Empirical models for predicting unsteady-state downburst wind speeds

Recent reports of failures of transmission line systems in several regions around the world point to downburst events rather than other types of thunderstorms as the primary cause of failure. Downburst winds are becoming the governing type of design wind event in many areas around world. There is an essential need for developing analytical or empirical models to apply these types of loads to structural systems. Most of the available models, whether analytical or empirical, are restricted to steady state downburst flow and do not consider the changes in wind speeds with time. Downburst wind speeds continuously change with time through the life cycle of the event. The current study uses estimated ages for downburst events from several recorded field events combined with numerical simulation to establish a pair of intensity decay functions. These functions have the ability to depict the changes in the temporal profile of wind speeds with space and can be added to the earlier empirical and analytical steady state models to update them from steady state to unsteady state “time dependent” simulation. Once the parent storm speed has been determined and added to the model, a full scale transient downburst wind speed model in the 4-dimensions is developed. Finally, several field cases are studied to show the application and accuracy of the presented model.