Upper-Level Frontogenesis

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Upper-Level Frontogenesis

• Cliff Mass
• University of Washington

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Early Days

• In the first half of the 20th century there was
  no concept of upper-level fronts.
• Most studies described a polar front that
  extended from the surface to the tropopause.
• The tropopause was considered an extensive and
  impenetrable barrier between the troposphere and
  stratosphere.

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1920s-1950 Polar Front
Bjerknes and Palmen 1937
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Conceptual Models of the Tropopause
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During the 50s and 60s Some Nations Conducted
Upper-Ground Testing of Nuclear Weapons
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Radioactivity

• It was thought the above ground tests were not a
  problem
• Radioactivity injected into the stratosphere
  would stay there.
• Radioactivity injected into troposphere (in
  remote areas!!) would fall out rapidly or would
  be removed by precipitation.
• But that did not prove to be the case. High
  concentrations of radioactivity showed up in the
  U.S. and other locations. High levels of
  strontium-90 were found in milk, for example.

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Somehow radiation from Pacific tests were getting
into the troposphere over the U.S. and then
either dry deposited or were scavenged out by
rain.
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How did the radioactive material get into the
midlatitude troposphere if the tropopause was
like a plastic sheath??
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Connection to Upper Level Fronts
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Upper- level fronts and stratosphere-troposphere
transport

• To answer this question, a number of synoptic
  studies and field experiments took place in the
  1950s and 1960s.
• They described new meteorological animals
• the upper level front
• Tropopause folding and gaps
• Stratosphere-troposphere exchange

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The First Study of Upper Level Fonts Reed 1957
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Vertical Cross Section
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A Series of Aircraft-Based Field Experiments
Described the Structure of Upper Level Fronts for
A First Time
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Radioactivity

• Measurements of radioactivity showed that high
  levels of radioactivity in the stratosphere were
  entering the troposphere through upper level
  fronts.

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Potential Vorticity As a Tracer of Air Parcel
Origin

• Potential vorticity is high in the stratosphere
  because of the large stability there. Ertel
  Potential Vorticity (PV)
• The aircraft studies found stratospheric values
  of potential vorticity transported into the
  troposphere through upper level fronts.

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Additional Aircraft Data Showed the Details of
Upper-Level Fronts/Trop Fold
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Official Definition
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Upper Level Front Characteristics

• Usually associated with midlatitude jet.
• Can extend down to 900-800 mb
• Close association with upper-level troughs.
• Often associate with substantial clear air
  turbulence.
• Associated with a folding or extrusion of the
  tropopause.
• Can also be associated with high ozone values,
  particularly in mountain stations.

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Schmatic of upper level trop folding
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Tropopause Folding
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Stratospheric Air Injected into the Troposphere
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Ozone and Upper Level Fronts

• Ozone levels are generally higher in the
  stratosphere than the troposphere.
• Ozone can be injected into the troposphere
  through upper level fronts
• The enhanced upper-level fronts can frequently be
  observed at the surface, particularly at mountain
  and higher-elevation observation sites.

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Ozone Measured By Aircraft
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Many studies have document such stratospheric
ozone in the troposphere
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Remotely Sensed Ozone During an Upper Level Front
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Simulated PV Structure at Same Time
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Good correlation between Ozone and PV not a
surprise!
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Trop Pressure A Good Toolhttp//www.atmos.washing
ton.edu/hakim/tropo/
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Clear-Air Turbulence (CAT) Associated with
Upper-Level Fronts
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Can be damaging and cause injuries and even death
(unbelted)
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CAT Associated With Upper Level Front and the
Lower Stratosphere
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Richardson Number (small less stable)
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Turbulence
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And why is highest PV near the jet core?
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Turbulence is Maximum Above and Below the Jet Due
to Large Shear
Large Shear
Jet Core Level
Large Shear
Impact on PV insert
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Upper Level Frontogenesis for the December 14-16,
1987 Storm
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400 hPa
1000 hPa
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Current Case
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Why Upper Level Fronts?

• What is the relative importance of tilting and
  confluence/deformation?
• Upper level short waves/jet streaks are key
  players.
• As we shall see, three dimensional motions are
  essential.

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Upper Level Frontal Frontogenesis

• Tilting frontogenesis associated with
  differential vertical motion is often dominant,
  particularly for shortwaves in NW flow
• Horizontal confluent frontogenesis is also
  important, but usually secondary.

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Frontogenesis
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Tilting Frontogenesis
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Tilting Frontogenesis
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Keyser et al (1986) 2-D Primitive Equation
Simulation
Clear differential Vertical motion Across upper
front
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Subsidence is centered at the warm side of the
front under the jetWHY?Several mechanisms
discussed1. Modified Sawyer-Eliassen
ageostrophic circulations2. Large scale
convergence/divergence argument
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Theoretical Studies Have Shown That A Jet
Streak/Shortwave Associated with Cold Advection
Shows the Appropriate Vertical Motion Pattern
(Sinking on the Warm Air Side)
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Did we see this in the fully 3D simulations of
the 1987 Event?
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Yes
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Tilting Appears to Be Dominant for Upper Fronts
Associated with Upper-Level Short Waves in NW Flow
Schultz and Doswell 2008
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Conceptual Model Tilting first and then
confluence
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Under Southwesterly Flow, Upper Frontogenesis Can
Be Produced by Confluence
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The End
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