Transcript Lecture #12: Mid-latitude Cyclogenesis

Midlatitude Cyclogenesis
Advanced Synoptic
M. D. Eastin
Midlatitude Cyclogenesis
Climatology
Understanding Cyclogenesis
• Vorticity perspective
• Pressure perspective
• QG perspective
• PV perspective
Explosive Cyclogenesis
Cyclone Classifications
• Petterssen Type A and Type B
• Miller Type A and Type B
• Zipper Lows
• Thermal Lows
Advanced Synoptic
M. D. Eastin
Cyclone Climatology
What is the role of mid-latitude cyclones?
• Variations in incoming solar radiation
and surface albedo (via cloud, ice, and
vegetation heterogeneity) produce a
latitudinal gradient in net heating
• Heat surplus in the tropics
• Heat deficit in the polar regions
• Since the long-term global mean temperature
changes VERY slowly, we know there must
be a relatively RAPID transfer of heat from
the tropics toward the poles
• Oceans do ~20% of the total
• Atmosphere does ~80% of total
via sensible and latent heat fluxes
at the surface and their vertical
transport by convection
• Mid-latitude cyclones are the most
efficient method of transfer
Advanced Synoptic
M. D. Eastin
Cyclone Climatology
Where do Surface Cyclones Form?
• More frequent in the winter than the summer
• Occurs further south during the winter and
further north in the summer
1. In the lee of the northern Rockies from
Alberta to Montana (“Alberta Clippers”)
2. In the lee of the southern Rockies near
Colorado (“Colorado Lows”)
These two locations are related to flow
over mountains (topographic forcing)
1
2
3
4
3. Off the east coast from the mid-Atlantic
states to New England (“Hatteras Lows”)
4. Off the Texas coast in the Gulf of Mexico
(“Longhorn Lows”)
These two locations are related to cold air
flowing over relatively warm waters
(diabatic forcing)
From Zishka and Smith (1980)
Advanced Synoptic
M. D. Eastin
Cyclone Climatology
Where do Surface Cyclones Move?
• Most surface cyclones are short waves and
move east (progress) with the mean flow
 Initial motions may be southeasterly (due to
topographic influences) but mature cyclones
almost always move northeasterly
 Related to motion toward maximum surface
pressure decreases (via QG theory)
• WAA maximum is often to the northeast
• An upper-level PVA maximum is often
to the northeast
• The warm front and its associated
convection (or diabatic heating) is
often to the northeast
From Zishka and Smith (1980)
Advanced Synoptic
M. D. Eastin
Cyclone Climatology
Where do Surface Cyclones Dissipate?
1. Far off the East Coast over the north-central
Atlantic → become “Icelandic Lows”
Related to occlusion and being cut-off from
their source of warm moist (tropical) air
1
2
3
2. Along the Pacific northwest coast
Related to flow toward / over topography
(topographic forcing)
3. Over New England and eastern Canada
Related to warm air flowing over a relatively
cold land surface (diabatic forcing)
From Zishka and Smith (1980)
Advanced Synoptic
M. D. Eastin
Understanding Cyclogenesis
Vorticity Perspective:
• Given that surface cyclones are always characterized by cyclonic vorticity maxima,
cyclogenesis can be explained through analysis of the vertical vorticity equation:
D a
Dt
  u  v 
 



