AOS 101
Cyclone Structure
April 22/24
April 29/May 1
Cyclone
• Symbols:
• Point in
direction of
front
movement
COLD
WARM
OCCLUDED
STATIONARY
Warm Front
WARM
COOL
Associated Weather (WF)
• Gradual Slope
• Stratiform rain
– long lasting light rain
– occurs on cool side of front
• Temperature increases prior to frontal
passage
• Wind becomes southerly after passage
Cold Front
WARM
COOL
Associated Weather (CF)
• Much Steeper Slope
• More intense (convective) rain
– Thunderstorms for a shorter period
– occurs on warm side of front
• Temperature decreases after frontal
passage
• Wind becomes northerly after passage
COOL AIR
L
LIGHTER RAIN
COLD
AIR
WARM
AIR
HEAVIER
RAIN
Finding a Front
• Temperature (dewpoint) Gradient
• Change in wind direction
– Converging winds at the front
• “Kink” or “trough” in isobars (lower pressure)
• Banded precipitation
Horizontal Cyclone Structure
Cyclone Life Cycle
•
•
•
•
•
•
Stationary Front
Wave
Cyclogenesis
Maximum Intensity
Occluding
Dissipation
Stationary Front
• North-to-south
temperature gradient
• Winds parallel to the
front
COLD
WARM
Wave
• Winds gain a
northerly/southerly
component
• Wave forms on the
front
• Usually caused by an
upper-level trough
(next week)
COLD
WARM
Cyclogenesis
• Closed cyclonic
circulation froms
• Surface pressure
minimum is observed
• Cold fronts and warm
fronts become
distinct
COLD
L
WARM
Maximum Intensity
• Warm air overruns
cold air at warm front
• Cold air quickly moves
around to the south
• Cold front moves
faster, “catches”
warm front
• Storm deepens to
lowest pressure
L
COLD
WARM
Comma Head
Occlusion
• Cold front catches
warm front leaving
warm air aloft
• Cold air encircles
cyclone center
• Another cyclone may
form at triple point
L
L
COLD
WARM
Occluded Front
WARM
COLD
COOL
Dissipating
• Cold air completely
encircles low
• All warm air is pushed
aloft
• Cyclone can no longer
feed off of
temperature
differences
Vertical Cyclone Structure
Upper-level terminology
• TROUGH: area of lower heights
• RIDGE: area of higher heights
L
H
Vorticity Advection
• VORTICITY: how much the flow curves
• Trough = positive; Ridge = negative
L
POSITIVE
VORTICITY
NEGATIVE
VORTICITY
H
• Downwind of trough axis = Positive V.A.
• Downwind of ridge axis = Negative V.A.
L
NEGATIVE
VORTICITY
POSITIVE VA
H
NEGATIVE VA
Mechanisms for Deepening Cyclones
• Warm air advection (WAA) near the ground
– Look at surface temperature and winds
• Positive vorticity advection (PVA) at upper
levels
– Look at upper-level (e.g. 500 hPa) winds and
trough/ridge axes.
• Both promote upward vertical motion (ascent).
Vertical Structure: Beginning
PVA
POSITIVE
VORTICITY
STATIONARY FRONT
Vertical Structure: Cyclogenesis
WAA
L
POSITIVE
VORTICITY
Westward tilt
with height
Vertical Structure: Occlusion
L
PVA
POSITIVE
VORTICITY
WAA
EXAMPLE 0000 UTC 10 November 1998
300 mb flow
which resulted in
a massive
cyclone
development
over the
midwest.
PVA downwind of
trough
PVA
TROUGH AXIS
300 hPa Height, Wind Speed
EXAMPLE 0000 UTC 10 November 1998
Weak cyclone
downwind of
trough
Fronts are
relatively weak
L
TROUGH AXIS
Surface Pressure, 500-1000 Thickness
EXAMPLE 1200 UTC 10 November 1998
12 hours later…
Trough moves
west, strong jet
rotates around
trough
PVA
TROUGH AXIS
EXAMPLE 1200 UTC 10 November 1998
Cyclone has
deepened to 977
hPa, has moved
closer to trough
axis.
Intense WAA
north of cyclone
L
TROUGH AXIS
EXAMPLE 0000 UTC 11 November 1998
12 hours later…
Trough is now
cutoff
PVA
TROUGH AXIS
EXAMPLE 0000 UTC 11 November 1998
Surface cyclone
practically under
trough axis
i.e. The cyclone
is “vertically
stacked.”
L
TROUGH AXIS
No PVA or WAA =
no more
intensification
Main Points
• Cyclones will tend to form downwind of an
upper-level trough in an area of PVA.
• As the cyclone develops, WAA will occur on the
warm front further deepening the cyclone
• Whole system will tilt westward with height
• While developing, cyclone will move closer to
trough axis
• While occluding, cyclone will move nearly
under trough axis in an area void of PVA/WAA.