Bow Echo Mesovortices' Part II: Genesis Mechanisms

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Bow Echo Mesovortices. Part II Genesis
Mechanisms

• Michael St.Laurent
• and
• Nolan T. Atkins

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Motivation
Trapp and Weisman 2003
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Motivation
Wakimoto et al. 2006
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Motivation
Tilting Mechanism
South
North
South
North
Downdraft
Anticyclonic
Cyclonic
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Motivation

• Very Little evidence of couplets in single
  Doppler studies
• Wakimoto et al 2006
• Atkins et al 2004, 2005
• Cyclonic only mesovortices are observed
• Suggests that their may be some other genesis
  mechanism.

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Model Simulation Details

• 10 June 2003 St.Louis Bow Echo
• Horizontal domain size 225km x 320km
• Horizontal resolution 500m
• Ice cloud microphysics

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Areas of Interest

• Two main types
• Cyclonic Mesovortices
• Mesovortex Couplets

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Cyclonic Mesovortices
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Vorticity Equation Approach
Advection of vertical vorticity
Tilting of horizontal vorticity
Stretching of vertical vorticity
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Time Integrated Tendency Terms

• Descending parcel from mid-levels.
• Positively tilting of streamwise horizontal
  vorticity.
• Vertical vorticity stretching by updraft at gust
  front.
• Cyclonic only mesovortices are observed.

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Supercell Low Level Mesocyclone Similarity
Rotunno and Klemp 1985
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Mesovortex Couplets

• Convective-scale downdraft
• Bulge in gust front
• Local maximum in updraft

Rain Water Mixing Ratio (g/kg) Streamlines Downdra
ft (m/s) U Component (m/s)
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Mesovortex Couplet Formation
Updraft (m/s) Vertical Vorticity (gt 1200
s-1) Gust front Vortex Lines
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Bow Echo Book-End Vortex Similarity
Weisman and Davis 1998
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Summary

• Cyclonic Only Mesovortices
• Tilting of baroclincally generated streamwise
  vorticity, which is then stretched by updraft at
  the gust front.
• Similar to the Rotunno and Klemp supercell low
  level mesocyclone formation.
• Mesovortex Couplets
• Convective scale downdraft causes a local updraft
  maximum at the gust front.
• The updraft maximum then tilts north-south
  oriented vortex lines causing a cyclonic
  mesovortex to the north, and anticyclonic
  mesovortex to the south.
• Similar to the process that forms the bow echo
  book-end vortices.

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Acknowledgements

• National Science Foundation under Grant
  ATM-0630445
• Vermont ESPCoR EPS-0236976