ATSC 5007

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ATSC 5007 Problems in Synoptic MeteorologyAn
introduction to mid-latitude cyclones and fronts
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Historical Background

• A. 19th century (pre-Bjerknes)
• Epsy (1840) recognized that most clouds form from
  expansion of air, and also recognized the role of
  release of latent heat. He suggested that the
  storm was basically a thermally-direct
  circulation driven by release of latent heat
  during condensation.
• Throughout the 19th century, the thermal theory
  of cyclones persists buoyancy driven, dependent
  on release of latent heat, and basically a
  vortex. A disturbance of the general
  circulation.
• Ferrel (1878)
• established that most midlatitude storms develop
  in statically stable conditions
• from the thermal wind equation, he deduced how
  the upper and lower wind fields must be related.
• hypothesizes that the storms energy is derived
  from UL kinetic energy, as a way to explain
  storms in statically stable conditions.

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Historical Development

• B. The Norwegian School
• Geophysical Institute at Bergen
• founded 1917
• included Vilhelm Bjerknes, Jacob Bjerknes, Thor
  Bergeron, C.G. Rossby, Erik Palmen, Petterssen
• established observing network analysis of the
  observations led to the Norwegian cyclone model.
• Their concept of the structure and evolution of
  baroclinic disturbances (the Norwegian cyclone
  model) was pivotal and remains very influential
  today
• At first they coined broad moving rain stripe,
  caused by gradual flow of warm air over cold.
  Similarly, they defined a narrow moving rain
  stripe.
• Later they came up with frontal labels as we know
  it.

Jacob Bjerknes
Vilhelm Bjerknes
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From Bjerknes 1919 Warm Front (Broad moving
rain stripe) tracked as it moved across Norway.
6 h later
Note 1. The broad area of light precipitation
(shaded) 2. The discontinuity in airflow
(and airmass properties) behind the rainband.
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From Bjerknes (1919) Cold Front (Narrow moving
rain stripe) tracked as it moved across Norway.
Cold air advances,lifting warm air. Result short
but intense precip over a narrow region.
Note intersecting flows, narrow area of
precipitation (shaded) that moved across Norway.
Now the discontinuity in airmass properties is at
the leading edge of the rain band.
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From Bjerknes (1919) vertical sections for warm
and cold fronts
Warm front
Cold front
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From Bjerknes (1919) large portion of cyclone in
observing network
Noted that the bands often alternated, with broad
band followed by narrow band.
Warm front
Cold front
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Bjerknes cyclone model. Dark shading
precipitation. Light shading middle/high
clouds. Note cloud types in vertical sections
through the storm.
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Fronts in a baroclinic disturbanceWhat is a
front?A front is a zone of pronounced horizontal
temperature contrast. A dividing line between
different airmasses (note fronts are always
analyzed on the warm edge of the transition zone)

• Cold Front
• Cold more dense air displaces warm less dense air
• Slope of the front is much steeper, so in warm
  unstable air there is significant lift and storms
• Warm Front
• Cold air retreats and warm air advances
• Widespread steady precipitation ahead of front
• Light drizzle and fog along the front
• Stationary Front
• No lateral movement
• Wind blows approximately parallel to isobars
• If precipitation occurs it is light and it occurs
  on the cold side
• Occluded Front
• Cold Occlusion
• Air behind the advancing cold front (cP) is
  colder than the cool air ahead of the warm front
  (mP)
• Warm Occlusion
• Air behind the advancing cold front (mP) is
  relatively mild compared to cold air ahead of the
  warm front (cP)
• Neutral Occlusion
• No temperature change, but showers present and a
  shift in winds

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Air masses
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Warm Front
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Cold Front
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Occluded Front
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Identification of FrontsWhere is a cold front?
What characteristics should we look for?

• Temperature gradient at surface
• Trough of surface low pressure (cyclonic
  circulation convergence)
• Wind shift at surface (clockwise shift)
• Dew point temperature (colder dew point
  temperatures behind front)
• Pressure tendency (rising pressures after FROPA)
• Temperature gradient at 850 mb (for sea level
  stations 700 mb for us)
• 1000-500 mb thickness
• Weather and clouds (clearing behind cold front,
  convective activity ahead)

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The Development of Cyclones (as deduced,
Norwegian school)

• initial stage
• Most start as waves on polar front
• Solberg first suggested that this is a
  near-continuous feature
• Cyclonic shear across polar front, even before
  the development of a wave
• Shear exists even if flow is westerly on the cold
  side
• Frontal surface is tilted towards the cold air

From Petterssen 1952, Weather Analysis and
Forecasting, p. 218
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The Development of Cyclones (as deduced,
Norwegian school)

• Open wave stage
• deepening of low
• strengthening of circulation and thermal
  advection
• formation or amplification of upper-level wave
  (with crest over warm front and trough over cold
  front)

From Petterssen, Weather Analysis and
Forecasting, p. 218
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The Development of Cyclones (as deduced,
Norwegian school)

• Temperature structure at 500 mb during the open
  wave stage

Frontal zone aloft
Uniform temperature south of front contd
cooling north of front 500 mb temperature
contrast is less sharp at this level than at the
surface Height contours tend to align with
isotherms
From Petterssen, Weather Analysis and
Forecasting, p. 229
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The Development of Cyclones (as deduced,
Norwegian school)

