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.