Geoscience Reference
In-Depth Information
rapidly, acquiring characteristics that resemble
tropical hurricanes. These have been termed
'bombs' in view of their explosive rate of
deepening; pressure falls of at least 24mb/24hr 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 950mb with hurricane-
force winds and an eye-like storm center within 24
hours (see Chapter 11B.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 favored
by an unstable lower troposphere and is often
located downstream of a traveling 500mb-level
trough. Bombs are characterized by strong vertical
motion, associated with a sharply defined level of
non-divergence near 500mb, and large-scale
release of latent heat. Wind maxima 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 10mb/24hr, or three times
greater than over the continental United States
(3mb/24hr). Hence, it is suggested that explosive
cyclogenesis represents a more intense version of
typical maritime cyclone development.
The movement of depressions is determined
essentially by the upper westerlies and, as a rule of
thumb, a depression center travels at about 70
percent of the surface geostrophic wind speed in
the warm sector. Records for the United States
indicate that the average speed of depressions is
32km hr
-1
in summer and 48km hr
-1
in winter.
The higher speed in winter reflects the stronger
westerly circulation. Shallow depressions are
mainly steered by the direction of the thermal
wind in the warm sector and hence their path
closely follows that of the upper jet stream (see
Chapter 6A.3). Deep depressions may greatly
distort the thermal pattern, however, as a result of
the northward transport of warm air and the
southward transport of cold air. In such cases, the
depression usually becomes slow moving. The
movement of a depression may be additionally
guided by energy sources such as a warm sea
surface that generates cyclonic vorticity, or by
mountain barriers. The depression may cross
obstacles, such as the Rocky Mountains or the
Greenland Ice Sheet, as an upper low or trough,
and subsequently redevelop, aided by the lee
effects of the barrier or by fresh injections of
contrasting air masses.
Ocean surface temperatures can critically
influence the location and intensity of storm
tracks.
Figure 9.25B
indicates that an extensive,
relatively warm surface in the north-central Pacific
in the winter of 1971-1972 caused a northward
displacement of the westerly jet stream together
with a compensating southward displacement
over the western United States, bringing in cold air
there. This pattern contrasts with that observed
during the 1960s (see
Figure 9.25A
), when a
persistent cold anomaly in the central Pacific, with
warmer water to the east, led to frequent storm
development in the intervening zone of strong
temperature gradient. The associated upper
airflow produced a ridge over western North
America with warm winters in California and
Oregon. Models of the global atmospheric
circulation support the view that persistent
anomalies of sea surface temperature exert an
important control on local and large-scale weather
conditions.
H NON-FRONTAL
DEPRESSIONS
Not all depressions originate as frontal waves.
Tropical depressions are indeed mainly non-
frontal and these are considered in Chapter 11.
In middle and high latitudes, four types that
develop in distinctly different situations are
of particular importance and interest: the lee
cyclone, the thermal low, the polar low, and the
cold low.
