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Friction in Mid-latitude Cyclones

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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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slide1

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
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