is the development or strengthening
of cyclonic circulation in the atmosphere (a low pressure area).
Cyclogenesis is an umbrella term for several different processes,
all of which result in the development of some sort of cyclone
. It can occur at various scales, from the
microscale to the synoptic scale. Extratropical cyclones form as
waves along weather fronts
occluding later in their life cycle as cold core cyclones. Tropical
cyclones form due to latent heat driven by significant thunderstorm
activity, and are warm core. Mesocyclones form as warm core
cyclones over land, and can lead to tornado formation. Waterspouts
can also form from mesocyclones, but more often develop from
environments of high instability and low vertical wind shear
. Cyclogenesis is the opposite of
cyclolysis, and has an anticyclonic (high pressure system)
equivalent which deals with the formation of high pressure areas
There are four main scales, or sizes of systems, dealt with in
meteorology: the planetary scale, the synoptic scale, the
mesoscale, and the microscale. The planetary scale deal with
systems with global size, such as the Madden-Julian Oscillation
Synoptic scale systems cover a portion of a continent, such as
dimensions of 1,000-2,500 km (620-1,550 mi) across. The
mesoscale is the next smaller scale, and often is divided into two
ranges: meso-alpha phenomena range from 200-2,000 km
(125-1,243 mi) across (the realm of the tropical cyclone
), while meso-beta
phenomena range from 20–200 km (12-125 mi) across (the
scale of the mesocyclone
microscale is the smallest of the meteorological scales, with a
size under two kilometers (1.2 mi) (the scale of tornadoes
). It should be noted that these
horizontal dimensions are not rigid divisions but instead reflect
typical sizes of phenomena having certain dynamical
characteristics. For example, a system does not necessarily
transition from meso-alpha to synoptic scale when its horizontal
extent grows from 2,000 to 2,001 km (1,243 mi).
Norwegian Cyclone Model
An upper level jet streak.
DIV areas are regions of divergence aloft, which will lead to
surface convergence and aid cyclogenesis
The Norwegian Cyclone Model is an idealized formation model of
cold-core cyclonic storms developed by Norwegian meteorologists
during the First World War
. The main
concept behind this model, relating to cyclogenesis, is that
cyclones progress through a predictable evolution as they move up a
frontal boundary, with the most mature cyclone near the northeast
end of the front and the least mature near the tail end of the
Precursors for development
A preexisting frontal boundary, as defined in surface weather analysis
required for the development of a mid-latitude cyclone. The
cyclonic flow begins around a disturbed section of the stationary
front due to an upper level disturbance, such as a short wave
or an upper-level trough,
near a favorable quadrant of the upper level jet.
Modes of development
The surface low could have a variety of causes for forming.
Topography can force a surface low when dense low-level high
pressure system ridges in east of a north-south mountain barrier.
can spawn surface lows which are initially warm core.
The disturbance can grow into a wave-like formation along the
and the low will be
positioned at the crest. Around the low, flow will become cyclonic,
by definition. This rotational flow will push polar air equatorward
west of the low via its trailing cold front, and warmer air will
push poleward low via the warm front. Usually the cold front will
move at a quicker pace than the warm front and “catch up” with it
due to the slow erosion of higher density airmass located out ahead
of the cyclone and the higher density airmass sweeping in behind
the cyclone, usually resulting in a narrowing warm sector. At this
point an “occluded front” forms where the warm air mass is pushed
upwards into a trough of warm air aloft, which is also known as a
arm air al
oft). All developing
low pressure areas share one important aspect, that of upward
vertical motion within the troposphere. Such upward motions
decrease the mass of local atmospheric columns of air, which lower
Maturity is after the time of occlusion when the storm has
completed strengthening and the cyclonic flow is at its most
intense. Thereafter, the strength of the storm diminishes as the
cyclone couples with the upper level trough or upper level low,
becoming increasingly cold core. The spin-down of cyclones, also
known as cyclolysis, can be understood from an energetics
perspective. As occlusion occurs and the warm air mass is pushed
upwards over a cold air airmass, the atmosphere becomes
increasingly stable and the centre of gravity of the system lowers.
As the occlusion process extends further down the warm front and
away from the central low, more and more of the available potential
energy of the system is exhausted. This potential energy sink
creates a kinetic energy source which injects a final burst of
energy into the storm's motions. After this process occurs, the
growth period of the cyclone, or cyclogenesis, ends, and the low
begins to spin down (fill) as more air is converging into the
bottom of the cyclone than is being removed out the top since
upper-level divergence has decreased.
Occasionally, cyclogenesis will re-occur with occluded cyclones.