y

p

x

p


Total Change
Tilting
 a

p
Stretching

 Fy Fx 




x

y


Friction
• For a uniform frontal zone near the surface (see below), scale analysis suggests
we can neglect the tilting and friction terms
 Any approaching source of synoptic-scale ascent (e.g. an upper-level trough) will produce
stretching in the lower troposphere and an increase in surface cyclonic vorticity
Advanced Synoptic
M. D. Eastin
Understanding Cyclogenesis
Pressure Perspective:
546
• For the surface pressure to fall
near the center of a developing
low pressure system, there must
be a net mass divergence aloft
in the overlying air column
552
558
L
• This can be accomplished through
the approach of a diffluent trough
Pressure Perspective
Vorticity Perspective
Note: The vorticity and pressure
views are exactly consistent
with one another, as each
perspective emphasizes
different aspects of the
same circulation
Advanced Synoptic
M. D. Eastin
Understanding Cyclogenesis
QG Perspective:
• Through analysis of BOTH the QG omega equation (surface system evolution) and
the QG height tendency equation (upper-level system evolution) for a given surface
frontal zone with an approaching upper-level trough, we can view the cyclogenesis
process as “mutual amplification”
Low-levels: CAA (WAA) behind (ahead of)
the surface cold (warm) front
decreases (increases) the
thicknesses west (east) of
the surface low & intensifies
the upper-level trough (ridge)
at the same time
Upper-levels: PVA downstream of the trough
forces ascent directly over the
surface system, lowering the
surface pressure & increasing
the low-level WAA / CAA
at the same time.
• “Sutcliffe-Petterssen self development”
(see Section 5.3.5 in your text)
Advanced Synoptic
M. D. Eastin
Understanding Cyclogenesis
PV Perspective:
• Through the PV invertibility principle, we can view the cyclogenesis process as the vertical
extensions of the flows associated with an upper-level positive PV anomaly (a trough)
and a low-level positive temperature anomaly (a weak surface low)
• The vertical extent of the flow
associated with either feature
is a function of (1) the ambient
static stability, (2) the magnitude
of each anomaly, and (3) the
horizontal scale of each anomaly
• Stronger upper-level troughs
are more likely to intensify any
given surface low
• Intensification occurs through
combination of “diabatic growth”
and “mutual amplification”
Advanced Synoptic
M. D. Eastin
Understanding Cyclogenesis
PV Perspective:
Diabatic Growth:
• The release of latent heat
produces a PV maximum
below the heating max and
a PV minimum above
Mutual Amplification:
• If the upper-level trough and
surface low exhibit westward
tilt with height & their vertical
flow extensions overlap, then
their respective cyclonic flows
can mutually amplify each
other as they become
“phase locked”
• Described as the “essence” of
baroclinic instability
Advanced Synoptic
M. D. Eastin
Understanding Cyclogenesis
PV Perspective:
• Developing cyclones always exhibit
four (4) distinct PV anomalies:
1.
2.
3.
4.
Stratospheric cyclonic PV maximum
Surface warm temperature maximum
Low-level diabatic PV maximum
Upper-level diabatic PV minimum
 The cyclogenesis process can be viewed
as a manifestation of interactions between
these PV anomalies and the processes
that cause these anomalies to either:
1. Amplify individually
2. Superimpose upon one another
3. Constructively interfere with
one another
Advanced Synoptic
M. D. Eastin
Explosive Cyclogenesis
“Bomb” Cyclones:
• In certain situations, the dynamical mechanisms important to cyclogenesis (including
the upper-level trough, the jet core, the surface low, and diabatic energetics) align
in such a manner to permit RAPID intensification
Definition: A mid-latitude low pressure system where the central surface pressure drops
24-mb over the course of 24-hr (a rate of 1 mb/hr)
Common Characteristics:
• Occur primarily in the winter
• Often produce severe blizzards
• Occur along eastern coasts were cold-dry
continental air interacts with warm ocean
currents (Gulf Stream)
Location of Bombs
(1979-1999)
Kuroshio
Current
• Often triggered by an upper-level trough
approaching a strong coastal baroclinic zone
• The “Perfect Storm” is a partial example
• Also called “nor-easters”
Gulf
Stream
Advanced Synoptic
M. D. Eastin
Explosive Cyclogenesis
“Bomb” Cyclones:
Important Physical Processes:
• Strong coastal baroclinic zone
Diffluent Trough
Strong WAA
Heat and Moisture Fluxes
Coastal Front
• Strong pre-existing low-level vorticity
• Strong surface energy fluxes due
to cold-dry air moving over a warm
ocean current (Gulf Stream)
L
• Diffluent trough with strong PVA
through a deep layer (500-200mb)
• Unusually strong WAA at upper
levels (500-200 mb) downstream
