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

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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

DRY LINE

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 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.

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