• Occluded stage
• Cold front overtakes warm front, or rather, LL
  vortex moves into the cold air
• Warm air is lifted by intersection of two
  cold-air wedges. The final deepening of the low
  is often enhanced by strong latent heat release
  aloft.
• This stage also ushers in decay, as diff CVA and
  WAA vanish, at least at the location of the low

Trough forms north of tripple point (often
with heavy precip), Cold front tends to remain
frontogenetic, but becomes more shallow
(katafront -rope cloud)
From Petterssen, Weather Analysis and
Forecasting, p. 218
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Frontal cyclone lifecycle
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Frontal Cyclone lifecycle
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cross section through fronts
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cross section through fronts
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Idealized Mature Frontal Cyclone
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3D conveyor belts
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Surface isobars (solid) and 1000-500 mb thickness
(dashed)
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3

• Note
• thickness contours concentrated between sfc
  front and upper level frontal zone

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From Petterssen, Weather Analysis and
Forecasting, Vol. I, pp. 230-231
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500 mb height contours
Note displacement of upper-level trough to the
west of surface low
1
3
2
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From Petterssen, Weather Analysis and
Forecasting, Vol. I, p. 231
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Relationship to Upper-Level Structure
500 mb 1000 mb height (thick thin lines) and
1000-500 mb thickness (dashed). The deflection
of the upper-level wave contributes to deepening
of the surface low.
Palmen and Newton, p. 326 cf. Houze p. 448
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Why do developing baroclinic systems tilt
westward with height?
Source Holton (2004) chap 6
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Westward tilt with height implies QG uplift over
surface low, i.e. development
Source Bluestein (1993) p. 134
temperature advection differential vorticity
advection vertical motion
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eastward tilt with height implies QG sinking over
surface low, i.e. decay
Source Bluestein (1993) p. 148
temperature advection differential vorticity
advection vertical motion
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QG perspective synergy between low-level
upper-level flow
WAA
CVA
AVA
CAA
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example Cyclone evolution 4 Jan 1989
A new model for the evolution of baroclinic
lows (Shapiro et al.)
316 K high PV blob
340 K low PV area
SLP and 950 mb z
q, wind speed, and PVgt2 PVU (shaded)
Shapiro et al 1999
q340K wind speed and 950 mb z
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Observed mesoscale low level T structure (T-bone
structure)
Shapiro et al 1999
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18 Z on 4 Jan
observations
350 m AGL qe and winds
950 mb q
model
grey zgt1 10-4s-1
black zgt2 10-4s-1
SLP and abs vort
Shapiro et al 1999
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seclusion
T-bone
slp, fronts, precip
frontal fracture
incipient
bent-back warm front
fracture
seclusion
850 mb temperature, LL jets
Shapiro 1990 Conceptual model of lifecycle
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Idealized cyclone evolution, Shapiro et al 1999
305K PV (shaded) and 3 PVU contour at 340 K
surface q and q
za and winds at the surface
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Idealized cyclone evolution, Shapiro et al 1999
305K PV (shaded) and 3 PVU contour at 340 K
surface q and q
za and winds at the surface
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ATSC 5007 Problems in Synoptic MeteorologyAn
introduction to mid-latitude cyclones and fronts
Benjamin Franklin In 1735, "Poor Richard," aka
Ben Franklin, wroteSome are weatherwise, some
are otherwise. When he was 37 (1743),
Benjamin Franklin observed that northeast storms
begin in the southwest. He thought it was odd
that storms travel in an opposite direction to
their winds. After further observations and
performing studies of storms, he predicted that a
storm's course could be plotted. He then printed
weather forecasts in his Poor Richard's Almanac.
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Thomas Jefferson, Notes on the State of Virginia,
1781
A change in our climate however is taking place
very sensibly. Both heats and colds are becoming
much more moderate within the memory even of the
middle-aged. Snows are less frequent and less
deep. They do not often lie, below the mountains,
more than one, two, or three days, and very
rarely a week. They are remembered to have been
formerly frequent, deep, and of long continuance.
The elderly inform me the earth used to be
covered with snow about three months in every
year. The rivers, which then seldom failed to
freeze over in the course of the winter, scarcely
ever do now. This change has produced an
unfortunate fluctuation between heat and cold, in
the spring of the year, which is very fatal to
fruits. In an interval of twenty-eight years,
there was no instance of fruit killed by the
frost in the neighborhood of Monticello The
accumulated snows of the winter remaining to be
dissolved all together in the spring, produced
those overflowings of our rivers, so frequent
then, and so rare now.
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Mark Twain (Samuel T. Clemens -
18351910) The coldest winter I ever spent was
a summer in San Francisco. Of course weather
is necessary to a narrative of human
experience. Thunder is good, thunder is
impressive but it is lightning that does the
work. Climate is what we expect, weather is
what we get. It is your human environment that
makes climate. And, of course Don't let
school interfere with your education. I could
never learn to like her, except on a raft at sea
with no other provisions in sight.

• Sir Arthur Conan Doyle, His Last Bow (1917)
• "There's an east wind coming, Watson.
• "I think not, Holmes. It is very warm."
• "Good old Watson! You are the one fixed point in
  a changing age. There's an east wind coming all
  the same, such a wind as never blew on England
  yet. It will be cold and bitter, Watson, and a
  good many of us may wither before its blast. But
  it's God's own wind none the less, and a cleaner,
  better, stronger land will lie in the sunshine
  when the storm has cleared."