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Theories of Blocking

Theories of Blocking. Brian Hoskins. B. Bolin (1949) in Beggren et al, Tellus 1. What is a block?. Chicago School 1940s Rossby, Bolin, Rex. A block for the westerlies with their weather systems. H. H. Z 500. L. L. A block for the westerlies with their weather systems. +. +.

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Theories of Blocking

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  1. Theories of Blocking Brian Hoskins B. Bolin (1949) in Beggren et al, Tellus 1

  2. What is a block? Chicago School 1940s Rossby, Bolin, Rex A block for the westerlies with their weather systems H H Z500 L L

  3. A block for the westerlies with their weather systems + + PV on θ - -

  4. A block for the westerlies with their weather systems W W θ on PV2 C C

  5. Some Theories • Hydraulic jump, Rossby • State with amplified Rossby wave ridge • e.gCharney & Devore (1979) Mulitipleequilibria in the presence of topography • Self-stabilisation of a vortex dipole in a westerly flow • e.g. Modon (McWilliams, 1980, 1981) • Soliton • Instability of a basic wavy westerly flow • e.gFrederiksen (1982) W C

  6. Transient eddy feedbacks Shutts (1983) eddy straining Hoskins, James & White (1983) W C PV inversion, e.g. HMR (1985)

  7. Group velocity considerations • Wave activity leakage, • e.g. On uniform westerly flow , U, • barotropic stationary waves exist & have eastward group velocity 2U • Modons constructed by McWilliams on equivalent barotropic westerly flow for which stationary waves do not exist. • Swanson (2001,2) considered an equivalent barotropic atmosphere with 2 regions of uniform PV separated by a discontinuity: • Possibility of R waves with zero group velocity & phase speed

  8. Initial value problem Swanson 2002

  9. B = north - south Onset Pelly & H (2003) Tyrlis & H (2008) θ on PV2

  10. Central latitude: Line of latitude (Tibaldi & Monteni, 1990), mean jet or storm-track latitude (x) (Pelly & H, 2003) or 2-D e.g. Berrisford et al (2007) for SH, Woollings et al (2008) for NH By interaction with subsequent weather systems Drip feed or big meal? Reinforcement

  11. Importance of diabatic processes? Erosion of blocking high by radiative damping ~ 1 week Erosion of cut-off low by latent heat release ~ days Anchoring by underlying surface? Importance of ambient planetary waves & forcing of them e.g. Rockies but also anomalous SSTs, sea ice,.... How non-linear is the behaviour of blocking? persistence Masato et al (2009) frequency of occurrence Scaife et al (2010)

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