Bow Echo Sensitivity to Ambient Moisture and Cold Pool Strength

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Bow Echo Sensitivity to Ambient Moisture and Cold
Pool Strength

• Richard P. James, Paul M. Markowski, and J.
  Michael Fritsch,
• 2006 Mon. Wea. Rev., 134, 950964.

Image source http//www.hpc.ncep.noaa.gov/researc
h/roth/072120031203zrad.gif
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Introduction

• Bow echoes are a common form of convective
  organization often associated with severe wind
  damage (Klimowski et al. 2003).
• Previous studies (Johns and Hirt 1987) examined
  warm-season derecho environments.
• Numerical modeling results have previously
  focused on sensitivity of bow echoes to CAPE and
  low-level shear.
• This paper attempts to address the role of
  ambient moisture even when CAPE is held constant.

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Methodology

• Numerical model (James et al. 2005)
• 360 x 120 km domain
• 500 m horizontal resolution
• 200 m vertical resolution below 5 km, increasing
  to 500 m above 8.5 km
• The model was initialized with warm bubbles
  spaced 20 km apart.
• Ice physics were included in all simulations.

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Methodology (cont.)

• Three different mixing ratio lapse rates were
  used (1 g kg-1 km-1, 3 g kg-1 km-1, and 4 g kg-1
  km-1)
• CAPE was held constant (4600 J kg-1) with a fixed
  temperature, pressure, and relative humidity at
  the top of the mixed layer.
• Simulations used 02.5-km shear magnitudes of 16,
  20, and 24 m s-1.
• All simulations had an integration length of 6 h.

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Results
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4 g kg-1 km-1
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3 g kg-1 km-1
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1 g kg-1 km-1
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Cold Pool Strength (C) and C/?U Ratio
t 4 h
t 6 h
CAPE 4600 J kg-1 02.5-km shear 24 m s-1
Low level mixing ratio lapse rate 4 g kg-1
km-1 (Most Moist)
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Cold Pool Strength (C) and C/?U Ratio
t 4 h
t 6 h
CAPE 4600 J kg-1 02.5-km shear 24 m s-1
Low level mixing ratio lapse rate 3 g kg-1 km-1
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Cold Pool Strength (C) and C/?U Ratio
t 4 h
t 6 h
CAPE 4600 J kg-1 02.5-km shear 24 m s-1
Low level mixing ratio lapse rate 1 g kg-1
km-1 (Least Moist)
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Conclusions

• Stronger cold pools are associated with larger
  scale three-dimensional features.
• An intermediate amount of ambient moisture
  results in the most robust bow echo formation.
• The cold pool strength balances the shear only
  locally allowing bowing segments to develop.
• Large CAPE and strong low-level shear constitute
  may represent necessary, but not sufficient,
  conditions for bow echo formation.