Advanced SynopticM. D. Eastin 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

Midlatitude Cyclogenesis


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

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)

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)

Cyclone Climatology

From Zishka and Smith (1980)

1

2

4

3


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

Cyclone Climatology



Where do Surface Cyclones Die?

1. Many do not decay until well off the East Coast over the north-central Atlantic when they become “Icelandic Lows”

Related to occlusion and being cut-off from their source of warm moist (tropical) air

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)

Cyclone Climatology


2

1

3


Understanding CyclogenesisVorticity Perspective:

• Given that surface cyclones are always characterized by cyclonic vorticity maxima, cyclogenesis can be explained through analysis of the vertical vorticity equation:

• 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

y

F

x

F

pp

v

xp

u

yDt

D xya

a

Total Change Tilting Stretching Friction


Pressure Perspective:

• 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

• This can be accomplished through the approach of a diffluent trough

Note: The vorticity and pressure views are exactly consistent with one another, as each perspective emphasizes different aspects of the same circulation

L

546

552

558

Pressure Perspective Vorticity Perspective



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)



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”



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



PV Perspective:

• Developing cyclones always exhibit four (4) distinct PV anomalies:

1. Stratospheric cyclonic PV maximum2. Surface warm temperature maximum3. Low-level diabatic PV maximum4. 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 individually2. Superimpose upon one another3. Constructively interfere with one another



“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)

• Often triggered by an upper-level trough approaching a strong coastal baroclinic zone

• The “Perfect Storm” is a partial example• Also called “noreasters”


Location of Bombs (1979-1999)

KuroshioCurrent

GulfStream


“Bomb” Cyclones:

Important Physical Processes:

• Strong coastal baroclinic zone

• Strong pre-existing low-level vorticity

• Strong surface energy fluxes due to cold-dry air moving over a warm ocean current (Gulf Stream)

• 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)

Diffluent Trough Strong WAA

Heat and Moisture Fluxes

Coastal Front

L



15-16 April 2007 “Bomb Cyclone”

850 mbHeights Temps

SurfacePress

15 April 1200ZMSLP = 993 mb

300 mbHeights Winds

GOES - IR




850 mbHeights Temps

SurfacePress


300 mbHeights Winds

GOES - IR



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


East Coast – Miller “Type A” and “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”



Type A Type B


“Zipper” Lows:


• 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




Thermal Lows:


• 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



ReferencesBluestein, 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 surroundingoceans environs for January and July, 1950-1977. Mon. Wea. Rev., 108, 387-401.

Advanced SynopticM. D. Eastin Midlatitude Cyclogenesis.

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