Midlattitude cyclones Flashcards
The single most common weather element in the middle-latitudes is the
frontal cyclone (also called the midlatitude cyclone).
Extratropical cyclones are
large low-pressure systems named by their cyclonic rotation and latitude of formation.
Extratropical cyclones are synoptic-scale phenomena that has a life span
on the order of one week.
Cyclogenesis
the process by which a surface cyclone initially develops and subsequently intensifies.
Intensification is often measured in terms of
negative sea level pressure tendency
or
positive low-level vorticity tendency.
Including the effects of …………………and …………………………., the four QG forcing functions that are associated with
friction
diabatic heating
Including the effects of friction and diabatic heating, the four QG forcing functions that are associated with …………………. and ……………………..at the surface
rising motion and pressure falls
Including the effects of friction and diabatic heating, the four QG forcing functions that are associated with rising motion and pressure falls at the surface are
- Increasing positive vorticity advection with height
- Local maximum in temperature advection
- Local maximum in diabatic heating
- Curl of the frictional force becoming more cyclonic with height
Type B cyclones
Surface cyclones that form mainly in response to vorticity advection
Type A cyclones
urface cyclones that form mainly in response to temperature advection along frontal zones
By QG theory, geostrophic temperature advection increasing ………………… is associated with …………………
upward is associated with height falls
for a developing cyclone, there is …………………………… and …………………………….
strong low-level warm advection below and downstream of the trough axis.
In a developing midlatitude cyclone, height rises usually occur to the
east of the sea-level pressure minimum (in the vicinity of the cyclone’s warm front).
Height falls usually occur to
the west of the sea-level pressure minimum (in the vicinity of the cold front)
Diabatic heating alone may be responsible for the……………………………….. if the ….
formation of a surface cyclone, if the region of diabatic heating represents a local maximum
What may form a low or trough?
Intense localized diabatic heating owing to latent heat release from cumulus convection
what may form a high or ridge?
region of diabatic cooling owing to evaporation of rain in an unsaturated layer below
Movement of surface low pressure
- toward a region of rising motion
- towards regions in which vorticity advection is becoming more cyclonic with height.
- regions of local maxima in warm advection.
- towards regions of local maxima in diabatic warming.
The two ……………………..that are important for the formation of upper-level troughs and ridges are
QG forcing terms
The two QG forcing terms that are important for the formation of upper-level troughs and ridges are
- Geostrophic vorticity advection
- differential temperature advection
geostrophic vorticity advection will
not amplify the upper level systems
………………………………… and ……………………………can affect the height field so that upper- level system form or intensify.
Differential temperature advection and diabatic heating
low on surface causes
ridge
When the strongest winds are ………………from the trough, the trough tends to ”dig” ……………………..
upstream
equatorward
When the strongest winds are …………………….from the trough, the trough tends to ”lift out”……………………..
downstream
poleward
The lifecycle of a midlatitude cyclone consists of four basic stages
- Perturbation stage
- Cyclogenesis stage
- Mature stage
- Decay stage
Nearly all cyclogenesis events proceed from a
precursor upper-level disturbance in the flow
Nearly all cyclogenesis events proceed from a precursor upper-level disturbance in the flow. This disturbance manifests itself as a
relative vorticity maxima
The disturbance will propagate in the direction of the ………………… by ……………..
flow by vorticity advection.
Cyclogenesis Stage:
Since the disturbance is often initially largest at ………………………….levels
middle and upper tropospheric levels,
Since the disturbance is often initially largest at middle and upper tropospheric levels, there will be
upward-increasing PVA downstream of the disturbance
Since the disturbance is often initially largest at middle and upper tropospheric levels, there will be upward-increasing PVA (NVA) downstream (upstream) of the disturbance
By QG theory, this circumstance is associated with
upward vertical motion downstream of the trough axis.
Under the influence of the ………………………..associated with the developing lower tropospheric disturbance
cyclonic circulation
Under the influence of the cyclonic circulation associated with the…………………….
developing lower tropospheric disturbance
Under the influence of the cyclonic circulation associated with the developing lower tropospheric disturbance, ………………………….will occur ……………………………..of the…………………………..
low-level warm advection
downstream
upper-level trough axis.
QG theory also predicts that such a circumstance will serve to raise the ……………………………….in the middle troposphere to the………………………….. of the surface low
geopotential heights
east
The distribution of upper tropospheric convergence and divergence associated with the………………………………….
vertical motion couplet
The distribution of upper tropospheric convergence and divergence associated with the vertical motion couplet will tend to
increase the upper tropospheric vorticity in the vicinity of the trough axis while decreasing it in the vicinity of the downstream ridge.
A more intense upper-level vorticity maximum leads to …………………………. by …………………………………. and ……………………………………. which……………………………..
greater PVA by the thermal wind and attendant upward vertical motions which further intensify the surface cyclone downstream of the upper feature.
As the upper disturbance continues to develop and progress eastward, it begins to
outrun its surface reflection.
As the upper disturbance continues to develop and progress eastward, it begins to outrun its surface reflection.
• As a result,
the convergence at the surface (maximized at the location of the sea-level pressure minimum) gradually becomes disconnected from its divergence valve aloft and the surface cyclone can no longer intensify.
During the mature stage of the cyclone, the axis of…………………….. is tilted to the …………………………..
minimum geopotential height is tilted to the west.
As the cyclone matures, the vertical tilt of the geopotential minimum axis gradually becomes
more vertical by the time of occlusion.
As the cyclone matures, the vertical tilt of the geopotential minimum axis gradually becomes more vertical by the time of occlusion.
As a consequence of the eastward displacement of the upper-level divergence at this stage of the life cycle, there is no mechanism available to
evacuate the accumulating mass near the center of the surface cyclone and the surface pressure rises as a consequence.
By QG theory, this rise in surface pressure is associated with a
decrease in the near surface geostrophic vorticity
By QG theory, this rise in surface pressure is associated with a decrease in the near surface geostrophic vorticity and therefore qualifies as a
cyclolysis event.
