Frontal cyclones
-otherwise known
as mid-latitude or wave cyclones. A cyclone is a cyclonically rotating system
which, in mid- and high-latitudes is normally associated with the boundary
between air masses of different temperature. Such boundaries are called fronts
and typically they are related into a wave-like pattern when seen on a surface
weather map. Below is a sketch of a frontal cyclone with some of its usual
features.
Air masses
An air mass is a
body of air having large horizontal extent (several 100’s to 1000’s km) of
nearly uniform properties (temperature, humidity).
Air masses are
labeled according to their source region in the following manner:
Arctic A
Polar P
(really sub-polar)
Tropical T
(really sub-tropical)
Equatorial E
These labels are
modified according to the surface type of the source region:
maritime m
continental
c
Resulting in the
following:
Arctic A
maritime polar mP
continental polar cP
maritime tropical mT
continental
tropical cT
Equatorial
E
The first four
are common over the N-American continent.
Fronts
A front is the
boundary between two air masses. In reality, we should think of a frontal zone
of relatively rapid transition between two air masses. Fronts are sometimes
sharp, particularly in the vicinity of a surface low, and at other times are
quite diffuse and difficult to identify on a weather map.
As a convention,
a front is marked at the northerly limit of the warmer air mass, so that the
zone of transition occurs north of the front, e.g.,
The front is
named according to the colder air mass. Thus, the boundary between mT and mP
air masses would be called the polar front, while that between mP and A would
be called the Arctic front.
Fronts are
classified according to their direction of motion. A warm front indicates that
warm air is replacing cold air.
A cold front
represents a situation in which cold air is advancing to replace warm air at
the surface:
If a frontal boundary
shows no significant motion in either direction, it is stationary and is marked
thus:
A frontal
boundary is continuous in the vertical although fronts are not marked on upper
air charts. Because cold air is denser than warm air, it sinks beneath the warm
air, and frontal boundaries are sloping boundaries. Generally, the slopes are
very gently, ranging from
Note that in the
diagram the steepness of the frontal slopes are greatly exaggerated.
The attached
figures present characteristic patterns of cloudiness and precipitation at warm
and cold fronts. Typically, warm fronts are associated with stratified clouds
and with steady light to moderate rain, while cold fronts often have cumuliform
clods and showery precipitation. Individual cases can, of course, differ
considerably from this normal pattern.
Development of a wave cyclone
A logical
question to ask is “why are fronts associated with mid-latitude cyclones?”.
There are basically two parts to the answer:
1)
A frontal boundary between warm and
cold air masses is a zone of energy conversion in which gravitational potential
energy is converted into the kinetic energy of motion.
Consider a container
with a vertical partition separating warm and cold air. As the partition is
removed the cold dense air sinks beneath the warm air which rides over the top.
The center of gravity of the system lovers (reducing PE) and motion is created
(KE).
2)
A cyclone is a region of low level
convergence which brings together warm air from the south and cold air from the
north, enhancing or at least maintaining the frontal boundary.
The following
diagrams show, first, the hypothetical case of a vertical boundary between air
masses inducing motion which is influenced by the Coriolis force to produce
cyclonic rotation. The second diagram illustrates three stages in the
development of a wave cyclone as would be seen on a surface weather map.
Generally, the
advancing cold front moves more quickly than the warm front and eventually
clips off and lifts up the warm air near the center of circulation. This
process is called occlusion. The warm sector at the surface is now removed from
the low center and a boundary is left, as shown in the diagram, between a fresh
outbreak of cold air from the northwest and retreating cold air to the east of
the low. When the warm air is moved sufficiently far from the strong
circulation, a source of energy is lost and the cyclone dissipates. An example
of an occluded system is shown in the attached satellite photograph of clouds
spiraling in towards a low center in the
Occluded fronts
can be of two types: the more usual cold type occlusion, or the less frequent
warm type occlusion. These are illustrated in the following vertical
cross-sections, west to east through an occlusion.
In both cases,
the warm air mass is lifted off the ground. On the left, a fresh outbreak of
cold air also lifts the modified cool air to the east. On the right, the air
coming from the NW is not as cold as the existing air mass to the east and
rides up over the top of it. This latter situation might occur when air from
the NW has a trajectory over the ocean, while the retreating cold air ahead of
the system was a wintertime continental and very cold air mass.
Examples of
observed cyclone tracks (March 1989) are also shown on an attached diagram.
They generally migrate eastward as dictated by the upper air flow patterns.
Tracks range in length from a few hundred km to several thousand km. Preferred
locations for their formation are (a) off the eastern coasts of continents as
relatively cool air masses are warmed from below by relatively warm waters, and
(b) east of N-S mountain ranges such as the Rockies.
Relationship of the wave cyclone to upper air trough
Through the quarter we have seen many examples of frontal
cyclones migrating ahead of an upper air trough. There is a good reason for
this relationship because the divergence in the upper troposphere at the
downwind (east) side of an upper air trough removes mass horizontally creating
a center of low pressure at the surface. This “upper air support” is an
essential component of any significant wave cyclone.
Between the
surface and the upper troposphere there is a continuous slope from the surface
low upwards and westward to the trough aloft. This is illustrated in the
following diagram.
Superimposing
surface and 500ml charts the relationship between trough and low might look
like the following.
In the final
stages of development of a frontal cyclone, the system occludes and the surface
low loses the support from upper air divergence. Typically, the trough aloft
closes off into a closed circulation pattern, while the surface low falls back
and comes into alignment with it. Thus, we are left with an almost vertically
aligned column of rotating cold air which gradually loses its energy. The
surface and 500 mb charts might then appear as below
