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Friction in Mid-latitude Cyclones

Friction in Mid-latitude Cyclones. Ian Boutle Bob Plant, Stephen Belcher, Bob Beare, Andy Brown. Motivation. Many studies have shown the significance of friction in formation and dissipation of cyclones Up to 50% reduced growth rate

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Friction in Mid-latitude Cyclones

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  1. Friction in Mid-latitude Cyclones Ian Boutle Bob Plant, Stephen Belcher, Bob Beare, Andy Brown

  2. Motivation • Many studies have shown the significance of friction in formation and dissipation of cyclones • Up to 50% reduced growth rate • Met Office Unified Model still has trouble with cyclone depth • Underlying physical mechanism not well understood

  3. 0 h Model Setup Near-surface Temperature 24 h Tropopause Upper Level Trough 48 h Idealised, dry setup – only dynamics and boundary layer scheme operating

  4. Ekman Pumping • Boundary layer forces convergence • Continuity forces ascent: • Ascent squashes • vortex, reducing • vorticity • What about • temperature? L

  5. Potential Vorticity • Frictional Term: • Ekman pumping • Baroclinic mechanism • Diabatic Term: • Surface heat fluxes • Latent heat fluxes

  6. Warm Conveyor Belt Baroclinic Mechanism Depth averaged PV generation in boundary layer: Depends on alignment of surface and thermal winds

  7. Baroclinic Mechanism 2 Mainly Static Stability Anomaly PV accumulates above Low centre

  8. ~PV ~Ekman Boundary Layer Stability PV mainly in SBL Unstable (Convective) BL Stable BL Surface stress mainly In CBL

  9. Summary • Boundary layer friction has a large affect on cyclone development • Ekman pumping is significant, but not the only process acting • PV perspective provides another mechanism, which appears similarly important

  10. Switching off BL • Surface stress parameterised in terms of “eddy diffusivities” • Define SBL by • Set here

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