Boundary Layer Friction in

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Boundary Layer Friction in Mid-latitude Cyclones
Ian Boutle Bob Plant Stephen Belcher Bob
Beare Andy Brown
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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
• Recent papers have disagreed on mechanism

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Ekman Pumping

• Boundary layer forces convergence
• Continuity forces ascent
• Ascent squashes
• vortex, reducing
• vorticity

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Potential Vorticity

• Frictional Term
• Ekman pumping
• Baroclinic mechanism

• Diabatic Term
• Surface heat fluxes
• Latent heat fluxes

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Baroclinic Mechanism
PV flux from Boundary Layer
From Adamson et. al. (2006)
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Baroclinic Mechanism 2
Mainly Static Stability Anomaly
PV accumulates above Low centre
L
From Adamson et. al. (2006)
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Contrasting Views

• Ekman
• Robust in tropical atmosphere ocean
• Neglects temperature gradients in mid-latitudes

• Baroclinic PV
• Includes temperature gradients and fronts
• Neglects regions of constant temperature, e.g.
  Convective Boundary Layer

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Model Setup
0 h
24 h
PV2
Upper Level Trough
48 h
20m Temperature
SST constant at 290K
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Heat Flux Effect
BL depth averaged PV
Little motion ? PV stays here
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Low Level Jet
Cold Conveyor Belt
Low level jet
Maximum Ekman Pumping 9 cm/s
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Low Level Jet 2
Weak Low Level Jet
Strong Low Level Jet
LC1
UM
Strongest Winds
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Varying Sea Surface Temp
Peak Friction Velocity Due to Low Level Jet
Baroclinic PV generation In WCB region
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Story so far

• UM simulations didnt see baroclinic mechanism
  due to surface heat fluxes
• LC1 lifecycle has no cold conveyor belt, so
  maximum surface stress corresponds to region of
  PV generation
• Which mechanism dominates?
• When and why?

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East-West PV Cross Section
LC1
UM
max 3PVU
max 3.5PVU
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Zonal Mean Static Stability
LC1
UM
L
max 4.5x10-4 s-2
max 3x10-4 s-2
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What is the effect of theincreased Static
Stability?
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Boundary Layer Stability
PV mainly in SBL
Friction velocity mainly In CBL
Unstable BL
Stable BL
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Sensitivity to BL
PV
u
Average of u over cyclone
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Conjecture

• Ekman pumping is direct mechanism
• Stops pre-existing vortex by reducing angular
  momentum
• e.g. UM with finite amplitude perturbation
• Baroclinic mechanism is indirect
• Prevents growth of baroclinic wave
• e.g. LC1 where wave grows from normal mode

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Other Ideas

• Finite amplitude initial condition helps promote
  the cold-conveyor belt?
• Timing of UM runs 2-3 days, much quicker than
  LC1 lifecycle (5-10 days) transport and
  accumulation of PV is not effective?
• UM does not impose wavenumber 6 symmetry how
  does this affect cyclone?

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LC1-type cyclone in UM
Higher PV values
Increased static stability
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Future Work

• Investigate different types of cyclone, eg.
  Normal modes in UM
• Determine importance of each mechanism by
  switching of
• Remove PV anomaly and re-run experiment
• Other effects surface roughness, moisture,
  diurnal cycle etc.