Friction in Mid-latitude
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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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Friction in mid latitude cyclones

Friction in Mid-latitude

Cyclones

Ian Boutle

Bob Plant, Stephen Belcher, Bob Beare, Andy Brown


Motivation

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


Model setup

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


Ekman pumping

Ekman Pumping

  • Boundary layer forces convergence

  • Continuity forces ascent:

  • Ascent squashes

  • vortex, reducing

  • vorticity

  • What about

  • temperature?

L


Potential vorticity

Potential Vorticity

  • Frictional Term:

  • Ekman pumping

  • Baroclinic mechanism

  • Diabatic Term:

  • Surface heat fluxes

  • Latent heat fluxes


Baroclinic mechanism

Warm Conveyor Belt

Baroclinic Mechanism

Depth averaged PV generation in boundary layer:

Depends on alignment of surface and thermal winds


Baroclinic mechanism 2

Baroclinic Mechanism 2

Mainly Static

Stability Anomaly

PV accumulates above

Low centre


Boundary layer stability

~PV

~Ekman

Boundary Layer Stability

PV mainly

in SBL

Unstable

(Convective) BL

Stable BL

Surface

stress

mainly

In CBL


Summary

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


Switching off bl

Switching off BL

  • Surface stress parameterised in terms of “eddy diffusivities”

  • Define SBL by

  • Set here


  • Login