Transcript Cyclone and Frontal Structure

Cyclone and Frontal Structure
and Evolution
Professor Cliff Mass
Department of Atmospheric Sciences
University of Washington
For much of the 20th century the dominant
paradigm for cyclone/frontal evolution has been
the Norwegian Cyclone Model (Bergen School)
Bjernkes, 1919
Concept
of
Evolution
of
Cyclones
Bjerknes and
Solberg
1922
Stationary Polar Front
Wave Forming on Polar Front
Wave Amplifies
Occlusion as Cold Front Catches Up to
Warm Front
Occlusion Lengthens and System Weakens
Warm and Cold Occlusions
Norwegian Cyclone Model (NCM)
• It was an important and revolutionary advance at
the time.
• First to connect three dimensional trajectories
with clouds and precipitation.
• Still found in many textbooks today
• Over flat land away from water and terrain,
reality often approximates gross characteristics of
the NCM.
• However, there are some major problems with
the Norwegian Cyclone model that have been
revealed by modern observations and modeling.
Some Problems With The Norwegian
Cyclone Model
• Different structures and evolutions of fronts and
cyclones often observed over water and
over/downstream of mountain barriers.
• Does not properly consider the role of the middle
to upper troposphere.
• No upper levels fronts.
• Major deficiencies regarding the occlusion
process.
• Does not properly consider that cyclogenesis and
frontogenesis occur simultaneously.
Consider one problem area:
the occlusion process
Classic Idea: Occlusion Type Determined By
Temperature Contrast Behind Cold Front and in
Front of Warm Front (“the temperature rule”
But reality is very different
From Stoelinga et al 2002, BAMS
Literature Review
• Schultz and Mass (1993) examined all published cross
sections of occluded fronts. Found no relationship
between the relative temperatures on either side of
the occluded front and the resulting structure. Of 25
cross sections, only three were cold-type occlusions.
• Of these three, one was a schematic without any
actual data, one had a weak warm front, and one
could be reanalyzed as a warm-type occlusion
• Cold-type occlusions appear rare.
But what controls the slope?
• Virtually all fronts are first-order fronts (which
the horizontal temperature gradient changes
discontinuously with frontal passage) rather
than zero-order fronts (where temperature
varies discontinuously across the front)
• Historical note: in the original Norwegian
Cyclone Model they suggested all fronts were
zero-order fronts.
Basic Relationship
The relative value of the
vertical potential temperature
derivative will determine the
slope
• Occluded frontal surfaces generally mark a
maximum in potential temperature on a
horizontal surface, so the numerator on the
right side of (2) is always positive.
• Therefore, the sign of the slope of the
occluded front is determined only by the
denominator on the right-hand side of (2),
that is, only by the static stability contrast
across the front, and not by the contrast in
horizontal potential temperature gradient.
An Improved View: The Static
Stability Rule of Occluded Front Slope
• An occluded front slopes over the statically
more stable air, not the colder air.
– A cold occlusion results when the statically more
stable air is behind the cold front.
– When the statically more stable air lies ahead of
the warm front, a warm occlusion is formed.
An Example
Another Example
According to the Norwegian Cyclone Model
Cyclones Begin to Weaken When They Start to
Occlude
• In reality, observations often show that cyclones
continue to deepen for many hours after the
formation of the occluded front, reaching central
pressures many hPa deeper than at the time often
occluded-front formation.
• Example: 29 of the 91 northeast United States
cyclones for which surface analyses appear in
Volume 2 of Kocin and Uccellini (2004) deepen 8–24
mb during the 12–24 h after formation of the
occluded front
Is Frontal Catch-Up the Essential
Characteristic of Occluded Front
Development?
• Not all occluded fronts developed from the
cold fronts overtaking warming fronts.
• Far more fundamental is the distortion of
warm and cold air by vortex circulations.
Even in a nondivergent barotropic model where “isotherms” are
passively advected by the flow, occluded-like warm-air and coldair tongues can develop
Oceanic Cyclone Structure
Shapiro-Keyser Model of
Oceanic Cyclones
Major Elements of S-K Model
• Weak cold front
• Northern part of cold front is very weak
(“fractured”)
• Not much evidence of classis occlusion (well
defined tongue of warm air projected to low
center).
• “T-Bone” structure: cold front intersects the
warms front at approximately a right angle
• Strong back bent (or bent back) warm front.
• Warm air seclusion near the low center.
Simulation of the QE-II Storm
Neiman
and
Shapiro
1993
Air-Sea Interactions Warm the Cold Air,
Weakening the Cold Front
Cross Section Across Cold Front
Cross Section Across Warm Front
Warm
Seclusion
Stage
Cross Section Across Warm Front and
Associated Low-level Jet
Cross Section Across Warm-Air Seclusion:
Circulation Weakens With Height
Strongest Winds With Back-Bent
Warm Front
How Does Different Drag Between
Ocean and Land Change Cyclone and
Frontal Structures?
Adiabatic, Primitive Equation Model
The Impact of Mountains Barriers
on Cyclone Structure
• Major topographic barriers can have a
profound influence on cyclone and frontal
structure.
