Geoscience Reference
In-Depth Information
and thunderstorms. Thermal lows normally disappear
at night when the heat source is cut off, but in fact those
of India and Arizona persist.
isotherms about the low centre. Surface charts may
show little or no sign of these persistent systems, which
are frequent over northeastern North America and
northeastern Siberia. They probably form as the result
of strong vertical motion and adiabatic cooling in
occluding baroclinic lows along the Arctic coastal
margins. Such lows are especially important during the
Arctic winter in that they bring large amounts of
medium and high cloud, which offsets radiational
cooling of the surface. Otherwise, they usually cause no
'weather' in the Arctic during this season. It is important
to emphasize that tropospheric cold lows may be linked
with either low- or high-pressure cells at the surface.
In middle latitudes, cold lows may form during
periods of low-index circulation pattern (see Figure
7.23) by the cutting off of polar air from the main body
of cold air to the north (these are sometimes referred to
as
cut-off lows
). This gives rise to weather of polar
airmass type, although rather weak fronts may also be
present. Such lows are commonly slow moving and give
persistent unsettled weather with thunder in summer.
Heavy precipitation over Colorado in spring and autumn
is often associated with cold lows.
3 Polar air depressions
Polar air depressions are a loosely defined class of
mesoscale to subsynoptic-scale systems (a few hundred
kilometres 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 19), 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 (Figure 9.17). 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 disturbance 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 800 mb or 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 centre. This organization accentuates
the general instability of the cold airstream to give
considerable precipitation, often as snow. Heat input to
the cold air from the sea continues by night and day, so
in exposed coastal districts showers may occur at any
time.
In the southern hemisphere, polar low mesocyclones
appear to be most frequent in the transition seasons, as
these are the times of strongest meridional temperature
and pressure gradients. In addition, over the Southern
Ocean the patterns of occurrence and movement are
more zonally distributed than in the northern hemi-
sphere.
I MESOSCALE CONVECTIVE SYSTEMS
Mesoscale convective systems (MCSs) are intermediate
in size and life span between synoptic disturbances and
individual cumulonimbus cells (see Figure 9.26). Figure
9.27 shows the movement of clusters of convective
cells, each cell about 1 km in diameter, as they crossed
southern Britain with a cold front. Each individual cell
may be short-lived, but cell clusters may persist for
hours, strengthening or weakening due to orographic
and other factors.
MCSs occur seasonally in middle latitudes (partic-
ularly the central United States, eastern China and South
Africa) and the tropics (India, West and Central Africa
and northern Australia) as either nearly circular clusters
of convective cells or linear squall lines. The
squall line
consists of a narrow line of thunderstorm cells, which
may extend for hundreds of kilometres. It is marked by
a sharp veer of wind direction and very gusty conditions.
The squall line often occurs ahead of a cold front,
maintained either as a self-propagating disturbance or
by thunderstorm downdrafts. It may form a pseudo-cold
front between rain-cooled air and a rainless zone within
the same airmass. Mid-latitude squall lines appear to
4 The cold low
The cold low (or
cold pool
) is usually most evident in
the circulation and temperature fields of the middle
troposphere. Characteristically, it displays symmetrical
