Upper-Level Frontogenesis
Cliff Mass
University of Washington
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.
1920s-1950: Polar Front
Bjerknes and Palmen 1937
Conceptual Models of the
Tropopause
During the 50’s and 60’s Some
Nations Conducted Upper-Ground
Testing of Nuclear Weapons
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.
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.
How did the radioactive material
get into the midlatitude
troposphere if the tropopause
was like a plastic sheath??
Connection to Upper Level Fronts
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
The First Study of Upper Level
Fonts: Reed 1957
Vertical Cross Section
A Series of Aircraft-Based Field
Experiments Described the
Structure of Upper Level Fronts for
A First Time
Radioactivity
• Measurements of
radioactivity
showed that high
levels of
radioactivity in
the stratosphere
were entering the
troposphere
through upper
level fronts.
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.
Additional Aircraft Data Showed the
Details of Upper-Level Fronts/Trop Fold
“Official” Definition
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.
Schmatic of upper level trop folding
Tropopause Folding
Stratospheric Air Injected into the
Troposphere
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.
Ozone Measured By Aircraft
Many studies have document such stratospheric
ozone in the troposphere
Remotely Sensed Ozone During an
Upper Level Front
Simulated PV Structure at Same
Time
Good correlation between Ozone
and PV: not a surprise!
Trop Pressure A Good Tool
http://www.atmos.washington.edu/~hakim/tropo/
Clear-Air Turbulence (CAT)
Associated with Upper-Level Fronts
Can be damaging and cause
injuries and even death (unbelted)
CAT Associated With Upper Level
Front and the Lower Stratosphere
Richardson Number (small less stable)
Turbulence
And why is highest PV near the jet core?
Turbulence is Maximum Above and
Below the Jet Due to Large Shear
Large Shear
Jet Core Level
Large Shear
Impact on PV insert
Upper Level Frontogenesis for the
December 14-16, 1987 Storm
400 hPa
1000 hPa
Current Case
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.
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.
Frontogenesis
Tilting Frontogenesis
Tilting Frontogenesis
Keyser et al (1986) 2-D Primitive
Equation Simulation
Clear differential
Vertical motion
Across upper
front
Subsidence is centered at the
warm side of the front under the
jet
WHY?
Several mechanisms discussed:
1. Modified Sawyer-Eliassen ageostrophic circulations
2. Large scale convergence/divergence argument
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)
Did we see this in the fully 3D
simulations of the 1987 Event?
Yes
Tilting Appears to Be Dominant for Upper Fronts
Associated with Upper-Level Short Waves in NW
Flow
Schultz and Doswell 2008
Conceptual
Model
Tilting first and
then confluence
Under Southwesterly Flow, Upper
Frontogenesis Can Be Produced by
Confluence
The End