Advancing Our Understanding of Intraseasonal United States Severe Convective Storm Variability

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Severe thunderstorms and associated hazardous weather are often concentrated in time, where outbreaks of severe weather are clustered over short periods on the order of days to weeks. The frequency of favorable weather patterns for severe thunderstorms suggests the connection to large scale weather patterns. This award will allow for further study of the physical mechanisms that drive intraseasonal variability in severe convective storms. The outcomes from this award will be based on the potential for improvement in the prediction of severe weather outbreaks at the subseasonal scale, which is relevant to stakeholders in the agriculture, aviation, and insurance sectors of the economy. Outreach will be conducted to improve public understanding of science, and education and training will take place through the inclusion of multiple graduate and undergraduate students in the project. Students at the US Naval Academy will also be involved in the project as collaborators.The research team will conduct a study to advance community understanding of the physical mechanisms that drive observed intraseasonal variability in severe convective storms and their associated hazards. The researchers hypothesize that extremely active and inactive episodes of severe convective storms in the continental US may result from the projection of intraseasonal teleconnections onto synoptic and mesoscale severe weather parameters. The three sources of forcing for US weather variability are the Madden-Julian Oscillation (MJO), Atmospheric angular momentum (AAM) or Global Wind Oscillation (GWO), and the El Nino Southern Oscillation (ENSO). The following science questions will be addressed through the use of reanalysis data, historical records of severe storm activity, and compositing techniques: 1) What are the physical pathways by which the intraseasonal frequency of severe convective storms is modulated by the MJO and GWO, what fraction of intraseasonal severe storm variability is explained by the MJO and GWO, and at what time lags does the modulation pathway tend to be strongest and weakest? 2) What observational evidence, taken from Rossby wave packet tracing and composites of synoptic- and mesoscale variables known to be associated with severe weather in the U.S. physically supports intraseasonal severe weather variability? 3) How does the modulation of severe weather events and favorable environmental conditions vary by geographic region, by time of year, by diurnal cycle, and by phase of ENSO and how does the time lag identified in question 1 vary with any of the above factors? 4) How common are intraseasonal Forecasts of Opportunity, null events, and extended periods with little-to-no severe convective storm activity and on what do they depend?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.