The Diffuse Source at the Center of LMC SNR 0509-67.5 is a Background Galaxy at z = 0.031

Type Ia supernovae (SNe Ia) are well-known for their use in the measurement of cosmological distances, but our continuing lack of concrete knowledge about their progenitor stars is both a matter of debate and a source of systematic error. In our attempts to answer this question, we presented unambiguous evidence that LMC SNR 0509-67.5, the remnant of an SN Ia that exploded in the Large Magellanic Cloud 400 +/- 50 years ago, did not have any point sources (stars) near the site of the original supernova explosion, from which we concluded that this particular supernova must have had a progenitor system consisting of two white dwarfs (Schaefer & Pagnotta 2012). There is, however, evidence of nebulosity near the center of the remnant, which could have been left over detritus from the less massive WD, or could have been a background galaxy unrelated to the supernova explosion. We obtained long-slit spectra of the central nebulous region using GMOS on Gemini South to determine which of these two possibilities is correct. The spectra show H-alpha emission at a redshift of z = 0.031, which implies that the nebulosity in the center of LMC SNR 0509-67.5 is a background galaxy, unrelated to the supernova.Comment: 2 figures, accepted for publication in Ap

Spatial and Temporal Analysis of Tornado Fatalities in the United States: 1880-2005

A dataset of killer tornadoes is compiled and analyzed spatially in order to assess region-specific vulnerabilities in the United States from 1880 to 2005. Results reveal that most tornado fatalities occur in the lower–Arkansas, Tennessee, and lower–Mississippi River valleys of the southeastern United States—a region outside of traditional “tornado alley.” Analysis of variables including tornado frequency, land cover, mobile home density, population density, and nocturnal tornado probabilities demonstrates that the relative maximum of fatalities in the Deep South and minimum in the Great Plains may be due to the unique juxtaposition of both physical and social vulnerabilities. The spatial distribution of these killer tornadoes suggests that the above the national average mobile home density in the Southeast may be a key reason for the fatality maximum found in this area. A demographic analysis of fatalities during the latter part of the database record illustrates that the middle aged and elderly are at a much greater risk than are younger people during these events. Data issues discovered during this investigation reveal the need for a concerted effort to obtain critical information about how and where all casualties occur during future tornado and hazardous weather events. These new, enhanced data, combined with results of spatially explicit studies exploring the human sociology and psychology of these hazardous events, could be utilized to improve future warning dissemination and mitigation techniques

Flood Fatalities in the United States

This study compiles a nationwide database of flood fatalities for the contiguous United States from 1959 to 2005. Assembled data include the location of fatalities, age and gender of victims, activity and/or setting of fatalities, and the type of flood events responsible for each fatality report. Because of uncertainties in the number of flood deaths in Louisiana from Hurricane Katrina, these data are not included in the study. Analysis of these data reveals that a majority of fatalities are caused by flash floods. People between the ages of 10 and 29 and 60 yr of age are found to be more vulnerable to floods. Findings reveal that human behavior contributes to flood fatality occurrences. These results also suggest that future structural modifications of flood control designs (e.g., culverts and bridges) may not reduce the number of fatalities nationwide. Spatially, flood fatalities are distributed across the United States, with high-fatality regions observed along the northeast Interstate-95 corridor, the Ohio River valley, and near the Balcones Escarpment in south-central Texas. The unique distributions found are likely driven by both physical vulnerabilities for flooding as well as the social vulnerabilities

The Storm Morphology of Deadly Flooding Events in the United States

This study investigates the synoptic and mesoscale environments associated with deadly flooding events in the United States from 1996 to 2005. A manual environment classification scheme, which includes analyses of surface charts, 500 hPa maps, and composite radar data (where available), is utilized to ascertain the primary ascent mechanisms and storm types producing these fatal flood events. Of the ten classifications in the scheme, the two most dominant ascent mechanisms associated with deadly floods include frontal boundaries (45%) and tropical systems (22%). Findings illustrate that mesoscale convective systems were responsible for 36% of the total number of flood fatalities over the period. The ten classifications are spatially and temporally analysed in order to assess region-specific risks associated with deadly flooding events

The relationship between tornadic and nontornadic convective wind fatalities and warnings

