in the boundary layer. The effi-
cacy of boundary layer turbulence in producing parcel ascent to the LFC clearly
depends on the temperature and humidity of the boundary layer environment. In
the tropics it is difficult to initialize deep convection unless the boundary layer is
brought toward saturation and destabilized, which may occur if there is large-scale
(or mesoscale) ascent in the boundary layer. Thus, convection tends to be con-
centrated in regions of large-scale low-level convergence. This concentration does
not arise because the large-scale convergence directly “forces” the convection, but
rather because it preconditions the environment to be favorable for parcel ascent
to the LFC.
Cumulus convection and the large-scale environmental motion are thus often
viewed as cooperatively interacting. In this viewpoint, diabatic heating due to latent
heat released by cumulus clouds produces a large-scale (or mesoscale) cyclonic
disturbance; this disturbance, in turn, through boundary layer pumping, drives the
low-level moisture convergence necessary to maintain an environment favorable
for the development of cumulus convection. There have been attempts to formalize
these ideas into a linear stability theory [often referred to as
conditional instability
of the second kind
(CISK)], which attributes hurricane growth to the organized
interaction between the cumulus scale and the large-scale moisture convergence.
This interaction process is indicated schematically in Fig. 9.15a. The CISK model
for hurricane development has not been very successful, as there is little evidence
