ATSC 5007 – Problems in Synoptic
Meteorology:
An introduction to mid-latitude
cyclones and fronts
Historical Background
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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):
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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 storm’s energy is derived from UL kinetic
energy, as a way to explain storms in statically stable conditions.
Historical Development
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B. The Norwegian School
– Geophysical Institute at Bergen:
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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
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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
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.
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.
From Bjerknes (1919): vertical sections for warm and cold fronts
Warm front
Cold front
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
Bjerknes cyclone model.
Dark shading: precipitation.
Light shading: middle/high clouds.
Note cloud types in vertical
sections through the storm.
Fronts in a baroclinic disturbance
What 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)
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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
Air masses
Warm Front
Cold Front
Occluded Front
Identification of Fronts
Where is a cold front? What characteristics should we look for?
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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)
The Development of Cyclones
(as deduced, Norwegian school)
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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
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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
The Development of Cyclones
(as deduced, Norwegian school)
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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
The Development of Cyclones
(as deduced, Norwegian school)
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Temperature structure at 500 mb during the open wave stage:
Uniform temperature south of front;
cont’d cooling north of front
Frontal zone aloft
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
The Development of Cyclones
(as deduced, Norwegian school)
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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
Frontal cyclone lifecycle
Frontal Cyclone lifecycle
cross section through fronts
cross section through fronts
Idealized Mature Frontal Cyclone
3D conveyor belts
Surface isobars (solid)
and 1000-500 mb
thickness (dashed)
1
Note:
• thickness contours
3
concentrated between sfc
front and upper level frontal
zone
2
4
From Petterssen, Weather Analysis and Forecasting, Vol. I, pp. 230-231
500 mb height
contours
Note displacement of
upper-level trough to
he west of surface low
1
3
2
4
From Petterssen, Weather Analysis and Forecasting, Vol. I, p. 231
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
Why do developing baroclinic systems tilt westward with height?
Source: Holton (2004) chap 6
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
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
QG perspective:
synergy between low-level & upper-level flow
WAA
CVA
AVA
CAA
A new model for the evolution of baroclinic lows
(Shapiro et al.)
316 K
high PV
blob
example: Cyclone evolution 4 Jan 1989
340 K low PV area
SLP and 950 mb z
q, wind speed, and PV>2 PVU (shaded)
q=340K wind speed and 950 mb z
Shapiro et al 1999
Observed mesoscale low level T structure (T-bone structure)
Shapiro et al 1999
18 Z on 4 Jan
observations
350 m AGL
qe and winds
950 mb q
model
grey: z>1 10-4s-1
black: z>2 10-4s-1
SLP and abs vort
Shapiro et al 1999
T-bone
seclusion
slp, fronts, precip
frontal fracture
incipient
bent-back warm front
fracture
seclusion
850 mb temperature,
& LL jets
Shapiro 1990
Conceptual model
of lifecycle
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
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
ATSC 5007 – Problems in Synoptic Meteorology:
An introduction to mid-latitude cyclones
and fronts
Benjamin Franklin:
In 1735, "Poor Richard," aka Ben Franklin, wrote:
“Some 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.
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.
Mark Twain (Samuel T. Clemens - 1835–1910):
“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."
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ATSC 5004 – Problems in Dynamic Meteorology