Geoscience Reference
In-Depth Information
CONDUCIVE
DETRIMENTAL
Anticyclonic shear of U
MEAN WIND SPEED (U)
U
WARM
UPPER
LEVELS
Small (U
.
c):warm air
stays above surface low
Large (U
.
c):warm
area does not appear
Solenoidal effect
intensifies
radial vertical
circulation
Stable stratification
gives cooling effect
to rising motion
MIDDLE
LEVELS
CUMULUS
CONVECTION
Cyclonic shear
of basic flow
MOISTURE CONVERGENCE
LOW
PLANETARY
BOUNDARY
LAYER
Sufficient moisture
essential condition
c
PROPAGATION SPEED (c)
Low sea-level temp.
reduces evaporation
High sea surface temp.
enhances evaporation
OCEAN
Figure 11.10
A schematic model of the conditions conducive (left) or detrimental (right) to the growth of a tropical
storm in an easterly wave; Uis the mean upper-level wind speed and cis the rate of propagation of the system. The
warm vortex creates a thermal gradient that intensifies both the radial motion around it and the ascending air
currents, termed the solenoidal effect.
Source: From Kurihara (1985). Copyright © Academic Press. Reproduced by permission.
distinctive feature of the hurricane is the warm
vortex, since other tropical depressions and
incipient storms have a cold core area of shower
activity. The warm core develops through the
action of 100-200 cumulonimbus towers releasing
latent heat of condensation; about 15 percent of
the area of cloud bands is giving rain at any one
time. Observations show that although these 'hot
towers' form less than 1 percent of the storm area
within a radius of about 400km, their effect is
sufficient to change the environment. The warm
core is vital to hurricane growth because it
intensifies the upper anticyclone, leading to a
'feedback' effect by stimulating the low-level
influx of heat and moisture, which further
intensifies convective activity, latent heat release
and therefore the upper-level high pressure. This
enhancement of a storm system by cumulus
convection is termed conditional instability of
the second kind, or CISK (cf. the basic parcel
instability described on p. 114). The thermally
direct circulation converts the heat increment
into potential energy and a small fraction of
this - about 3 percent - is transformed into
kinetic energy. The remainder is exported by the
anticyclonic circulation that exists at about
the 12km (200mb) level. A major driver is the
temperature difference between the ocean surface
