Cyclone and Frontal Structure and Evolution
Professor Cliff Mass
Department of Atmospheric Sciences
University of Washington

For much of the 20 th century the dominant paradigm for cyclone/frontal evolution has been the Norwegian Cyclone Model (Bergen School)
Bjernkes, 1919

Concept of
Evolutio n 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

Intensification after Occluded
Frontogeneis
• This makes sense since cyclogenesis depends on three-dimensional dynamics and dynamics.
• Such mechanisms for cyclogenesis can be undertood from quasigeostrophic, Petterssen–
Sutcliffe development theory, baroclinic instability ideas, or potential-vorticity.

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

Occlusion
• This gradient in tangential wind speed takes the initially straight isotherms and differentially rotates them.
• The differential rotation of the isotherms increases the gradient (i.e., frontogenesis)
• The lengthening and spiraling of the isotherms brings the cold- and warm-air tongues closer

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

The Norwegian Cyclone Model Was
Developed over the Eastern Atlantic and Europe, Might Development be
Different In Other Midlatitude
Locations Where the Large Scale Flow is Different?

Confluent
Diffluent

Confluent Diffluent

There is considerable literature demonstrating different cyclonefrontal evolutions in differing synoptic environments.

Confluent versus diffluent synoptic flow
• The Norwegian Cyclone model was developed in a region of generally diffluent flow (eastern
Atlantic and Europe).
• How does confluent and diffluent flow influence evolution?

Add a vortex to various synoptic flows and simulate the thermal evolution

Just Vortex

Confluence-Like Western Side of Oceans

Looks Like Shapiro-Keyser Model of
Oceanic Cyclones
• S-K developed over western oceans during the
Erica field experiment.
• Fractured cold front, strong bent-back warm/occluded front.

Summary

Diffluent Flow: Like Eastern Side of
Oceans

Confluent Flow
• Strong cold front and weaker warm front
• Resembles Norwegian Cyclone Model (NCM)
• NCM devised over a region of confluent flow.

Summary

LC1 and LC2 Cyclone Evolutions: The
Influence of Changing the Horizontal
Shear Across the Midlatitude Jet

Primitive Equation Model Run with
Two Shear Profiles
LC1
LC2
Plus cyclonic shear

• LC1, LC2, and LC3 depending on whether added barotropic shear is zero, positive (i.e., cyclonic), or negative (i.e., anticyclonic)

LC1

LC2
• With cyclonic shear

LC1 and LC2 Cyclone Evolutions
• The LC1 is more comparable to the Norwegian lifecycle with strong temperature gradients in the cold frontal region. The cold front eventually pinches off the warm sector, which decreases in area reminiscent of a Norwegian occlusion.
• In LC2 one sees the effects of stronger cyclonic mean shear. The strongest temperature gradients in the warm frontal zone with warm-core seclusion occurs as baroclinicity associated with the extended bentback warm front encircles the low-pressure center.

LC3: Anticyclonic Shear
• A third category LC3 refers to open wave
cyclones (i.e., cyclones that never develop occluded fronts) in which the cold front is dominant.

Major Mountain Barriers and
Land/Water Configurations Can Have a
Large Impact on Cyclone and Frontal
Structures

How Does Different Drag Between
Ocean and Land Change Cyclone and
Frontal Structures?

Adiabatic, Primitive Equation Model

Ocean Drag Land Drag

No drag

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

Southern
Plains Dry
Line
©1993 Oxford
University Press --
From: Bluestein,
Synoptic-Dynamic
Meteorology in
Midlatitudes,
Volume II
Temperatures in degrees Celsius
Dry Line

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.

L
Warm, Moist

L

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 mixed 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 .

West East
Note weak inversion or “cap” over low-level moist layer east of the surface dry line

Sounding
West of the
Dryline
West Winds
Very Dry
Albuquerque, NM
12Z -- 26 June 1998
NCAR

Sounding
East of the
Dryline
South Winds
Moist
Oklahoma City, OK
12Z -- 26 June 1998
NCAR

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.
• Troughing on the eastern side can occur on two scales:
– small mesoscale trough locked to the terrain associated with downslope warming.
– QG leeside cyclogenesis

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.
• Eventually cold air is able to move into the eastward moving trough, producing a more classical cold frontal structure.

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. (can think of interacting upper and lower
PV centers as an alternative).

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.