A database of tornado fatalities, nontornadic convective wind fatalities, severe thunderstorm warnings, and tornado warnings was compiled for the period 1986–2007 to assess the spatial and temporal distribution of warned and unwarned fatalities. The time of fatality and location as reported in Storm Data was compared to tornado and severe thunderstorm warnings to determine if a warning was in effect when the fatality occurred. Overall, 23.7% of tornado fatalities were unwarned, while 53.2% of nontornadic convective wind fatalities were unwarned. Most unwarned tornado fatalities occurred prior to the mid-1990s—coinciding with modernization of the National Weather Service—while unwarned nontornadic convective wind fa- talities remained at a relatively elevated frequency throughout the study period. Geographic locations with high numbers of unwarned tornado and nontornadic convective wind fatalities were associated with one high-magnitude event that was unwarned rather than a series of smaller unwarned events over the period. There are many factors that contribute to warning response by the public, and the issuance of a severe thunderstorm or tornado warning is an important initial step in the warning process. A better understanding of the characteristics of warned and unwarned fatalities is important to future reduction of unwarned fatalities

Recipe for Disaster: How the Dynamic Ingredients of Risk and Exposure Are Changing the Tornado Disaster Landscape

Tornado disasters and their potential are a product of both hazard risk and underlying physical and social vulnerabilities. This investigation appraises exposure, which is an important component and driver of vulnerability, and its interrelationship with tornado risk in the United States since the mid-twentieth century. The research demonstrates how each of these dynamic variables have evolved individually and interacted collectively to produce differences in hazard impact and disaster potential at the national, regional, and local scales. Results reveal that escalating tornado impacts are driven fundamentally by growing built-environment exposure. The increasing tornado disaster probability is not uniform across the landscape, with the mid-South region containing the greatest threat based on the juxtaposition of an immense tornado footprint risk and elevated exposure/development rates, which manifests—at least for one important impact marker—in the area’s high mortality rate. Contemporary, high-impact tornado events are utilized to emphasize how national- and regional-level changes in exposure are also apparent at the scale of the tornado. The study reveals that the disaster ingredients of risk and exposure do vary markedly across scales, and where they have increasing and greater overlap, the probability of disaster surges. These findings have broad implications for all weather and climate hazards, with both short- and long-term mitigation strategies required to reduce future impacts and to build resilience in the face of continued and amplifying development in hazard-prone regions.Tornado disasters and their potential are a product of both hazard risk and underlying physical and social vulnerabilities. This investigation appraises exposure, which is an important component and driver of vulnerability, and its interrelationship with tornado risk in the United States since the mid-twentieth century. The research demonstrates how each of these dynamic variables have evolved individually and interacted collectively to produce differences in hazard impact and disaster potential at the national, regional, and local scales. Results reveal that escalating tornado impacts are driven fundamentally by growing built-environment exposure. The increasing tornado disaster probability is not uniform across the landscape, with the mid-South region containing the greatest threat based on the juxtaposition of an immense tornado footprint risk and elevated exposure/development rates, which manifests—at least for one important impact marker—in the area’s high mortality rate. Contemporary, high-impact tornado events are utilized to emphasize how national- and regional-level changes in exposure are also apparent at the scale of the tornado. The study reveals that the disaster ingredients of risk and exposure do vary markedly across scales, and where they have increasing and greater overlap, the probability of disaster surges. These findings have broad implications for all weather and climate hazards, with both short- and long-term mitigation strategies required to reduce future impacts and to build resilience in the face of continued and amplifying development in hazard-prone regions

A Climatology of Fatal Convective Wind Events by Storm Type

There are still hundreds of casualties produced by thunderstorm hazards each year in the United States despite the many recent advances in prediction and mitigation of the effects of convective storms. Of the four most common thunderstorm hazards (wind, hail, flooding, and lightning), convective winds (tornadic and nontornadic) remain one of the most dangerous threats to life and property. Using thunderstorm fatality and Weather Surveillance Radar-1988 Doppler (WSR-88D) data, this research illustrates a spatial and temporal analysis of the storm morphological characteristics, or convective mode, of all fatal tornadic and nontornadic convective wind events from 1998 to 2007. The investigation employs a radar-based morphology classification system that delineates storm type based on an organizational continuum, including unorganized cellular, quasi-organized cellular (either a cluster of cells or a broken line of cells), organized cellular (supercells and supercells embedded in an organized linear system), and organized linear (either squall lines or bow echoes). Results illustrate that over 90%% of the 634 recorded tornado deaths were associated with supercells, with 78%% of the deaths due to isolated tornadic supercells and 12%% linked to tornadic supercells embedded within an organized linear convective system. The morphologies responsible for the 191 nontornadic convective wind fatalities vary substantially, with bow echoes (24%%), squall lines (19%%), and clusters of cells (19%%) the most prominent convective modes producing fatalities. Unorganized and quasi-organized convection accounted for nearly half (45%%) of all nontornadic convective wind fatalities. Over half of all fatal tornadoes (53%%) occurred between 0000 and 0600 UTC, and most (59%%) fatalities from nontornadic convective winds occurred in the afternoon between 1800 and 0000 UTC. Two corridors of nontornadic convective wind fatalities were present: the lower Great Lakes region and the mid-South. Tornado fatalities were greatest in a zone extending from southeastern Missouri, through western Tennessee, northeastern Arkansas, Mississippi, Alabama, and Georgia. The methods employed and results found in this study are directly applicable in the further development of storm classification schemes and provide forecasters and emergency managers with information to assist in the creation and implementation of new convective wind mitigation strategies

