Upper-Level Frontogenesis

Cliff MassUniversity 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 Toolhttp://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

Jet Core Level

Large Shear

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 differentialVertical motionAcross upper front

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

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