Geography Reference
In-Depth Information
that such interaction leads to a growth rate maximum on the observed scale of
hurricanes.
In recent years a dramatically different view of the stability of the tropical
atmosphere has come into vogue. This view, which is referred to as
wind-induced
surface heat exchange
(WISHE), is based on air-sea interactions. According to
the WISHE view, illustrated schematically in Fig. 9.15b, the potential energy for
hurricanes arises from the thermodynamic disequilibrium between the atmosphere
and the underlying ocean. The efficacy of air-sea interaction in providing potential
energy to balance frictional dissipation depends on the rate of transfer of latent heat
from the ocean to the atmosphere. This is a function of surface wind speed; strong
surface winds, which produce a rough sea surface, can increase the evaporation rate
greatly. Thus, hurricane development depends on the presence of a finite amplitude
initiating disturbance, such as an equatorial wave, to provide the winds required to
produce strong evaporation. Given a suitable initial disturbance, a feedback may
occur in which an increase in inward spiraling surface winds increases the rate of
moisture transfer from the ocean, which by bringing the boundary layer toward
saturation increases the intensity of the convection, which further increases the
secondary circulation.
The air-sea interaction theory is consistent with observations that hurricanes can
develop only in the presence of very warm ocean surface temperatures. For surface
temperatures less than 26˚C, the converging flow in the boundary layer apparently
cannot acquire a high enough equivalent potential temperature to sustain the intense
transverse circulation needed to maintain the hurricane. Thus, it appears that hurri-
canes do not arise from linear instability associated with latent heating of the tropi-
cal atmosphere, but develop from preexisting large-scale disturbances under rather
special conditions that permit a rapid flux of moisture from the ocean surface. The
role of convection is then not to provide an internal heat source, but rather to rapidly
establish a moist adiabatic lapse rate, tied to the boundary layer θ
e
. This produces
a warm core structure in the region of enhanced surface saturation θ
e
(Fig. 9.15b).
PROBLEMS
9.1.
Show by transforming from θ coordinates to height coordinates that the Ertel
potential vorticity P is proportional to F
2
N
s
-S
4
. [See (9.27).]
9.2.
Show that (9.17), the quasi-geostrophic version of the equation for the
streamfunction of the cross-frontal circulation, is equivalent to the omega
equation (6.53) in the Boussinesq approximation for adiabatic flow inde-
pendent of the x coordinate.
9.3.
Starting with the linearized Boussinesq equations for a basic state zonal flow
that is a function of height, derive (9.31) and verify the form given for the
Scorer parameter.
