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Upper-Level Frontogenesis
Cliff MassUniversity 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 50’s and 60’s 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.washington.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
Jet Core Level
Large Shear
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 differentialVertical motionAcross upper front
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Subsidence is centered at the warm side of the front under the
jet
WHY?Several mechanisms discussed:
1. 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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