Download CycloneStructure_3

ATS/ESS 452: Synoptic Meteorology
- Cyclone Structure
- QG Theory
NAM Forecast Sounding
for this Saturday at 03 UTC
(i.e., tonight near 9pm)
a) In the sfc to ~700-mb
layer, how are the
winds turning with
height?
Counter-clockwise
b) Is this a veering or
backing wind profile?
Backing
c) What type of
temperature advection
is this indicative of?
Cold Air Advection
NAM Forecast Sounding
for this Saturday at 03 UTC
(i.e., tonight near 9pm)
d) Notice the
temperature and dew
point profile from
~850-mb to just above
the sfc. Describe
what’s going on here
and does it make sense
based on the
temperature advection
that’s occurring within
that layer.
CAA  sinking air 
compressional
warming in that layer
 air dries and temp
warms / parallels a
dry adiabat
2. (4 pts) In an absolute vorticity conserving flow, if an air parcel is displaced to the north, what
happens to the relative vorticity (ζ) and in what direction does the parcel begin to rotate?
Assume initial ζ = 0. An illustration may be used to describe this process.
If a parcel is displaced to the north, then f increases, so through the conservation of
absolute vorticity, the relation vorticity must decrease.
ζ < 0; the parcel begins to rotate clocks, or anti-cyclonically
3. (2 pts) In the Northern Hemisphere, shortwave troughs and ridges are dominated by
( planetary / relative ) vorticity advection, which causes them to ( prograde / retrograde)
Convergence;
sinking air
Divergence;
rising air
Divergence;
rising air
Convergence;
sinking air
Healthy Westward Tilt of Pressure Systems with
Height
H
L
Z
H
W
L
E
Healthy Westward Tilt of Pressure Systems with
Height
•
Note that the thermal ridges and troughs depicted in Fig. 6.6
tilt eastward with height and are out of phase with the
pressure ridges and troughs. This means that:
 Baroclinic ridges (highs) are cold at the surface and warm
aloft
 Baroclinic troughs (lows) are warm at the surface and cold
aloft
Vertically Stacked Systems
• As cyclones mature and become occluded, they become vertically stacked with their upperlevel system
H
L
H
L
Z
W
E
Vertically Stacked Systems
•
As a result, the pressure trough and temperature trough lose
their vertical tilt. They also become aligned. (This is common as
cyclones move poleward toward 60N and 60S latitudes)
•
The same happens with the pressure ridge and temperature ridge
as an aging polar high moves equatorward and merges with the
subtropical high near 30N and 30S
i.e. systems began to lose their baroclinic nature
•
As a consequence, the thermal advection with these systems
becomes quite weak (or non-existent). Thus, energy conversion
from the jet stream to these surface systems becomes negligible
 No thermal contrasts, then the jet stream retreats
Vertically Stacked Systems
•
These vertically-stacked systems become more barotropic in
nature (equivalent barotropic) with the wind direction being
uniform with height (no wind shear)
Westward tilt of pressure systems
with height
Vertically stacked
Unhealthy Eastward Tilt
• If the pressure trough displays an eastward tilt with height, there is an upscale energy cascade whereby the eddies must give their energy back to the
mean flow
H
L
Z
H
W
L
E
• Thus, individual cyclones and anticyclones will
weaken or dissipate as they lose energy to the
larger jet-stream westerlies
• The surface cyclone or anticyclone must give
up its kinetic and potential energy to the jet
stream
• The surface system rapidly dissipates
• This is characterized on weather maps by the
upper trough (ridge) tending to “outrun” the
surface low (high)
• This is common with landfalling cyclones on
the West Coast
• A simplistic diagram of baroclinic troughs and ridges
is displayed in your handout
– The surface trough (i.e. convergence) is overlain by the
upper-tropospheric ridge (i.e. divergence)
– Thus, mass continuity requires that upward vertical
motion occur in the vicinity of baroclinic surface lows
– The surface ridge (i.e. divergence) is overlain by the
upper-tropospheric trough (i.e. convergence)
– Thus, mass continuity requires that downward vertical
motion occur in the vicinity of baroclinic surface highs
• BUT, in actuality, this vertical view of baroclinic
systems is too simplistic. In reality:
– The greatest upper-tropospheric convergence and
divergence occurs in between upper-tropospheric
troughs and ridges (via the gradient wind relationship):
• Super-geostrophic flow in the ridges
• Sub-geostrophic flow in the troughs
And other factors not yet discussed
• What is the gradient wind?
