Bow Echo Sensitivity to Ambient Moisture and Cold Pool Strength

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.