Download SO441 Synoptic Meteorology

SO441 Synoptic Meteorology
Fronts
Lesson 8: Week 13
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 2009
Sea-level pressure (mb)
950-mb relative vorticity (s-1)
Potential temp (k)
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
  u    v        d 
F
 
 

 

x  y  y  y  p  y  y  dt 
Shearing
Confluence
• Let’s examine each term in turn
Tilting
Diabatic
heating
Shearing term
• Shear frontogenesis describes the change in front strength
due to differential temperature advection by the front-parallel
wind component

x
u
y
– 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 term
• Shear frontogenesis describes the change in front strength
due to differential temperature advection by the front-parallel
wind component

u
– Along the warm front, x is positive, but y 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 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.

v
– Along the front, y is negative. Here y 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=0
t=+24
Example: positive contribution to F along the front: confluence frontogenesis
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, p is negative

(potential temperature increases above the surface), and y
is negative (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
• 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
 d
– In that example, y  dt  would be positive, and F positive
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
•
Arrows represent direction of upper-level winds;
hatching in katafront figure indicates precipitation area
Front slopes backward
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