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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, 950 – 964. Image source: http://www.hpc.ncep.noaa.gov/research/roth/072120031203zrad.gif. Introduction.
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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, 950–964. Image source: http://www.hpc.ncep.noaa.gov/research/roth/072120031203zrad.gif
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.
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.
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.
Cold Pool Strength (C) and C/ΔU Ratio t = 4 h t = 6 h 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)
Cold Pool Strength (C) and C/ΔU Ratio t = 4 h t = 6 h 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
Cold Pool Strength (C) and C/ΔU Ratio t = 4 h t = 6 h 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)
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.