Mid-latitude synoptic and mesoscale systems - Atmosphere, Weather and Climate - page 222

Figure 9.24 The relations between surface fronts and isobars, surface precipitation ( 25 mm vertical hatching; >25 mm cross-

hatching), and jet streams (wind speeds in excess of about 45 m s -1 shown by stipple) over the United States on 20 September 1958
and 21 September 1958. This illustrates how the surface precipitation area is related more to the position of the jets than to that of
the surface fronts. The air over the south-central United States was close to saturation, whereas that associated with the northern jet
and the maritime front was much less moist.
Source : After Richter and Dahl (1958), by permission of the American Meteorological Society.

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upstream of the core), divergence causes lower-level
air to rise on the equatorward (i.e. right) side of the
jet, whereas in the exit zone (downstream of the core)
ascent is on the poleward side. Figure 9.24 shows how
precipitation is related more often to the position of the
jet stream than to that of surface fronts; maximum
precipitation areas are in the right entrance sector of the
jet core. This vertical motion pattern is also of basic
importance in the initial deepening stage of the depres-
sion. If the upper-air pattern is unfavourable (e.g.
beneath left entrance and right exit zones, where there
is convergence) the depression will fill.
The development of a depression can also be con-
sidered in terms of energy transfers. A cyclone requires
the conversion of potential into kinetic energy. The
upward (and poleward) motion of warm air achieves
this. The vertical wind shear and the superimposition of
upper tropospheric divergence drive the rising warm air
over a baroclinic zone. Intensification of this zone
further strengthens the upper winds. The upper diver-
gence allows surface convergence and pressure fall to
occur simultaneously. Modern theory relegates the
fronts to a subordinate role. They develop within depres-
sions as narrow zones of intensified ascent, probably
through the effects of cloud formation.

Recent research has identified a category of mid-
latitude cyclones that develop and intensify rapidly,
acquiring characteristics that resemble tropical hurri-
canes. These have been termed 'bombs' in view of their
explosive rate of deepening; pressure falls of at least 24
mb/24 hr are observed. For example, the ' QE II storm',
which battered the ocean liner Queen Elizabeth II off
New York on 10 September 1978, developed a central
pressure below 950 mb with hurricane-force winds and
an eye-like storm centre within twenty-four hours (see
Chapter 11C.2). These systems are observed mainly
during the cold season off the east coast of the United
States, off Japan, and over parts of the central and
northeastern North Pacific, in association with major
baroclinic zones and close to strong gradients of sea-
surface temperature. Explosive cyclogenesis is favoured
by an unstable lower troposphere and is often located
downstream of a travelling 500-mb-level trough. Bombs
are characterized by strong vertical motion, associated
with a sharply defined level of non-divergence near 500
mb, and large-scale release of latent heat. Wind maxim
a in the upper troposphere, organized as jet streaks,
serve to amplify the lower-level instability and upward
motion. Studies reveal that average cyclonic deepening
rates over the North Atlantic and North Pacific are about