S11 Evolution of Weather Systems
S11.1 Vertical motion and weather systems development
Vertical motion is crucial for the development of weather systems, as is the
ageostrophic wind. If the flow was purely horizontal and geostrophic we would have
no weather!
Before the advent of numerical weather prediction models, it was still necessary to
predict the intensification or decay of weather systems. Given that vertical motion
was (and remains) extremely difficult to measure, forecasters used the position of
jets and their knowledge of thermal advection associated with fronts etc to predict
regions where systems would be likely to intensify or decay. (Similar to the physical
arguments we’ve been through relating vertical motion to jet exits, jet entrances and
regions of thermal advection).
Ascent is associated with warm thermal advection and also occurs to the North of jet
exits and the south of jet entrances
Descent is associated with cold thermal advection and also occurs to the South of jet
exits and the North of jet entrances.
So, what has this to do with the intensification and decay of weather systems?
Cyclones form and evolve as they move from west to east. The best predictions of
their behaviour involve cyclone dynamics (see below) and practical experience over
many case studies.
S11.2 Cyclogenesis
Cyclogenesis is the birth and growth of cyclones. Intensification can be defined by:
 Sea-level pressure decrease
 Upward motion increase
 Vorticity increase
These characteristics are not independent; upward motion can reduce surface
pressure  draws in air that rotates due to the Coriolis force.
We have looked at some of these quantities in the Case study we have been doing
during the practical sessions. In the session following this we will complete the
picture by thinking about vertical motion some more.
We can consider each one in more detail.
Sea-level pressure decrease:
Changes in sea-level pressure with time are caused by changes in the total air mass
above the surface, which in turn can result from:
 upper level divergence (removing mass from the column)
 boundary layer pumping (turbulent drag causes flow across isobars)
 advection (wind blows in a column of air having less mass than the original
column)
 diabatic heating (latent heat release causes the column to expand and the
developing horizontal temperature gradients push air out of the column)
Upward motion increase:
If ascent occurs in the troposphere (usually only below the level of the jet) then
there must be divergence at upper levels (through mass continuity considerations).
Regions of divergence aloft remove mass from the column of air, thus lowering sealevel pressure and contributing to cyclogenesis.
Vorticity increase:
Advection changes vorticity by blowing in air with different vorticity from some other
location. Similar is the beta effect where winds from the north blow in vorticity
associated with the earth’s rotation. Stretching of vortex tubes due to ascent will
cause the spin up of positive (cyclonic vorticity). The balance of these and other
effects will either spin-up or spin-down the vorticity.
Cyclogensis can also be triggered by mountains.
S11.3 Self-development of cyclones
Sometimes the cyclone can enhance its own development (as opposed to
cyclogenesis being a response to some external driving)
Condensation:
Divergence of the upper level winds causes a broad region of upward motion in the
region of a trough. Rising air forms clouds and precipitation (an upper level
disturbance when such weather is not yet associated with a strong surface low).
Latent heating of the air enhances buoyancy and increases upward motion. The
resulting stretching leads to spin-up of vorticity and takes some air away from the
surface leaving lower pressure. Diabatic heating also increases the average
temperature of the column which strengthens a ridge west of the initial ridge axis
and leads to a shortening of the wavelength between trough and ridge. This leads to
tighter turning of the winds and therefore more vorticity etc. As the low strengthens
there can be more precipitation and latent heating and so the feedback continues,
giving strong development of the cyclone.
Temperature advection:
When warm air exists slightly to the west of a ridge axis it advects into the region
just to the west of the ridge causing ridge heights to increase. Cold air advects into
the trough causing heights to fall there. The net result is an intensification of the
wave amplitude which can cause stronger surface lows due to enhanced upper-level
divergence.
S11.4 Rapid cyclogenesis
The following conditions have been found to favour rapid cyclogenesis:
Strong horizontal temperature gradients
Weak static stability
Mid or high latitude location (increasing contribution to vorticity from earth’s
rotation)
Large moisture input
Large-amplitude wave in the jet stream (a trough to the west and ridge to the east
of the surface low enhance horizontal divergence aloft which strengthens updrafts)
Terrain elevation decrease towards the east (cyclogenesis in the lee of mountains)
S11.4 Cyclolysis
This is literally the death of a cyclone. Most cyclone lifetimes are about a week at
mid-latitudes but can range between less than a day and more than two weeks.