from the trough axis that helps
provide a deep column of ascent
• Very strong low-level WAA (CAA)
downstream (upstream)
From Bluestein (1993)
Advanced Synoptic
M. D. Eastin
Explosive Cyclogenesis
15-16 April 2007 “Bomb Cyclone”
15 April 1200Z
MSLP = 993 mb
850 mb
Heights
Temps
300 mb
Heights
Winds
Surface
Press
GOES - IR
Advanced Synoptic
M. D. Eastin
Explosive Cyclogenesis
15-16 April 2007 “Bomb Cyclone”
16 April 0000Z
MSLP = 979 mb
850 mb
Heights
Temps
300 mb
Heights
Winds
Surface
Press
GOES - IR
Advanced Synoptic
M. D. Eastin
Explosive Cyclogenesis
15-16 April 2007 “Bomb Cyclone”
16 April 1200Z
MSLP = 968 mb
850 mb
Heights
Temps
300 mb
Heights
Winds
Surface
Press
GOES - IR
Advanced Synoptic
M. D. Eastin
Explosive Cyclogenesis
15-16 April 2007 “Bomb Cyclone”
Advanced Synoptic
M. D. Eastin
Cyclone Classifications
Historic – Petterssen “Type A” and “Type B”
“Type A”
• Form without interaction from a clearly-defined, pre-existing, upper-level trough
• Recent research suggests this cyclone type rarely occurs (less than 5% of cases)
“Type B”
• Form when a pre-existing, finite-amplitude, upper-level trough overtakes a low-level frontal
zone with at least some pre-existing baroclinicity, convergence, and vertical vorticity
• Vast majority (over 95%) of cyclones intensify this way
Type B
Advanced Synoptic
M. D. Eastin
Cyclone Classifications
East Coast – Miller “Type A” and “Type B”
Type A
Type B
“Type A”
• Form along surface frontal zones located in
and near the Gulf of Mexico primarily during
the winter season
• Southerly flow off the Gulf provides the source
of warm conveyor air
• Most often move through the southeast states
and up along the east coast
“Type B”
• Form as a “secondary” low to the southeast
of the “primary” low, along a coastal front
• Often occur during cold-air damming events
when a high is located to the north causing
onshore flow
• Most often move north along the coast and
along the strong SST gradient of the Gulf
Stream → can develop into “bombs”
From Bluestein (1993)
Advanced Synoptic
M. D. Eastin
Cyclone Classifications
“Zipper” Lows:
Common Characteristics:
• Occur along coastal fronts
during the winter
• No upper-level support
• Low-level convergence
and weak WAA ahead
(northeast) of the low
produces pressure falls
• Low-level divergence
and weak CAA behind
(southwest) of the low
produces pressure rises
• Net result is motion along
the coastal front with little
to no intensification
• Looks like the opening
and closing of a zipper
Advanced Synoptic
From Bluestein (1993)
M. D. Eastin
Cyclone Classifications
Thermal Lows:
Common Characteristics:
• Occur in arid and semi-arid regions
during the warm season
• Develop in response to intense diabatic
heating at low-levels due to surface
sensible heat fluxes
• Often shallow systems (below 700 mb)
• Rarely develop as upper-level troughs
pass over due to lack of moisture to
support convection
Advanced Synoptic
M. D. Eastin
References
Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Mid-latitudes. Volume II: Observations and Theory of Weather
Systems. Oxford University Press, New York, 594 pp.
Brennan, M. J., G. M. Lackmann, and K. A. Mahoney, 2008: Potential vorticity (PV) thinking in operations: The utility
of non-conservation. Weather and Forecasting, 23, 168-182
Davis, C. A., 1992b: Piecewise potential vorticity inversion. Journal of Atmospheric Science, 49, 1397-1411
Hoskins, B. J., 1990: The theory of extra-tropical cyclones. Extra-tropical cyclones: The Erik Palmen Memorial Volume,
C. W. Newton and E. O. Holopainen, eds, American Meteorological Society, 129-153.
Hoskins B. J., and P. J. Valdes, 1990: On the existence of storm-tracks. J. Atmos. Sci., 47, 1854-1864.
Miller, J. E., 1946: Cyclogenesis in the Atlantic coastal region of the United States. J. Meteor., 3, 31-44.
Petterssen, S., 1956:, Weather Analysis and Forecasting 2nd, ed. McGraw-Hill, 428 pp.
Petterssen, S., and S. J. Smebye, 1971: On the development of extra-tropical cyclones. Quart J. Roy. Meteor. Soc.,
97, 457-482.
Roebber, P. J., 1984: Statistical analysis and updated climatology of explosive cyclogenesis. Mon. Wea. Rev.,
112, 1577-1589.
Sanders, F., 1988: Life history of mobile troughs in the upper westerlies. Mon. Wea. Rev., 116, 2629-2648.
Sanders, F., R. J. Gyakum, 1980: Synoptic dynamic climatology of the “bomb”. Mon. Wea. Rev., 108, 1589-1606.
Sutcliffe, R. C. and A. G. Forsdyke, 1950: The theory and use of upper air thickness patterns in forecasting.
Quart. J. Roy. Meteor. Soc., 176, 189-217.
Zishka, K. M., and P. J. Smith, 1980: the climatology of cyclones and anticyclones over North America and surrounding
oceans environs for January and July, 1950-1977. Mon. Wea. Rev., 108, 387-401.
Advanced Synoptic
M. D. Eastin