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The Dissipation of a Left-Moving Cell in a Severe Storm Environment

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  • 1 Cooperative Institute for Research in the Atmosphere, Fort Collins, Colorado
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Abstract

Observations have shown that thunderstorms sometimes undergo updraft splitting, where one updraft moves to the right of the mean tropospheric wind and the other to the left. Observations also show that the left-moving updraft tends to dissipate approximately 15 min after the splitting process. The right-moving cell, however, may exist for up to a few hours.

Idealized modeling studies suggest that this behavior is related to the clockwise turning of the environmental shear vectors with height. The interaction between the environmental shear and the storms updraft produces a high–low pressure couplet oriented downshear. This pressure pattern produces favorable vertical accelerations for the right mover. This same process inhibits upward motion for the left mover.

In this paper an idealized simulation is presented that suggests an additional process that contributed to the decay of the left-moving updraft. Analysis of low-level storm-relative winds for the left-moving cell indicated that the inflow was from the cool precipitating downdraft. This inflow was characterized by negatively buoyant air. Subsequently the updraft dissipated approximately 1500 s after the precipitating downdraft formed. In contrast, the inflow for the right-moving updraft was partly from the downdraft and the undisturbed environment. A second simulation was run in which no precipitation was allowed to form, thus no downdraft formed. In that simulation the left mover was long lived. These results suggest that the simulated left-moving cell dissipated by ingesting downdraft air.

Corresponding author address: Dr. Lewis D. Grasso, NOAA/NESDIS/RAMMT CIRA, Colorado State University, West Laporte Avenue, Fort Collins, CO 80523.

Email: grasso@genesis.cira.colostate.edu

Abstract

Observations have shown that thunderstorms sometimes undergo updraft splitting, where one updraft moves to the right of the mean tropospheric wind and the other to the left. Observations also show that the left-moving updraft tends to dissipate approximately 15 min after the splitting process. The right-moving cell, however, may exist for up to a few hours.

Idealized modeling studies suggest that this behavior is related to the clockwise turning of the environmental shear vectors with height. The interaction between the environmental shear and the storms updraft produces a high–low pressure couplet oriented downshear. This pressure pattern produces favorable vertical accelerations for the right mover. This same process inhibits upward motion for the left mover.

In this paper an idealized simulation is presented that suggests an additional process that contributed to the decay of the left-moving updraft. Analysis of low-level storm-relative winds for the left-moving cell indicated that the inflow was from the cool precipitating downdraft. This inflow was characterized by negatively buoyant air. Subsequently the updraft dissipated approximately 1500 s after the precipitating downdraft formed. In contrast, the inflow for the right-moving updraft was partly from the downdraft and the undisturbed environment. A second simulation was run in which no precipitation was allowed to form, thus no downdraft formed. In that simulation the left mover was long lived. These results suggest that the simulated left-moving cell dissipated by ingesting downdraft air.

Corresponding author address: Dr. Lewis D. Grasso, NOAA/NESDIS/RAMMT CIRA, Colorado State University, West Laporte Avenue, Fort Collins, CO 80523.

Email: grasso@genesis.cira.colostate.edu

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