Geography Reference
In-Depth Information
Noting that the basic state horizontal vorticity in this case is
i
∂v
∂z
j
∂u
∂z
ω
=
k
×
S
=−
+
we see that there is a contribution to the dynamic pressure in (9.57) of the form
∇·
U
×
ω
≈−
∇·
w
S
From (9.57) the sign of the pressure perturbation due to this effect may be deter-
mined by noting that
∂x
w
S
x
−
∂y
w
S
y
∂
∂
2
p
dyn
∼−
∇
p
dyn
∼−
(9.60)
which shows that there is a positive dynamical pressure perturbation upshear of
the cell and a negative perturbation downshear (analogous to the positive pressure
perturbation upwind and negative perturbation downwind of an obstacle). The
resulting pattern of dynamical pressure perturbations is shown in Fig. 9.13. In the
case of unidirectional shear (Fig. 9.13a), the induced pressure pattern favors updraft
growth on the leading edge of the storm. However, when the shear vector rotates
clockwise with height as in Fig. 9.13b, (9.60) shows that a dynamical pressure
disturbance pattern is induced in which there is an upward directed vertical pressure
gradient force on the flank of the cyclonically rotating cell and a downward directed
pressure gradient force on the flank of the anticyclonic cell. Thus, in the presence
of clockwise rotation of the environmental shear stronger updrafts are favored in
the right-moving cyclonic vortex to the south of the initial updraft.
9.7
HURRICANES
Hurricanes, which are also referred to as tropical cyclones and typhoons in some
parts of the world, are intense vortical storms that develop over the tropical oceans
in regions of very warm surface water. Although the winds and convective clouds
observed in hurricanes are not truly axisymmetric about the vortex center, the
fundamental aspects of hurricane dynamics can be modeled by idealizing the hur-
ricane as an axisymmetric vortex. Typically hurricanes have radial scales of several
hundred kilometers, similar to those of some midlatitude synoptic systems. How-
ever, the horizontal scale of the region of intense convection and strong winds in a
hurricane is typically only about 100 km in radius. Thus, it is reasonable to classify
hurricanes as mesoscale systems.
Unlike the rotating convective storms treated in the previous section, the hurri-
cane vortex cannot be understood without including the rotation of the earth in the
vorticity balance. The rapid rotation observed in hurricanes is produced by con-
centration of the vertical component of absolute vorticity by vortex stretching, not
