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Stephanie M. Verbout, Harold E. Brooks, Lance M. Leslie, and David M. Schultz

Abstract

Over the last 50 yr, the number of tornadoes reported in the United States has doubled from about 600 per year in the 1950s to around 1200 in the 2000s. This doubling is likely not related to meteorological causes alone. To account for this increase a simple least squares linear regression was fitted to the annual number of tornado reports. A “big tornado day” is a single day when numerous tornadoes and/or many tornadoes exceeding a specified intensity threshold were reported anywhere in the country. By defining a big tornado day without considering the spatial distribution of the tornadoes, a big tornado day differs from previous definitions of outbreaks. To address the increase in the number of reports, the number of reports is compared to the expected number of reports in a year based on linear regression. In addition, the F1 and greater Fujita-scale record was used in determining a big tornado day because the F1 and greater series was more stationary over time as opposed to the F2 and greater series. Thresholds were applied to the data to determine the number and intensities of the tornadoes needed to be considered a big tornado day. Possible threshold values included fractions of the annual expected value associated with the linear regression and fixed numbers for the intensity criterion. Threshold values of 1.5% of the expected annual total number of tornadoes and/or at least 8 F1 and greater tornadoes identified about 18.1 big tornado days per year. Higher thresholds such as 2.5% and/or at least 15 F1 and greater tornadoes showed similar characteristics, yet identified approximately 6.2 big tornado days per year. Finally, probability distribution curves generated using kernel density estimation revealed that big tornado days were more likely to occur slightly earlier in the year and have a narrower distribution than any given tornado day.

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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
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Bogdan Antonescu, David M. Schultz, Hugo M. A. M. Ricketts, and Dragoş Ene

Abstract

Tornadoes and waterspouts have long fascinated humankind through their presence in myths and popular beliefs and originally were believed to have supernatural causes. The first theories explaining weather phenomena as having natural causes were proposed by ancient Greek natural philosophers. Aristotle was one of the first natural philosophers to speculate about the formation of tornadoes and waterspouts in Meteorologica (circa 340 BCE). Aristotle believed that tornadoes and waterspouts were associated with the wind trapped inside the cloud and moving in a circular motion. When the wind escapes the cloud, its descending motion carries the cloud with it, leading to the formation of a typhon (i.e., tornado or waterspout). His theories were adopted and further nuanced by other Greek philosophers such as Theophrastus and Epicurus. Aristotle’s ideas also influenced Roman philosophers such as Lucretius, Seneca, and Pliny the Elder, who further developed his ideas and also added their own speculations (e.g., tornadoes do not need a parent cloud). Almost ignored, Meteorologica was translated into Latin in the twelfth century, initially from an Arabic version, leading to much greater influence over the next centuries and into the Renaissance. In the seventeenth century, the first book-length studies on tornadoes and waterspouts were published in Italy and France, marking the beginning of theoretical and observational studies on these phenomena in Europe. Even if speculations about tornadoes and waterspouts proposed by Greek and Roman authors were cited after the nineteenth century only as historical pieces, core ideas of modern theories explaining these vortices can be traced back to this early literature.

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David M. Schultz, Timothy M. DelSole, Robert M. Rauber, and Walter A. Robinson
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David A. Williams, Kevin J. Horsburgh, David M. Schultz, and Chris W. Hughes

Abstract

On the morning of 23 June 2016, a 0.70-m meteotsunami was observed in the English Channel between the United Kingdom and France. This wave was measured by several tide gauges and coincided with a heavily precipitating convective system producing 10 m s−1 wind speeds at the 10-m level and 1–2.5-hPa surface pressure anomalies. A combination of precipitation rate cross correlations and NCEP–NCAR Reanalysis 1 data showed that the convective system moved northeastward at 19 ± 2 m s−1. To model the meteotsunami, the finite element model Telemac was forced with an ensemble of prescribed pressure forcings, covering observational uncertainty. Ensembles simulated the observed wave period and arrival times within minutes and wave heights within tens of centimeters. A directly forced wave and a secondary coastal wave were simulated, and these amplified as they propagated. Proudman resonance was responsible for the wave amplification, and the coastal wave resulted from strong refraction of the primary wave. The main generating mechanism was the atmospheric pressure anomaly with wind stress playing a secondary role, increasing the first wave peak by 16% on average. Certain tidal conditions reduced modeled wave heights by up to 56%, by shifting the location where Proudman resonance occurred. This shift was mainly from tidal currents rather than tidal elevation directly affecting shallow-water wave speed. An improved understanding of meteotsunami return periods and generation mechanisms would be aided by tide gauge measurements sampled at less than 15-min intervals.

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David A Williams, David M Schultz, Kevin J Horsburgh, and Chris W Hughes

Abstract

Meteotsunamis are shallow-water waves that, despite often being small (~ 0.3 m), can cause damage, injuries and fatalities due to relatively strong currents (> 1 m s−1). Previous case studies, modelling and localised climatologies have indicated that dangerous meteotsunamis can occur across northwest Europe. Using 71 tide gauges across northwest Europe between 2010–2017, a regional climatology was made to understand the typical sizes, times and atmospheric systems that generate meteotsunamis. A total of 349 meteotsunamis (54.0 meteotsunamis per year) were identified with 0.27–0.40 m median wave heights. The largest waves (~ 1 m high) were measured in France and the Republic of Ireland. Most meteotsunamis were identified in winter (43–59%), and the fewest identified meteotsunamis occurred in either spring or summer (0–15%). There was a weak diurnal signal, with most meteotsunami identifications between 1200–1859 UTC (30%) and fewest between 0000–0659 UTC (23%). Radar-derived precipitation was used to identify and classify the morphologies of mesoscale precipitating weather systems occurring within 6 h of each meteotsunami. Most mesoscale atmospheric systems were quasi-linear systems (46%) or open-cellular convection (33%), with some non-linear clusters (17%) and a few isolated cells (4%). These systems occurred under westerly geostrophic flow, with Proudman resonance possible in 43 out of 45 selected meteotsunamis. Because most meteotsunamis occur on cold winter days, with precipitation, and in large tides, wintertime meteotsunamis may be missed by eyewitnesses, helping to explain why previous observationally-based case studies of meteotsunamis are documented predominantly in summer.

