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SO441 Synoptic Meteorology. Fronts Lesson 8: Weeks 12-13. Courtesy: Lyndon State College. What is a front ?. Early meteorological theory thought that “fronts” led to development of low pressure systems (cyclones)

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so441 synoptic meteorology

SO441 Synoptic Meteorology

Fronts

Lesson 8: Weeks 12-13

Courtesy: Lyndon State College

what is a front
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
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
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 2009
Example of a front: 17 Nov 2009

Potential temp (k)

Sea-level pressure (mb)

Cross-section of potential temp (k) and wind

950-mb relative vorticity (s-1)

frontogenesis function
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
shearing term
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.

t=0

t=+24

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

shearing term1
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

t=0

t=+24

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

confluence term
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

t=+24

t=0

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

tilting term
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)

z

z

y

y

Example: positive contribution to F along a front: tilting

diabatic heating term
Diabatic heating term
  • The differentialdiabatic 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 positive

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

frontal circulations
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
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

Anafront

Katafront

warm fronts
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
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