On the episodic nature of derecho-producing convective systems in the United States

Convectively generated windstorms occur over broad temporal and spatial scales; however, one of the larger-scale and most intense of these windstorms has been given the name ‘derecho’. This study illustrates the tendency for derechoproducing mesoscale convective systems to group together across the United States – forming a derecho series. The derecho series is recognized as any succession of derechos that develop within a similar synoptic environment with no more than 72 h separating individual events. A derecho dataset for the period 1994–2003 was assembled to investigate the groupings of these extremely damaging convective wind events. Results indicate that over 62% of the derechos in the dataset were members of a derecho series. On average, nearly six series affected the United States annually. Most derecho series consisted of two or three events; though, 14 series during the period of record contained four or more events. Two separate series involved nine derechos within a period of nine days. Analyses reveal that derecho series largely frequent regions of the Midwest, Ohio Valley, and the south–central Great Plains during May, June, and July. Results suggest that once a derecho occurred during May, June, or July, there was a 58% chance that this event was the first of a series of two or more, and about a 46% chance that this was the first of a derecho series consisting of three or more events. The derecho series climatology reveals that forecasters in regions frequented by derechos should be prepared for the probable regeneration of a derecho-producing convective system after an initial event occurs

Climatological Radar Delineation of Urban Convection for Atlanta, Georgia

The distribution of warm season (June through August) thunderstorm activity surrounding Atlanta, Georgia from 1997 to 2006 was determined utilizing composite reflectivity data obtained from the network of National Weather Service radars. The radar data, at 2 km and 5 min spatial and temporal resolutions, allows for high resolution analyses of urban convective trends when grid averaged over a 10-year period. Maxima of medium- to high-reflectivity episodes were identified to the north of and within downtown Atlanta and immediately east of the primary urban expansion of the central business district (CBD). Additional enhanced, high-reflectivity areas are found in southern Fulton and Clayton counties, located south of downtown Atlanta. These regions are also collocated with high-density urban expansion south of the Atlanta CBD. The research presented is the most comprehensive spatial and temporal analysis of grid averaged composite reflectivity data for urban convection conducted to date

Vulnerability Due to Nocturnal Tornadoes

This study investigates the human vulnerability caused by tornadoes that occurred between sunset and sunrise from 1880 to 2007. Nocturnal tornadoes are theorized to enhance vulnerability because they are difficult to spot and occur when the public tends to be asleep and in weak building structures. Results illustrate that the nocturnal tornado death rate over the past century has not shared the same pace of decline as those events transpiring during the daytime. From 1950 to 2005, a mere 27.3% of tornadoes were nocturnal, yet 39.3% of tornado fatalities and 42.1% of killer tornado events occurred at night. Tornadoes during the overnight period (local midnight to sunrise) are 2.5 times as likely to kill as those occurring during the daytime hours. It is argued that a core reason why the national tornado fatality toll has not continued to decrease in the past few decades is due to the vulnerability to these nocturnal events. This vulnerability is magnified when other factors such as escalating mobile (or “manufactured”) home stock and an increasing and spreading population are realized. Unlike other structure types that show no robust demarcation between nocturnal and daytime fatalities, nearly 61% of fatalities in mobile homes take place at night revealing this housing stock’s distinct nocturnal tornado vulnerability. Further, spatial analysis illustrates that the American South’s high nocturnal tornado risk is an important factor leading to the region’s high fatality rate. The investigation emphasizes a potential break in the tornado warning dissemination system utilized currently in the United States