SO441 Synoptic Meteorology

FrontsLesson 8: Weeks 13-14

Courtesy: Lyndon State College

What is a front?• Early meteorological theory thought that “fronts” led to development of

low pressure systems (cyclones)– However, in the 1940s, “baroclinic instability theory” found that cyclones can form

away from fronts, then develop frontal features• So what is a front?

– Several definitions exist:• Zone of “enhanced” temperature gradient (but what constitutes “enhanced”?)• Sharp transition in air masses

– The Great Plains dry line is a sharp change in air masses but is not considered a front• Zone of density differences

– But density is driven by not only temperature but also moisture and pressure

– Example:• Early a.m. clear skies, NW winds, & cold air over Oklahoma, and cloudy skies, SE winds, and

warm air over Arkansas. A cold front separates the two.• By mid-day, solar radiation has strongly heated the air over Oklahoma, and it is now warmer

than the moist air over Arkansas. Has the front disappeared? Changed to a warm front?

A basic definition

• Following Lackmann (2012), we will use the following definition of a front:– A boundary between air masses

• Recognize that all boundaries between air masses may not be fronts– Examples: semi-permanent thermal gradients locked in

place by topographic boundaries, land-sea contrasts• How do we proceed?– In weather chart analyzes, be sure to analyze temperature

• The important boundaries will then be evident on the chart

Properties of fronts• Most defining property (on a weather map): enhanced horizontal gradients of

temperature– Usually long and narrow: synoptic scale (1000 km) in the along-front direction, mesoscale (100

km) in the across-front direction• Other properties:

– Pressure minimum and cyclonic vorticity maximum along the front– Strong vertical wind shear

• Exists because of horizontal temperature gradients (required by “thermal wind balance”– Large static stability within the front– Ageostrophic circulations

• Rising motion on the warm side of the frontal boundary• Sinking motion on the cool side of the boundary

– Greatest intensity at the bottom, weakening with height• Fronts are mostly confined near the surface, but not always

– Upper-level fronts, i.e. gradients of temperature aloft, are associated with strong vertical wind shear• Clear-air turbulence and aviation hazards often occur there

Example of a front: 17 Nov 2009Sea-level pressure (mb) Potential temp (k)

950-mb relative vorticity (s-1) Cross-section of potential temp (k) and wind

Frontogenesis function• To examine whether a front is strengthening or weakening,

can look at the “Frontogenesis Function”– When F is positive, frontogenesis is occurring– When F is negative, frontolysis is occurring

• F allows for examination of the different physical mechanisms that lead to changes in temperature gradients

• Let’s examine each term in turn

DiabaticShearing Confluence Tiltingheating

u v dFx y y y p y y dt

Shearing term• Shear frontogenesis describes the change in front strength

due to differential temperature advection by the front-parallel wind component– Along the cold front, both and are negative, giving a positive

contribution to F (note the rotation of the coordinate system!!)– This means cold-air advection in the cold air, and warm-air advection

in the warm air.

x

uy

Example: positive contribution to F along the cold front: shearing frontogenesis

t=0 t=+24

Shearing term• Shear frontogenesis describes the change in front strength

due to differential temperature advection by the front-parallel wind component– Along the warm front, is positive, but is negative, giving a

negative contribution to F (again note the rotation of the coordinate system!!)

– This means along the warm front, shearing acts in a frontolytical sense

x

uy

Example: negative contribution to F along the warm front: shearing frontolysis

t=0 t=+24

Confluence term• Confluence frontogenesis describes the change in front

strength due to stretching. If the isotherms are stretching (spreading out), there is frontolysis. If they are compacting, frontogenesis is occurring.– Along the front, is negative. Here is also negative, giving a

positive contribution to F (again note the rotation of the coordinate system!!)

– This means along the front, confluence acts in a frontogenetical sense

y

vy

Example: positive contribution to F along the front: confluence frontogenesis

t=0 t=+24

Tilting term• Near the Earth’s surface, vertical motion is usually fairly small

– But higher aloft, it can be strong• Thus tilting usually acts to strengthen fronts above the Earth’s

surface• Consider the following example: here, is positive

(temperature decreases above the surface), and is also positive (rising motion in the cold air, sinking in the warm air)

p

y

y y

z z

Example: positive contribution to F along a front: tilting

Diabatic heating term• The differential diabatic heating term takes into account all

diabatic processes together:– Differential solar radiation, differential surface heating due to soil

characteristics, differential heat surface flux• One example: differential solar radiation

– Assume the diabatic heating rate in the warm air exceeds the diabatic heating rate in the cold air

– In that example, would be positive, and F positivedy dt

Example: positive contribution to F along a front: differential diabatic heating

Frontal circulations• Important terminology:

– Thermally direct: warm air rises, cold air sinks– Thermally indirect: warm air sinks, cold air rises– Ageostrophic: departure from geostrophic flow

• Because of the strong temperature contrasts along fronts, there are often thermally direct circulations: warm air rises, cold air sinks– The rising / sinking motions are ageostrophic, and by themselves, act to weaken fronts

• See the tilting term example• Also, lifting air cools it (so the warm air cools) and sinking air warms (so the cold air warms)

– But when ageostrophic circulations act together with geostrophic flow above the surface, they can act to strengthen the front at the surface

Example: geostrophic and ageostrophic flows strengthening a front at the surface

Cold fronts• Defined as:

– Clear advance of cold airmass with time• Usually characterized by:

– Abrupt wind shift from a southerly component to a westerly or northerly component

– Pressure falls before, then rises after, passage

– Showers and sometimes thunderstorms• Two types:

– Katafront, with precipitation ahead of the front• Usually preceeded by a cold front (or

boundary) aloft• Front slopes forward

– Anafront, with precipitation behind the front• Front slopes backward

Arrows represent direction of upper-level winds; hatching in katafront figure indicates precipitation area

Katafront

Anafront

Warm fronts

• Defined as:– Clear advance of warm airmass with time

• Usually characterized by:– Gradual wind shift from easterly to southerly

during passage– Turbulent mixing along the passage• Gives rise to risk of tornadic thunderstorms along front

– Shallow vertical slope

Occluded fronts• Cyclogenesis is favored along frontal boundaries

– Rich area of cyclonic vorticity– Rising motion (and vorticity stretching)

• Circulation around surface cyclone moves air masses– We call these boundaries fronts

• Cold front moves faster than warm front– What happens when the cold front “catches up” to the warm front?

• The resulting boundary (between cold and not so cold air) is called an occluded front• Noted on surface charts by purple symbol with both triangles and semi-circles in same direction