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

From Zishka and Smith (1980) 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 4 3 From Zishka and Smith (1980) Advanced Synoptic M. D. Eastin

From Zishka and Smith (1980) 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

From Zishka and Smith (1980) 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) 2 1 3 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: 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 Total Change Tilting Stretching Friction Advanced Synoptic M. D. Eastin

Vorticity Perspective 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 L 546 552 558 Pressure Perspective Vorticity Perspective 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: 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 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) 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 Advanced Synoptic M. D. Eastin

Heat and Moisture Fluxes 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) From Bluestein (1993) Diffluent Trough Strong WAA Heat and Moisture Fluxes Coastal Front L 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” 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) 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 From Bluestein (1993) Advanced Synoptic 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 Advanced Synoptic M. D. Eastin 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