–
–
–
–
Similar to geostrophic wind, but allows for curved flow
A balance between the pressure gradient, Coriolis AND centrifugal forces
Air still blows parallel to isobars (isoheights) with low values to the left
Example
• For low pressure cases, the PGF must be balanced by the Centrifugal +
Coriolis forces
– Because the PGF doesn’t change, this means that the Coriolis force must
decrease to achieve the desired balance
– This in turn decreases the overall wind speed
– In other words, the gradient wind blows parallel to the isobars at a less than
geostrophic wind speed (subgeostrophic)
• For high pressure cases, the Coriolis force must be balanced by the
Centrifugal + PG forces
– Again, the PGF doesn’t change, so now the Coriolis force must increase to
achieve the desired balance
– This in turn increases the overall wind speed
– So now, the gradient wind blows parallel to the isobars at a greater than
geostrophic wind speed (supergeostrophic)
Upper Level Gradient Wind Relationship
970
dam
(Supergeostrophic Winds)
(Supergeostrophic Winds)
Ridge
Ridge
990
dam
Speed
Convergence
Speed
Divergence
Trough
(Subgeostrophic Winds)
300-mb Isobaric Surface
970
dam
990
dam
Upper Level Gradient Wind Relationship
Aloft: Speed Convergence
970
dam
990
dam
Aloft: Speed Divergence
H
L
SFC
SFC
970
dam
990
dam
Region of Lifting
(dashed line)
Region of Subsidence
(dashed line)
300-mb Isobaric Surface
• Also, much of the upward and downward
vertical motion in a baroclinic system is slantwise
• So just by using the basic wave pattern
construct, we can determine where “stormy”
weather might be
– Example
Vertical Motions in Mid-Latitude Synoptic
Systems
• Evaluation of synoptic scale vertical motion fields are of
primary importance in analyzing and forecasting
synoptic scale weather
• Vertical motions in the atmosphere are extremely
difficult to observe and forecast
• Large-scale (i.e. synoptic scale) horizontal winds in the
troposphere usually have magnitudes between 5 and
50 m/s (10 – 100 kt)
• Synoptic scale vertical winds in the troposphere are
much smaller, with magnitudes generally between 0.05
and 5 cm/s (0.1 and 3 knot)
• The continuity equation for mass links divergence
(convergence) of the horizontal wind to vertical motion
Vertical Motions in Mid-Latitude Synoptic
Systems
• Small changes in the speed or direction of the
horizontal wind may have HUGE effects on the lifting or
sinking (you see this at the jet level).
– The actual horizontal wind direction and speed cannot be
measured with enough accuracy to allow direct calculations
of the resulting vertical wind. There are many disruptions to
the flow, such as storms, mountains and buildings, which
produce irregular wind flow and generate turbulence/eddies
– Thus, actual winds measured by rawinsonde balloons,
aircraft, surface weather stations, and satellite cloud motions
cannot be used for accurate calculations of divergence and
vertical motion in the classical sense
Vertical Motions in Mid-Latitude Synoptic
Systems
• Therefore, alternative “indirect” vertical motion
methods must be developed which may be partially
or totally independent of the actual wind.
• Two of the most popular indirect methods are the
Omega Equation and Q-Vector methods, both of
which are derived from Quasi-Geostrophic (Q-G)
wind relationships
• Another method, depicts wind flow, moisture, and
isobar patterns on an isentropic surface. Isentropic
vertical motions will be discussed at a later date in
Synoptic
Quasi-Geostrophic Theory
• On the synoptic scale (and outside of the tropics),
wind flow can be generally assumed to be quasigeostrophic.
– That is, wind flow is approximately in geostrophic
balance with only small amounts of
divergence/convergence occurring
• Q-G theory is designed for baroclinic systems within
the mid-latitude westerlies
Main Assumptions of Q-G Theory
• The atmosphere is in hydrostatic balance (i.e. the
atmosphere is thermodynamically stable)
– Hydrostatic balance is destroyed near thunderstorms where
the atmosphere is unstable
• The wind is quasi-geostrophic (i.e. flows essentially
parallel the height contours)
– Advantages to this assumption are that height contour
patterns may replace the actual wind flow and that no actual
wind observations are required
• No small-scale weather features required
• Only a single analysis of vorticity, isobaric heights, and
temperatures are required
Main Assumptions of Q-G Theory
• Divergence/Convergence is small
– This is representative of inactive weather regions and
small vertical motions
• Vorticity is represented by geostrophic vorticity
– Geostrophic vorticity can be derived from the height
pattern on isobaric surfaces
– Vorticity advection can be inferred from the intersection
of vorticity and height contours
• Thermal advection can be represented by the
intersection of isobars (or isoheights) with
isotherms
Main Limitations of Q-G Theory
• Provides only a first inference of where and why vertical motion
may be occurring in the atmosphere
• Should not be applied in the tropics where synoptic-scale flow is
more ageostrophic in nature
• Should be used with great caution in active weather regions where
divergence is large and non-hydrostatic conditions may exist
• Vertical motion produced by topography (i.e. friction), latent
heating (i.e. convection), evaporative cooling, radiational heating
and cooling, and small-scale processes will not be represented by
Q-G theory and the omega equation.
• Strictly a diagnostic equation (here and now – not future)
– Q-G theory can not be used to “predict” future vertical motions (no
time derivative)
– On the other hand, Q-G theory may be used to diagnose vertical motion
in model forecast patterns
• Some error is introduced in Q-G analyses when smoothed contour
patterns are used (all model analyses are smoothed)
The Q-G Approximations
Fundamental assumption for Q-G equations: Rossby number is small
(on the order of 0.1).
**Define the Rossby number**