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The Role of Forcing in Cell Morphology and Evolution within Midlatitude Squall Lines

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  • 1 Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois
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Abstract

This study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines. The forcing chosen was a cold front, simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front. Use of a realistic, balanced, and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing. Three-dimensional squall lines were modeled with and without the front present, in low and high bulk Richardson number environments. The forced convection evolved in significantly different ways than their isolated, unforced counterparts. In low-shear conditions, the line of isolated convective cells split, with the adjacent split cells interfering destructively with neighboring cells in the line. With forcing present, differences in anticyclonic cell intensity and propagation prevented this interaction from occurring, leading to longer-lived cyclonic convection despite a near-normal orientation between cloud-bearing shear and the convective line. The split-cell interaction also failed to occur under higher-shear conditions due to anticyclonic cell decay given the greater cyclonic hodograph curvature. In both low- and higher-shear environments, a strong bias toward cyclonic storms was noted with forcing present, due to shallower anticyclonic cells with the front present and to preexisting vorticity in the environment; updraft–vorticity correlations were skewed accordingly. Forcing also reduced the sensitivity of the evolving convection to detailed aspects of the initialization.

Corresponding author address: Brian F. Jewett, Department of Atmospheric Sciences, 105 S. Gregory St., Urbana, IL 61801. Email: bjewett@uiuc.edu

Abstract

This study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines. The forcing chosen was a cold front, simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front. Use of a realistic, balanced, and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing. Three-dimensional squall lines were modeled with and without the front present, in low and high bulk Richardson number environments. The forced convection evolved in significantly different ways than their isolated, unforced counterparts. In low-shear conditions, the line of isolated convective cells split, with the adjacent split cells interfering destructively with neighboring cells in the line. With forcing present, differences in anticyclonic cell intensity and propagation prevented this interaction from occurring, leading to longer-lived cyclonic convection despite a near-normal orientation between cloud-bearing shear and the convective line. The split-cell interaction also failed to occur under higher-shear conditions due to anticyclonic cell decay given the greater cyclonic hodograph curvature. In both low- and higher-shear environments, a strong bias toward cyclonic storms was noted with forcing present, due to shallower anticyclonic cells with the front present and to preexisting vorticity in the environment; updraft–vorticity correlations were skewed accordingly. Forcing also reduced the sensitivity of the evolving convection to detailed aspects of the initialization.

Corresponding author address: Brian F. Jewett, Department of Atmospheric Sciences, 105 S. Gregory St., Urbana, IL 61801. Email: bjewett@uiuc.edu

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