Geoscience Reference
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wedge of warm air is pinched out at the surface
and lifted bodily off the ground. This stage
of
occlusion
eliminates the wave form at the
surface (see
Figure 9.7
). The depression usually
achieves its maximum intensity 12-24 hours after
the beginning of occlusion. The occlusion
gradually works outwards from the center of the
depression along the warm front. Sometimes, the
cold air wedge advances so rapidly that, in the
friction layer close to the surface, cold air overruns
the warm air and generates a
squall line
(see
Chapter 4G).
By no means all frontal lows follow the
idealized life-cycle discussed above. It is generally
characteristic of oceanic cyclogenesis, although
the evolution of those systems has been re-
examined using aircraft observations collected
during North Atlantic meteorological field
programs in the 1980s. These suggest a different
evolution of maritime frontal cyclones (
Figure
9.9
). Four stages are identified: (1) cyclone
inception features a broad (400km) continuous
frontal zone; (2) frontal fracture occurs near
the center of the low with tighter frontal gradients;
(3) a T-bone structure and bent-back warm
front develop; and (4) the mature cyclone shows
seclusion of the warm core within the polar
airstream behind the cold front.
Over central North America, cyclones forming
in winter and spring depart considerably from the
Norwegian model. They often feature an outflow
of cold arctic air east of the Rocky Mountains
forming an arctic front, a lee trough with dry air
descending from the mountains, and warm, moist
southerly flow from the Gulf of Mexico (
Figure
9.10
). The trough superposes dry air over warm,
moist air, generating instability and a rain band
analogous to a warm front. The arctic air moves
southward west of the low center, causing lifting
of warmer, dry air but giving little precipitation.
There may also be an upper cold front advancing
over the trough that forms a rain band along its
leading edge. Such a system is thought to have
caused a record rainstorm at Holt, Missouri, on 22
June 1947, when 305mm fell in just 42 minutes!
(A)
Potentially
unstable region
CFA
Low-
θ
e
Air
Dry trough
Gulf of Mexico
High-
θ
e
Air
(B)
CFA
Upward
motion
Upward
motion
Dry trough
Gulf of Mexico
(C)
CFA
Dry trough
CFA rain band
at surface
Figure 9.10
Schematic model of a dry trough and
frontogenesis east of the Rocky Mountains. A:
Warm, dry air with low equivalent potential
temperature (u
e
) from the Rockies overrides warm,
moist, high u
e
air from the Gulf of Mexico, forming
a potentially unstable zone east of the dry trough.
B: Upward motion associated with the cold front
aloft (CFA). C: Location of the CFA rain band
at the surface. [Equivalent potential temperature is
the potential temperature of an air parcel that
is expanded adiabatically until all water vapor is
condensed and the latent heat released then
compressed adiabatically to 1000mb pressure.]
Source: After Locatelli et al. (1995). By permission of the
American Meteorological Society.
