Geoscience Reference
In-Depth Information
A
B
CLOUD COVERED
MOSTLY OPEN
C
D
E
Figure 9.8
Schematic patterns of cloud cover (white) observed from satellites, in relation to surface fronts and generalized isobars.
A, B, C and D correspond to the four stages in Figure 9.7.
Source
: After Boucher and Newcomb (1962), by permission of the American Meteorological Society.
twenty-four hours after the beginning of occlusion.
The occlusion gradually works outward from the
centre 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 9I).
By no means all frontal lows follow the idealized
life cycle discussed above (cf. the caption for Plate 18).
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 programmes in the
1980s. These suggest a different evolution of maritime
frontal cyclones (Figure 9.9). Four stages are identified:
(I) cyclone inception features a broad (400 km) con-
tinuous frontal zone; (II) frontal fracture occurs near the
centre of the low with tighter frontal gradients; (III) a T-
bone structure and bent-back warm front develop, and
(IV) 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 a 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 centre,
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 305 mm fell in just forty-two minutes!
E FRONTAL CHARACTERISTICS
The character of frontal weather depends upon the
vertical motion in the airmasses. If the air in the warm
sector is rising relative to the frontal zone the fronts are
usually very active and are termed
ana-fronts
, whereas
sinking of the warm air relative to the cold airmasses
gives rise to less intense
kata-fronts
(Figure 9.11).
