The evolution and structure of a "bow-echo-microburst' event. Part I: the microburst

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. -from Authors