Geography Reference
In-Depth Information
If we assume that for a typical hurricane the vertical scale is the scale height H ,
the azimuthal velocity scale is U
50 m s
−
1
, the horizontal scale is L
∼
∼
100 km,
10
−
5
s
−
1
(corresponding to a latitude of about 20˚), we find from
(9.63) that the radial temperature fluctuation must have a magnitude
and f
∼
5
×
∼
UL
R
f
2U
L
∼
10
◦
C
δT
+
This strong radial variation in temperature is one of the most important thermody-
namical characteristics of hurricanes.
The kinetic energy of hurricanes is maintained in the presence of boundary layer
dissipation by the conversion of latent heat energy acquired from the underlying
ocean. This potential energy conversion is carried out by a transverse secondary
circulation associated with the hurricane, as shown schematically in Fig. 9.14. This
circulation consists of boundary layer inflow into a region of enhanced convection
surrounding the storm center that is referred to as the
eyewall
, ascent within con-
vective cloud towers that tend to be concentrated in the narrow outward-sloping
eyewall, radial outflow in a thin layer near the tropopause, and gentle subsidence
at a large radius. Observations show that evaporation of water from the sea surface
into the inward flowing air in the boundary layer causes a large increase in θ
e
as
the air approaches the eyewall region. Within the eyewall the θ
e
and M
λ
surfaces
coincide so that parcel ascent in the eyewall (along the path labeled 1 in Fig. 9.14) is
neutral with respect to conditional symmetric instability, and thus does not require
external forcing. The eyewall surrounds a central eye of radius 5-50 km that is
often calm and nearly cloud free.
The energetics of the steady-state hurricane can be viewed as an example of a
Carnot cycle heat engine in which heat is absorbed (in the form of water vapor)
from the ocean at temperature T
s
and expelled by radiative cooling to space at
Fig. 9.14
Schematic cross section of secondary meridional circulation in a mature hurricane. Air
spirals in toward the eye (region 5) in the boundary layer (region 4), ascends along constant
M surfaces in the eyewall cloud (region 1), and slowly subsides and dries in regions 2 and
3. (After Emanuel, 1988.)
