Development of a physically-realistic model for thunderstorm downburst outflows for wind-resistant design of transmission line structures

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Downbursts are columns of air that descend and impact upon the ground, resulting in often-intense outflows containing a dominant vortex, whose underside is associated with the strongest wind speeds that may damage structures, notably electricity transmission line systems. In the field of wind engineering a model of downbursts has been adopted based upon an impulsively-started momentum jet, issuing from a nozzle, which then impinges upon the ground. The attraction of this model is that any "downburst" event may be linearly scaled, in both intensity and size, from a single axi-symmetric wind field realization (either from numerical or experimental modelling) by varying the jet velocity at the nozzle (intensity) or the nozzle diameter (spatial scaling). Recent work by the present applicant has shown that this model is fundamentally flawed because it contains none of the physics of a real event (in particular the baroclinic generation of vorticity is missing), with the result that the outflow wind velocity profile shape and magnitudes are incorrectly predicted. In addition, the impinging jet model is completely de-coupled from any of the physics of its parent storm. Finally, real downbursts are far from axi-symmetric and so any "design" wind speed taken from an impinging jet model will not represent the peak wind speed for that event. The objectives of the research are; (1) To develop a simple but physically-realistic model that describes the temporally and spatially varying near-ground outflow wind velocities of single thunderstorm downburst events, as well as multiple events that form a downburst line, incorporating the influence of local environmental (atmospheric) conditions, terrain roughness and topography; (2) To use this model to provide peak wind speed information for the design of transmission line structures. The objectives will be achieved by coupling results from world-leading, well-resolved, full thunderstorm cloud simulations, including precipitation microphysics, that produce downbursts with simpler numerical models based on Large-Eddy Simulations, supported by experimental data obtained from single and multiple density-driven translating downburst events in a novel large flume facility.