Geoscience Reference
In-Depth Information
10 mb/24 hr, or three times greater than over the
continental United States (3 mb/24 hr). Hence, it is
suggested that explosive cyclogenesis represents a more
intense version of typical maritime cyclone develop-
ment.
The movement of depressions is determined essen-
tially by the upper westerlies and, as a rule of thumb, a
depression centre travels at about 70 per cent of the
surface geostrophic wind speed in the warm sector.
Records for the United States indicate that the average
speed of depressions is 32 km hr
-1
in summer and 48
km 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 7A.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 in addition be 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
airmasses.
Ocean-surface temperatures can crucially 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 to 1972
caused a northward displacement of the westerly jet
stream together with a compensating southward dis-
placement 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, polar air depressions and
the cold low.
1 The lee cyclone
Westerly airflow that is forced over a north-south
mountain barrier undergoes vertical contraction over
the ridge and expansion on the lee side. This vertical
movement creates compensating lateral expansion and
contraction, respectively. Hence there is a tendency for
divergence and anticyclonic curvature over the crest,
and convergence and cyclonic curvature in the lee of
the barrier. Wave troughs may be set up in this way
on the lee side of low hills (see Figure 6.13) as well as
major mountain chains such as the Rocky Mountains.
The airflow characteristics and the size of the barrier
determine whether or not a closed low-pressure system
actually develops. Such depressions, which at least
initially tend to remain 'anchored' by the barrier, are
frequent in winter to the south of the Alps, when the
mountains block the low-level flow of northwesterly
airstreams. Fronts often develop in these depressions,
but the low does not form as a wave along a frontal zone.
Lee cyclogenesis is common in Alberta and Colorado,
in the lee of the Rocky Mountains, and in northern
Argentina in the lee of the Andes.
2 The thermal low
These lows occur almost exclusively in summer,
resulting from intense daytime heating of continental
areas. Figure 7.1C illustrates their vertical structure. The
most impressive examples are the summer low-pressure
cells over Arabia, the northern part of the Indian sub-
continent and Arizona. The Iberian Peninsula is another
region commonly affected by such lows. They occur
over southwestern Spain on 40 to 60 per cent of days in
July and August. Typically, their intensity is only 2 to
4 mb and they extend to about 750 mb, less than in other
subtropical areas. The weather accompanying them is
usually hot and dry, but if sufficient moisture is present
the instability caused by heating may lead to showers
