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SO441 Synoptic Meteorology

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SO441 Synoptic Meteorology. Fronts. Lesson 8: Weeks 13-14. ... The Great Plains dry line is a sharp change in air masses but is not considered a ... lifting air cools ... – PowerPoint PPT presentation

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Title: SO441 Synoptic Meteorology


1
SO441 Synoptic Meteorology
  • Fronts
  • Lesson 8 Week 14

Courtesy Lyndon State College
2
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, cool dry air
    over Maryland, and cloudy skies, SE winds, and
    warm air over Virginia. A cold front separates
    the two states.
  • By mid-day, solar radiation has strongly heated
    the air over Maryland, and it is now warmer than
    the moist air over Virginia. Has the front
    disappeared? Changed to a warm front?

3
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

4
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

5
Example of a front 17 Nov 2009
Potential temp (k)
Sea-level pressure (mb)
950-mb relative vorticity (s-1)
Cross-section of potential temp (k) and wind
6
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
  • Lets examine each term in turn

7
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.

t0
t24
-
-
Example positive contribution to F along the
cold front shearing frontogenesis
x
y
Fgt0
8
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

y
t0
t24
x
Example negative contribution to F along the
warm front shearing frontolysis

-
Flt0
9
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

Example positive contribution to F along the
front confluence frontogenesis
-
-
t24
t0
y
x
Fgt0
10
Tilting term
  • Near the Earths surface, vertical motion is
    usually fairly small
  • But higher aloft, it can be strong
  • Thus tilting usually acts to strengthen fronts
    above the Earths surface
  • Consider the following example here, is
    negative (potential temperature increases above
    the surface), and is negative (rising
    motion in the cold air, sinking in the warm air)

-
-
z
z
Fgt0
y
y
Example positive contribution to F along a
front tilting
11
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 negative
    (so positive) , and F positive

-
-
Fgt0
y
Example positive contribution to F along a
front differential diabatic heating
12
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
13
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
14
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

15
Occluded 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
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