A parametric study of downburst line near-surface outflows

Large Eddy Simulations (LES) of two identical downburst events, representing part of a downburst line, with a range of spatial (1, 2, and 3 km) and temporal (30, 60, 90, and 120 s) separations, are used to compare near-surface outflow features with those of an isolated event. Peak maximum wind speed and total damaging surface footprints are used to determine the expected critical wind loading conditions and likelihood of a surface structure being exposed to such an event. The results show that significant wind speed amplification factors of up to 1.55 and increases in damaging surface footprint of at least 70% can be expected from a two event downburst line. In addition, the peak maximum horizontal velocity in a downburst line is associated with smaller scale structures in the flow caused by downdraft interactions aloft, as opposed to the primary ring vortex expected for an isolated event. Subsequently, conventional downburst velocity profiles, used for an isolated event, cannot be applied in the collision region of a downburst line.