MT11B Weather Systems Analysis
Eleanor Highwood, December 2003
Lecture W5 Frontogenesis
W5 1 - The Norwegian model:
In the Norwegian cyclone model which we discussed last term, the “polar front” was seen
as a permanent feature of the atmosphere. Changeable weather comes as a result of
instabilities of the polar front, and the subsequent alternation of tropical and polar air
masses over a particular point on the Earth's surface.
Such a view is inadequate for a number of reasons:
The distribution of sunlight over the Earth's surface varies smoothly with latitude; we
would expect the heating which results from absorption of sunlight to produce a
smooth variation of temperature with latitude.
Synoptic experience shows that a “polar front” cannot always be detected in between
the depression systems. The polar front appears to be a broken, discontinuous
feature, strong in some locations but absent in others.
Even within a strongly developed weather system, it is quite rare to observe the full
set of features included in the Norwegian model. For example, there is generally a
well-developed warm front and an inactive cold front, or vice versa, but it is unusual
to observe both in the same system.
If we follow the development of a weather system for a period of several days, we find
that the fronts often cannot be followed for more than a few days. Then they have to
be dropped from the analysis, while new fronts appear in different locations.
Fronts can form (“frontogenesis”) and dissolve (“frontolysis”). The theory of how this
happens is an advanced topic which is beyond the scope of this course. But we can
indicate some basic processes which can initiate frontogenesis or frontolysis, based on the
concept of thermal advection.
MT11B Weather Systems Analysis
Eleanor Highwood, December 2003
W5 2 - Frontogenesis by wind shear:
A schematic illustration of the basic process is shown in the diagram. Suppose that the
wind blows across the temperature contours, and that the wind speed, but not its
direction, varies from place to place. The resulting wind shear tightens the temperature
gradients in the region of the wind reversal.
If the process continues indefinitely, then the temperature gradients will be become
increasingly tight. The region where the wind changes sign will be a favoured region for
frontogenesis.
W5 3 - Deformation wind fields:
The quantity:
F
u v

x y
is called the “deformation”. Note that it differs from the definition of divergence by the
sign of the second term. It measures the rate at which the shape of the air parcel is
changed by the wind field.
Consider an initially square parcel of air in a wind field with positive deformation. As the
air is advected by the wind, the parcel expands along the x-axis, which is called the “axis
of dilation”, but it contracts along the y-axis (the “axis of contraction”. After a time, the
parcel will become very long and thin.
If T contours are parallel to the axis of dilation, the thermal advection leads to
frontogenesis.
On the other hand, frontolysis will arise when the temperature contours run parallel to
the axis of contraction.
W5 4 - Theory of frontogenesis:
MT11B Weather Systems Analysis
Both shear and deformation will lead to a tightening of the temperature gradients. After
an infinite time, these gradients will become infinitely tight. But fronts form in a finite
time.
Modern theories of frontogenesis are based on the need to maintain geostrophic and
thermal wind balance as the temperature gradients are increased. Vertical circulations are
set up, with rising motion in the warm air and descent in the cold air. It can be shown
that deformation fields are associated with these circulations. The “secondary
deformation” acts to enhance the frontogenesis, and so a positive feedback occurs. As a
result, the temperature gradients near the ground become infinitely tight in a finite time.
Depressions and high pressure centres develop naturally in a situation where the
temperature varies smoothly with latitude by a process called “baroclinic instability”.
Associated with baroclinic instability are shear and deformation flow fields. These lead to
frontogenesis. Thus the formation of fronts is now seen as part of the life cycle of
developing depressions, not as a necessary precondition for the development of
depressions.
Eleanor Highwood, December 2003