The Evolution and Structure of a “Bow-Echo–Microburst” Event. Part I: The Microburst

Abstract

On 14 July 1982, a comprehensive multi-Doppler radar dataset was collected during the life cycle of an intense microburst-producing thunderstorm during the Joint Airport Weather Studies (JAWS) Project. This is believed to be one of the first attempts to study the temporal and spatial evolution of an entire microburst-producing thunderstorm. In addition, the radar echo of the parent storm evolved into a bow-shaped echo, thus providing the first detailed dataset on this phenomenon. This microburst was numerically simulated using Srivastava's one-dimensional downdraft model in an attempt to quantify microphysical processes and downdraft propagation in the subcloud layer. Analysis of the stormwide structure using a standard multi-Doppler kinematic analysis technique and the Gal-Chen thermodynamic retrieval technique is stressed in this paper, which is Part 1 of a two-part study. Part II examines the evolution of the stormwide vorticity during the formation of the bow echo. The microburst downdraft was initiated primarily by precipitation loading near the cloud base [4.2 km above ground level (AGL)]. The sedimentation of precipitation particles not only promotes the propagation of the downdraft but also contributes to the total negative buoyancy through loading, sublimation, melting, and evaporation. A high perturbation pressure, located at the microburst center, is consistent with strong horizontal divergence and vertical compression of the air near the surface. Low perturbation pressure, which surrounds the high pressure, is associated with the vertical shear within the outflow as suggested by the three-dimensional (3D) diagnostic pressure equation. The maximum acceleration of the downdraft occurs between 2.4 and 1.6 km where both melting and evaporation contribute equally to the diabatic cooling. Numerical simulation shows the sequence of events beneath this thunderstorm. These include the sedimentation of a few large hydrometeors, descent of a radar reflectivity maximum, descent of a negative thermal buoyancy maximum, occurrence of a precipitation maximum, and finally, the downdraft-outflow maximum. The air parcels that arrive at the surface microburst center come from the subcloud layer on the northeast and southeast quadrant of the storm.