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Paul J. Roebber, David M. Schultz, Brian A. Colle, and David J. Stensrud

Abstract

A large gap in skill between forecasts of the atmospheric circulation (relatively high skill) and quantitative precipitation (low skill) has emerged over the past three decades. One common approach toward closing this gap has been to try to simulate precipitation features directly by decreasing the horizontal grid spacing of the numerical weather prediction models. Also at this time, research has begun to explore the benefits of short-range ensemble forecast methods. The authors argue that each approach has benefits: high-resolution models assist in the development of a forecaster's conceptual model of various mesoscale phenomena, whereas ensembles help quantify forecast uncertainty. A thoughtful implementation of both approaches, in which this complementary nature is recognized, will improve the forecast process, empower human forecasters, and consequently add value relative to current trends. The science and policy issues that must be addressed in order to maximize this forecast potential are discussed.

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David M. Schultz, Derek S. Arndt, David J. Stensrud, and Jay W. Hanna

Abstract

A cold-air outbreak east of the Rocky Mountains on 23 January 2003 produced banded clouds and snow across the central and southeastern United States. The bands occurred through two processes: 1) thermal instability in the planetary boundary layer produced horizontal convective rolls (HCRs) over widespread areas, and 2) lake-effect processes downstream of small lakes (fetch < 100 km) produced localized bands. Characteristics of the observed bands associated with the HCRs, such as horizontal scale, depth of circulation, orientation, duration, and dynamics, are explored through observations, previous literature, and theoretical models. Snow from clouds produced by HCRs over land during the cold season has not been extensively studied previously. In this event, cold-air advection over the warm ground led to an upward sensible heat flux, promoting the occurrence of the HCR circulations. As the surface temperature decreased, the height of the lifting condensation level decreased, eventually forming cloud bands within the ascending portion of the HCR circulations. Ice crystals are inferred to have fallen from a large-scale precipitation system aloft into the cloud bands in the planetary boundary layer, which was within the favored temperature regime for dendritic growth of ice crystals. The ice crystals grew and reached the surface as light snow. This seeder–feeder process suggests one way to anticipate development of such snowbands in the future, as demonstrated by other similar events on other days in Oklahoma and Illinois. As the cloud bands were advected equatorward, they ingested drier air and dissipated. Among the several lake-effect bands observed on 23 January 2003, one notable band occurred downwind of Lake Kentucky. Midlake convergence of the land breeze may have initially produced a narrow cloud band that broadened as the land breeze ended. That the snowbands due to the HCRs and lake effect were both associated with heat and/or moisture fluxes from the earth's surface highlights the potential importance of ground- and water-surface temperature measurements for accurate numerical weather prediction.

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David A. Williams, David M. Schultz, Kevin J. Horsburgh, and Chris W. Hughes

Abstract

Meteotsunamis are shallow-water waves that, despite often being small (~0.3 m), can cause damage, injuries, and fatalities due to relatively strong currents (>1 m s−1). Previous case studies, modeling, and localized climatologies have indicated that dangerous meteotsunamis can occur across northwest Europe. Using 71 tide gauges across northwest Europe between 2010 and 2017, a regional climatology was made to understand the typical sizes, times, and atmospheric systems that generate meteotsunamis. A total of 349 meteotsunamis (54.0 meteotsunamis per year) were identified with 0.27–0.40-m median wave heights. The largest waves (~1 m high) were measured in France and the Republic of Ireland. Most meteotsunamis were identified in winter (43%–59%), and the fewest identified meteotsunamis occurred in either spring or summer (0%–15%). There was a weak diurnal signal, with most meteotsunami identifications between 1200 and 1859 UTC (30%) and the fewest between 0000 and 0659 UTC (23%). Radar-derived precipitation was used to identify and classify the morphologies of mesoscale precipitating weather systems occurring within 6 h of each meteotsunami. Most mesoscale atmospheric systems were quasi-linear systems (46%) or open-cellular convection (33%), with some nonlinear clusters (17%) and a few isolated cells (4%). These systems occurred under westerly geostrophic flow, with Proudman resonance possible in 43 out of 45 selected meteotsunamis. Because most meteotsunamis occur on cold winter days, with precipitation, and in large tides, wintertime meteotsunamis may be missed by eyewitnesses, helping to explain why previous observationally based case studies of meteotsunamis are documented predominantly in summer.

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David M. Schultz, Adam J. Durant, Jerry M. Straka, and Timothy J. Garrett

Abstract

Doswell has proposed a mechanism for mammatus called double-diffusive convection, the mechanism responsible for salt fingers in the ocean. The physics of salt fingers and mammatus are different. Unlike the ocean where the diffusivity is related to molecular motions within solution, the hydrometeors in clouds are affected by inertial and gravitational forces. Doswell misinterprets the vertical temperature profiles through mammatus and fails to understand the role of settling in volcanic ash clouds. Furthermore, given that mixing is a much more effective means of transferring heat in the atmosphere and given idealized numerical model simulations of mammatus showing that the destabilizing effect of subcloud sublimation is an effective mechanism for mammatus, this reply argues that double-diffusive convection is unlikely to explain mammatus, either in cumulonimbus anvils or in volcanic ash clouds.

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