• Barriers destroy low level front structures,
weaken cyclone circulations, create new
structures (e.g., lee troughs and windward
ridges), and restricts the motions of cold and
warm air.
• (show recent example)
Consider the U.S. Impacts
• When flow is relatively zonal synoptic
structures are greatly changed over and
downstream of the Rockies.
• Takes roughly 1000 km for structures to
appear more “classical”
• Classic reference: Palmen and Newton (1969)
Steenburgh and Mass (Mon. Wea.
Rev., 1994)
• Detailed modeling study of the cyclone/frontal
development east of the Rockies.
Conceptual Model
Boundary Between Descended Air off the Rockies
and Moist Air off the Gulf Produces Dry Lines
Dry Lines
or
Dry Lines
• Associated with large horizontal gradients in
moisture, but not necessarily temperature.
• Results from the interaction of cyclones and
fronts with large-scale terrain.
• Found over the U.S. Midwest, northern India,
China, central West Africa and other locations.
• Acts as a focus for convection, and particularly
severe convection.
• Most prevalent during spring/early summer
in U.S.
Dry Line
• Surface boundary between warm, moist air
and hot, dry air.
Surface dry line
Well-mixed warm air
Inversion or cap
Typical Dryline
Temperatures in
degrees Celsius
©1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology in Midlatitudes, Volume II
Dry Line
Southern
Plains Dry
Line
©1993 Oxford
University Press -From: Bluestein,
Synoptic-Dynamic
Meteorology in
Midlatitudes,
Volume II
Temperatures in
degrees Celsius
Trajectories
• Fundamentally the dry line represents a
trajectory discontinuity between moist
southerly flow and flow descending from
higher elevations.
• Can only happen relatively close to the
upstream barrier (no more than 1000 km)
since otherwise air would swing southward
behind the low system and thus would be cool
and somewhat moist.
DRY LINE
L
Warm, Moist
L
NO
DRY
LINE—
Get Cold
Front
Indian Dryline
Dew Point Gradients Associated
with Indian Dry Line
Dry Line: Tends to Move Eastward
During the Day and Westward At
Night
• After sunrise, the sun will warm the surface
which will warm the air near the ground.
• This air will mix with the air above the
ground.
• Since the air above the moist layer is dry,
the lower level air will dry out.
• The dry line boundary will progress toward
the deeper moisture.
Dry Line
Top of moist layer
before mixing
Boundary after mixing
Hot, Dry Air—Usually Well Mixed
Warm, Moist Air
Initial Position
of the Dry Line
Position of the
Dry Line after
mixing
Dry Line
• After sunset, a nocturnal inversion forms and the
winds in the moist air respond to surface pressure
features.
• The dry line may progress back toward the west .
NCAR
Sounding
East of the
Dryline
South Winds
Moist
Oklahoma City, OK
12Z -- 26 June 1998
Aircraft Study of the Dry Line
Convection Tends to Focus On the
Dryline
Simulation of a Thunderstorm Initiation
Along Dryline in TX Panhandle
Storm
Note converging winds and rising
motion
Storm Initiation Along a Dry Line
Why is a dry line conducive for
strong convection?
• Low level confluence and convergence
produce upward motion.
• The cap allows the build-up of large values of
Convective Available Potential Energy (CAPE)
• East of the surface dry line, the existence of a
layer of dry air over moist air enhances
convective/potential instability.
Greatest Potential for Convective
Development Exists at the
Intersection between the Dry Line
and Approaching Cold Front
Cold Fronts Aloft And Forward
Tilting Frontal Zones
Conceptual Model By Peter Hobbs
and Collaborators:
Terrain Effects Are Central
1986
0000 UTC 25 January 1986
Cold Front Aloft Model Comments
• The lower portions of Pacific weather
systems are destroyed as they cross
the Rockies.
• The upper short-wave and its
associated thermal structure can
pass across the barrier.
CFA
• The low level leeside troughing cannot
move eastward as fast as the upper trough
and a natural forward tilt develops.
• With upward motion leading the upper
trough, precipitation naturally leads the
slow-moving lower trough. Often a
potentially unstable environment exists
ahead of the trough, leading to convective
precipitation.
CFA
• Another way to look at this. There is a natural tendency
for normal wind shear with height to shear systems,
changing the phasing between lower and upper levels.
• Naturally occurs even without mountains: decrease in
slope with time.
• The mutual support of upper and lower structures and
interacting vertical motions tends to delay this
decoupling
CFA
• Shearing off the lower portion of a system,
and establishing a new low-level trough on the
eastern side that is not dynamically coupled to
the system aloft facilitates the rapid
development of a forward tilting system. (e.g.,
no low level cold air advection that supports
structure aloft)
Cold Air Damming
Cold Air Damming
• Found around the world, where there is stable
flow interacting with terrain.
• Dependent on a semi-geostrophic balance.
– Geostrophic normal to the barrier
– Ageostrophic parallel to the barrier.
Coastal Frontogenesis: Closely
Associated with Cold Air Damming