Vittorio A. Gensini

Climate and Hazardous Convective Weather

Substantial progress has been made recently relating the large-scale climate system and hazardous convective weather (HCW; tornadoes, hail, and damaging wind), particularly over the USA where there are large societal impacts and a long observational record. Despite observational data limitations, HCW has shown to be influenced by the climate system and the tropical atmosphere via the Madden-Julian Oscillation and El Niño-Southern Oscillation. Analysis of the atmospheric environments favorable to HCW (e.g., convective available potential energy and vertical wind shear) avoids observational and model limitations. While few robust trends are seen over recent decades, future climate projections indicate increased frequency of such environments over the USA, Europe, and Australia, suggesting increased future HCW activity. A recent increase in the year-to-year variability of US tornado occurrence is striking, but not yet understood. Dynamical downscaling to convection-permitting resolutions promises improved understanding of the relationships between large-scale climate and HCW occurrence.

Estimations of Hazardous Convective Weather in the United States Using Dynamical Downscaling

High-resolution (4km; hourly) regional climate modeling is utilized to resolve March‐May hazardous convective weather east of the U.S. Continental Divide for a historical climate period (1980‐90). A hazardous convectiveweathermodelproxyisusedtodepictoccurrencesoftornadoes,damagingthunderstorm windgusts, and large hail at hourly intervals during the period of record. Through dynamical downscaling, the regional climate model does an admirable job of replicating the seasonal spatial shifts of hazardous convective weather occurrence during the months examined. Additionally, the interannual variability and diurnal progression of observed severe weather reports closely mimic cycles produced by the regional model. While this methodology has been tested in previous research, this is the first study to use coarse-resolution global climate model data to forceahigh-resolutionregional modelwithcontinuousseasonal integration intheUnitedStates for purposes of resolving severe convection. Overall, it is recommended that dynamical downscaling play an integral role in measuring climatological distributions of severe weather, both in historical and future climates.

Tornado Frequency in the United States Related to Global Relative Angular Momentum

AbstractGlobal relative angular momentum and the first time derivative are used to explain nearly an order of magnitude of the variability in 1994–2013 U.S. boreal spring tornado occurrence. When plotted in a phase space, the global wind oscillation (GWO) is obtained. This global index accounts for changes in the global budget of angular momentum through interactions of tropical convection anomalies and extratropical dynamics including the engagement of surface torques. It is shown herein that tornadoes are more likely to occur in low angular momentum base states and less likely to occur in high angular momentum base states. When excluding weak GWO days, a maximum average of 3.9 (E)F1+ tornadoes per day were found during phase 1. This decreases to a minimum of 0.9 (E)F1+ tornadoes per day during phase 5. Composite environmental analysis suggests that increases/decreases in tornado occurrence are closely associated with anomalies in tropospheric ingredients necessary for tornadic storms. In addition, torna...

Tornado Debris Characteristics And Trajectories During The 27 April 2011 Super Outbreak As Determined Using Social Media Data

Using publicly available information gleaned from over 1700 found-and-returned objects on the “Pictures and Documents found after the 27 April 2011 Tornadoes” Facebook page, the authors have created a database of 934 objects lofted by at least 15 different tornadoes during the 27 April 2011 Super Outbreak in the southeast United States. Analysis of the takeoff and landing points of these objects using GIS and high-resolution numerical trajectory modeling techniques extends previous work on this subject that used less specific information for much smaller sets of tracked tornado debris. It was found that objects traveled as far as 353 km, exceeding the previous record for the longest documented tornado debris trajectory. While the majority of debris trajectories were 10° to the left of the average tornado track vector, the longest trajectories exhibited a previously undocumented tendency toward the right of the average tornado track vector. Based on results from a high-resolution trajectory model, a relati...

Modulation of Annual Cycle of Tornadoes by El Niño–Southern Oscillation

Long-term trends suggest shifts toward earlier tornado season peaks, and yet fail to examine the role of year-to-year climate variability. Here, El Niño–Southern Oscillation phase is demonstrated to influence annual cycle characteristics of United States tornadoes. Observations and favorable environments show substantial modification of the peak spatial distribution and the temporal onset of tornado occurrence. La Niña produces an earlier annual peak probability by 1.5–2 weeks, with a higher overall fraction of events in March and April. In contrast, El Niño leads to a week delay in the maximum probability and enhances a second peak in the fall months. Consequently, this suggests that climate change is not the sole driver of changes to seasonal onset and peak, and climate variability plays an important role in modulating the annual cycle. Plain Language Summary What drives the onset of tornado season, and can we point to a changing climate as the cause of a trend toward an earlier season start date? In this paper, climate variability driven by equatorial Pacific sea surface temperature variations (El Niño–Southern Oscillation) is shown to modulate the characteristics of when the majority of tornadoes in any given year occur, and the timing of the season peak. La Niña is shown to shift the bulk of tornado season earlier by 1.5–2 weeks relative to years when no Pacific sea surface temperature anomaly exists, while El Niño delays the maximum daily frequency by a week. This result contrasts earlier studies that show long-term trends of a similar or smaller magnitude, which had previously suggested that climate variability does not modulate the annual cycle. The results here provide the potential for guidance as to season onset in developing seasonal tornado outlooks.

Spatial trends in United States tornado frequency

Severe thunderstorms accompanied by tornadoes, hail, and damaging winds cause an average of 5.4 billion dollars of damage each year across the United States, and 10 billion-dollar events are no longer uncommon. This overall economic and casualty risk—with over 600 severe thunderstorm related deaths in 2011—has prompted public and scientific inquiries about the impact of climate change on tornadoes. We show that national annual frequencies of tornado reports have remained relatively constant, but significant spatially-varying temporal trends in tornado frequency have occurred since 1979. Negative tendencies of tornado occurrence have been noted in portions of the central and southern Great Plains, while robust positive trends have been documented in portions of the Midwest and Southeast United States. In addition, the significant tornado parameter is used as an environmental covariate to increase confidence in the tornado report results.ATMOSPHERIC HAZARDS: understanding tornado trendsWhile the number of tornadoes has remained fairly static in the United States over the past 40 years, strong geographic contrasts are apparent. Tornadoes have the potential to cause severe damage, yet understanding their changes in time—particularly the impact of anthropogenic warming—has been hampered by sparse observations. Vittorio Gensini and Harold Brooks, from Northern Illinois University and the National Severe Storms Laboratory, respectively, therefore use a proxy of tornado activity—the significant tornado parameter, STP—to investigate regional trends since 1979. Tornado counts have increased in the Southeast, Midwest, and Northeast, as represented by a robust upward trend in the STP. In the southern Great Plains, by contrast, negative trends are apparent. These regional differences emphasise the need to consider geographic variability when assessing projected shifts in tornado hazards.