Title: SO441 Synoptic Meteorology
1SO441 Synoptic Meteorology
Courtesy Lyndon State College
2What 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?
3A 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
4Properties 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
5Example 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
6Frontogenesis 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
7Shearing 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
8Shearing 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
9Confluence 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
10Tilting 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
11Diabatic 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
12Frontal 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
13Cold 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
14Warm 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
15Occluded 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