When this happens a new low center will form on the triple-point
(the point where the cold front, warm front, and occluded front
meet). During triple-point cyclogenesis, the occluded parent low
will fill as the secondary low deepens into the main
Tropical cyclones exist within a mesoscale alpha domain. As opposed
to mid-latitude cyclogenesis, tropical cyclogenesis is driven by
strong convection organised into a central core with no baroclinic
zones, or fronts, extending through
their center. Although the formation of tropical cyclones
is the topic of extensive
ongoing research and is still not fully understood, there are six
main requirements for tropical cyclogenesis: sea surface temperatures
warm enough, atmospheric instability, high humidity
in lower to middle levels of the troposphere
, enough Coriolis force
to develop a low pressure
center, a pre-existing low level focus or disturbance, and low
vertical wind shear
. These warm core
cyclones tend to form over the oceans between 10 and 30 degrees of
The updraft then starts rotating.
Mesocyclones range in size from mesoscale beta to microscale. The
term mesocyclone is usually reserved for mid-level rotations within
severe thunderstorms, and are warm core cyclones driven by latent
heat of its associated thunderstorm activity.
Tornadoes form in the warm sector of extratropical cyclones
where a strong
upper level jet stream exists. Mesocyclones are believed to form
when strong changes of wind speed and/or direction with height
") sets parts of the lower
part of the atmosphere spinning in invisible tube-like rolls. The
convective updraft of a thunderstorm is then thought to draw up
this spinning air, tilting the rolls' orientation upward (from
parallel to the ground to perpendicular) and causing the entire
updraft to rotate as a vertical column.
As the updraft rotates, it may form what is known as a wall cloud.
The wall cloud is a spinning layer of clouds descending from the
mesocyclone. The wall cloud tends to form closer to the center of
the mesocyclone. It should be noted the wall clouds do not
necessarily need a mesocyclone to form and do not always rotate. As
the wall cloud descends, a funnel-shaped cloud may form at its
center. This is the first stage of tornado formation. The presence
of a mesocyclone is believed to be a key factor in the formation of
the strong tornadoes associated with severe thunderstorms.
Tornadoes exist on the microscale or low end of the mesoscale beta
domain. The cycle begins when a strong thunderstorm develops a
rotating mesocyclone a few miles up in the atmosphere, becoming a
supercell. As rainfall in the storm increases, it drags with it an
area of quickly descending air known as the rear flank downdraft
downdraft accelerates as it approaches the ground, and drags the
rotating mesocyclone towards the ground with it.
As the mesocyclone approaches the ground, a visible condensation
funnel appears to descend from the base of the storm, often from a
rotating wall cloud. As the funnel descends, the RFD also reaches
the ground, creating a gust front that can cause damage a good
distance from the tornado. Usually, the funnel cloud begins causing
damage on the ground (becoming a tornado) within minutes of the RFD
reaching the ground.
Waterspouts exist on the microscale. While some waterspouts are
strong (tornadic) like their land-based counterparts, most are much
weaker and caused by different atmospheric dynamics. They normally
develop in moisture-laden environments with little vertical
along lines of convergence,
such as land breezes
, lines of
frictional convergence from nearly landmasses, or surface troughs.
Their parent cloud can be as innocuous as a moderate cumulus, or as
significant as a thunderstorm
Waterspouts normally develop as their parent clouds are in the
process of development, and it is theorized that they spin up as
they move up the surface boundary from the horizontal wind shear
near the surface, and then stretch
upwards to the cloud once the low level shear vortex aligns with a
developing cumulus or thunderstorm. Weak tornadoes, known as
landspouts, across eastern Colorado have been witnessed to develop
in a similar manner. An outbreak occurred in the Great Lakes in late September and early October 2003 along a
lake effect band. September is the peak month of landspout and
waterspout occurrence around Florida and for
waterspout occurrence around the Great Lakes.
Cyclogenesis is the opposite of cyclolysis, which concerns the
weakening of surface cyclones. The term has an anticyclonic (high
pressure system) equivalent—Anticyclogenesis
, which deals with the
formation of surface high pressure systems.
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Wendell A. Nuss. A Rapid Cyclogenesis Event during GALE IOP 9.
Retrieved on 2008-06-28.
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- Raymond D. Menard1, and J.M. Fritsch A Mesoscale Convective Complex-Generated Inertially
Stable Warm Core Vortex
- The Physics Factbook Density of Air
- St. Louis University What is a trowal?
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Online Tornado FAQ. Retrieved on 2006-10-25.
- Barry K. Choy and Scott M. Spratt. A WSR-88D
Approach to Waterspout Forecasting. Retrieved on
- Barry K. Choy and Scott M. Spratt. Using
the WSR-88D to Predict East Central Florida Waterspouts.
Retrieved on 2006-10-25.