Forcings, Characteristics, and Loadings of Damaging Winds in Derechos and Other High-Impact Thunderstorm Events

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Damaging thunderstorm-wind events in the US outnumber those of tornadoes by an order of magnitude, with greater cumulative losses. Most notably, the August 2020 derecho event in the Midwest US was the costliest thunderstorm event in US history at over $13Billion in losses. Critical questions regarding the mechanisms for such extreme wind generation remain unanswered, partly because, relative to tornadoes, thunderstorm winds have been understudied both by meteorologists and engineers. This Disaster Resilience Research Grants (DRRG) project will examine how characteristics of thunderstorm winds, including design-level peak wind speeds, relate to the different generating mechanisms of such winds. The project will also investigate spatial variabilities and temporal intermittencies associated with these mechanisms, which will contribute to the fundamental understanding of thunderstorm-wind generation and the variability observed in the field. The underlying focus of the project on hazardous thunderstorms will ultimately lead to better forecasts and warnings of thunderstorm-wind events, and to integration of the findings into building codes and standards. In addition, the project will train a new generation of interdisciplinary engineers and scientists who appreciate both the basic scientific problem, and the engineering solutions.Conceptual wind-damage models, especially for engineering, have focused on the “downburst” as being responsible for all extreme winds near the surface. However, there are other mechanisms for thunderstorm wind generation including the rear-inflow jet (RIJ) and mesovortices, which may be responsible for a significant proportion of damage from thunderstorms. RIJ-induced winds are suspected in the August 2020 derecho based on early results from numerical simulations. This project will examine alternative forcing mechanisms by combining existing observational data from the August 2020 derecho and other high-impact events, with post-event damage surveys, and to-be-developed high-resolution numerical simulations of convective-storm events. Three major research tasks will tease out the different mechanisms: (1) Numerical simulations of specific events, with resolution sufficient for determination of the wind-generation mechanism and associated forcings and reduced-complexity dynamical modeling to relate the wind forcings to the detailed characteristics of the thunderstorm winds. (2) Wind engineering analysis, which will match/compare the modeling results of specific events to existing thunderstorm wind data and post-event damage data as well as an analysis of the data generated from the higher resolution numerical models. This analysis will include four-dimensional assessment of engineering-relevant characteristics such as peak wind speed, wind speed profiles, and comparisons to existing engineering models of thunderstorm winds. (3) Climatological analysis of thunderstorm winds in the US, with specific interest in sub-classifying these winds by their generation mechanism (RIJ, downburst, mesovortex). Each mechanism will be assessed for their importance in wind loading of structures through the engineering characteristics in (2). This will connect the results from (1) and (2).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.