Geoscience Reference
In-Depth Information
Figure 9.15
Schematic diagrams showing airflows, relative to the moving frontal system, at an ana-cold front. A warm conveyor belt
(stippled) ascends above the front with cold air (dashed arrows) descending beneath it. (A) Plan view. (B) Vertical section along the line
X-Y, showing rates of vertical motion.
Source
: Browning (1990), by permission of the American Meteorological Society.
3 The occlusion
A different process occurs when there is interaction
between the cloud bands within a polar trough and the
main polar front, giving rise to an
instant occlusion
.
A warm conveyor belt on the polar front ascends as an
upper tropospheric jet, forming a stratiform cloud band
(Figure 9.17), while a low-level polar trough conveyor
belt at right angles to it produces a convective cloud
band and precipitation area poleward of the main polar
front on the leading edge of the cold pool.
Frontolysis
(frontal decay) represents the final phase
of a front's existence although it is not necessarily linked
with occlusion. Decay occurs when differences no
longer exist between adjacent airmasses. This may arise
in four ways: (1) through their mutual stagnation over
a similar surface; (2) as a result of both airmasses
moving on parallel tracks at the same speed; (3) through
their movement in succession along the same track at
the same speed, or (4) by the system entraining air of the
same temperature.
The cold front moves faster relative to the warm front,
eventually catching up with it, leading to
occlusion
,
where the warm sector air is lifted off the ground.
Occlusions are classified as either
cold
or
warm
,
depending on the relative states of the airmasses lying
in front and to the rear of the warm sector (Figure 9.16).
If airmass 2 is colder than airmass 1, then the occlusion
is warm, but if the reverse is so it is termed a
cold occlu-
sion
. The air in advance of the depression is likely to be
coldest when depressions occlude over Europe in winter
and very cold cP air is affecting the continent. Recent
work suggests that most occlusions are warm and that
the thermal definition is often misleading. A new
definition is proposed: a cold (warm) occlusion forms
when the air that is more statically stable lies behind the
cold front (ahead of the warm front) (Figure 9.16).
The line of the warm air wedge aloft is associated
with a zone of layered cloud (similar to that found with
a warm front) and often of precipitation. Hence its
position is indicated separately on some weather maps
and it is referred to by Canadian meteorologists as a
trowal
(trough of warm air aloft). The passage of an
occluded front and trowal brings a change back to polar
airmass weather.
4 Frontal-wave families
Observations show that frontal waves over the oceans,
at least, do not generally occur as separate units but in
families
of three or four (see Figure 9.9; Plate 18). The
depressions that succeed the original one form as
