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Midlatitude Cyclogenesis

Midlatitude Cyclogenesis. 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

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Midlatitude Cyclogenesis

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  1. Midlatitude Cyclogenesis M. D. Eastin

  2. 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 M. D. Eastin

  3. 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 M. D. Eastin

  4. 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) 1 2 3 4 From Zishka and Smith (1980) M. D. Eastin

  5. 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) M. D. Eastin

  6. 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) 1 2 3 From Zishka and Smith (1980) M. D. Eastin

  7. 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: • For a uniform frontal zonenear 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 • stretchingin the lower troposphere and an increase in surface cyclonic vorticity Total Change Tilting Stretching Friction M. D. Eastin

  8. Understanding Cyclogenesis • 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 546 552 558 L Pressure Perspective Vorticity Perspective M. D. Eastin

  9. 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) M. D. Eastin

  10. 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” M. D. Eastin

  11. 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 M. D. Eastin

  12. Understanding Cyclogenesis • PV Perspective: • Developing cyclones always exhibit • four (4) distinct PV anomalies: • 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: • Amplify individually • Superimpose upon one another • Constructively interfere with • one another M. D. Eastin

  13. 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) • 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) Kuroshio Current Gulf Stream M. D. Eastin

  14. Explosive Cyclogenesis • “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) Diffluent Trough Strong WAA Heat and Moisture Fluxes Coastal Front L From Bluestein (1993) M. D. Eastin

  15. 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 M. D. Eastin

  16. 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 M. D. Eastin

  17. 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 M. D. Eastin

  18. Explosive Cyclogenesis 15-16 April 2007 “Bomb Cyclone” M. D. Eastin

  19. 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 M. D. Eastin

  20. Cyclone Classifications • 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 From Bluestein (1993) M. D. Eastin

  21. 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 From Bluestein (1993) M. D. Eastin

  22. 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 M. D. Eastin

  23. 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. M. D. Eastin

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