Geoscience Reference
In-Depth Information
weather accompanying them is usually hot and
dry, but if sufficient moisture is present the
instability caused by heating may lead to showers
and thunderstorms. Thermal lows normally
disappear at night, when the heat source is cut off,
but in fact those in India and Arizona persist.
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 like 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. It also occurs off southeast
Greenland where the barrier effect of the ice sheet
promotes cyclogenesis in the Denmark Strait. The
development of such lee cyclones contributes to
the strength and position of the mean Icelandic
low.
3 Polar lows
Polar lows are a loosely defined class of mesoscale
to subsynoptic-scale systems (a few hundred
kilometers across) with a lifetime of one to two
days. On satellite imagery, they appear as a cloud
spiral with one or several cloud bands, as a comma
cloud (see
Figure 9.17
and
Plate 9.2
), or as a swirl
in cumulus cloud streets. They develop mainly in
the winter months, when unstable mP or mA air
currents stream equatorward along the eastern
side of a north-south ridge of high pressure,
commonly in the rear of an occluding primary
depression. They usually form within a baroclinic
zone, e.g., near sea-ice margins, where there are
strong sea surface temperature gradients, and their
development may be stimulated by an initial
upper-level disturbance.
In the Northern Hemisphere, the comma
cloud type (which is mainly a cold-core distur-
bance of the middle troposphere) is more
common over the North Pacific, while the
spiral-form polar low occurs more often in the
Norwegian Sea. The latter is a low-level warm-
core disturbance that may have a closed cyclonic
circulation up to about 800mb or it may consist
simply of one or more troughs embedded in the
polar airflow. A key feature is the presence of an
ascending, moist, southwesterly flow
relative
to
the low center. This organization accentuates the
general instability of the cold airstream to give
considerable precipitation, often as snow. Latent
heat release is an important mechanism for
generating polar lows in the southern Norwegian
Sea while stronger low-level baroclinicity and
weaker convection prevail in systems in the
northern Norwegian Sea and Barents Sea. Heat
input to the cold air from the sea continues by
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 Saudi
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 40-60
percent of days in July and August. Typically, their
intensity is only 2-4mb and they extend to about
750mb, less than in other subtropical areas. The
