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Structure and Evolution of a Long-Lived, Microburst-Producing Storm

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  • 1 Atmospheric Science Department, University of Alabama in Huntsville, Huntsville, Alabama
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Abstract

This paper describes an analysis of a long-lived, microburst-producing storm that evolved within a relatively dry environment having a relatively low CAPE value of 450 J kg−1. The storm displayed a variety of kinematic and echo formations over its 2.5-h lifetime, including 1) a near equality in the strength (∼10 m s−1) of updrafts and downdrafts, 2) strong downdrafts over an extended time period of greater than 60 min, 3) a prevalence of up-down-type downdraft trajectories associated with the strong downdrafts, 4) a prominent echo overhang during the early mature stage, 5) a spearhead-like echo protrusion during the mature storm phase that was indirectly associated with strong downdrafts, and 6) a narrow bow echo and associated weak inflow jet at midlevels during the latter storm stage.

An elongated ascending branch of the up-down downdraft circulation was associated with the echo protrusion. The prominence of the up-down trajectory is corroborated by surface data and 3D numerical simulations, both of which reveal comparable values of equivalent potential temperature in the low-level inflow and downdraft outflow air. Time series plots of saturation point reveal an evaporation line structure typical of evaporation of precipitation into the subcloud boundary layer. Thus, in this case there is little evidence to indicate that significant amounts of downdraft air originated above the atmospheric boundary layer during the sustained mature to dissipating stages.

Abstract

This paper describes an analysis of a long-lived, microburst-producing storm that evolved within a relatively dry environment having a relatively low CAPE value of 450 J kg−1. The storm displayed a variety of kinematic and echo formations over its 2.5-h lifetime, including 1) a near equality in the strength (∼10 m s−1) of updrafts and downdrafts, 2) strong downdrafts over an extended time period of greater than 60 min, 3) a prevalence of up-down-type downdraft trajectories associated with the strong downdrafts, 4) a prominent echo overhang during the early mature stage, 5) a spearhead-like echo protrusion during the mature storm phase that was indirectly associated with strong downdrafts, and 6) a narrow bow echo and associated weak inflow jet at midlevels during the latter storm stage.

An elongated ascending branch of the up-down downdraft circulation was associated with the echo protrusion. The prominence of the up-down trajectory is corroborated by surface data and 3D numerical simulations, both of which reveal comparable values of equivalent potential temperature in the low-level inflow and downdraft outflow air. Time series plots of saturation point reveal an evaporation line structure typical of evaporation of precipitation into the subcloud boundary layer. Thus, in this case there is little evidence to indicate that significant amounts of downdraft air originated above the atmospheric boundary layer during the sustained mature to dissipating stages.

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