Bow Echo Sensitivity to Ambient Bow Echo Sensitivity to Ambient Moisture and Cold Pool StrengthMoisture and Cold Pool Strength

Richard P. James, Paul M. Markowski, and J. Michael Fritsch,

2006: Mon. Wea. Rev., 134, 950–964.

Image source: http://www.hpc.ncep.noaa.gov/research/roth/072120031203zrad.gif

IntroductionIntroduction

• 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.

MethodologyMethodology

• 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.

Methodology (cont.)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 0–2.5-km shear magnitudes of 16, 20, and 24 m s−1.

• All simulations had an integration length of 6 h.

ResultsResults

4 g kg−1 km−1

3 g kg−1 km−1

1 g kg−1 km−1

Cold Pool Strength (C) and C/Cold Pool Strength (C) and C/ΔΔU RatioU Ratio

CAPE = 4600 J kg−1 0–2.5-km shear = 24 m s−1

Low level mixing ratio lapse rate = 4 g kg−1 km−1 (Most Moist)

t = 6 h

t = 4 h

Cold Pool Strength (C) and C/Cold Pool Strength (C) and C/ΔΔU RatioU Ratio

CAPE = 4600 J kg−1 0–2.5-km shear = 24 m s−1

Low level mixing ratio lapse rate = 3 g kg−1 km−1

t = 6 h

t = 4 h

Cold Pool Strength (C) and C/Cold Pool Strength (C) and C/ΔΔU RatioU Ratio

CAPE = 4600 J kg−1 0–2.5-km shear = 24 m s−1

Low level mixing ratio lapse rate = 1 g kg−1 km−1 (Least Moist)

t = 4 h

t = 6 h

ConclusionsConclusions

• 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.