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P. Grady Dixon

Abstract

Periodic surges of moisture from the Gulf of California are considered to be partly responsible for widespread showers and thunderstorms across the deserts of Arizona during the summer monsoon season. Presently, the primary method for detecting these surges is to look for changes in the surface observations at Yuma, Arizona. Unfortunately, these surface data are easily influenced by gust fronts, marine layers, and the dramatic diurnal patterns of the desert environment. Therefore, a new method for Assessing Low-level Atmospheric Moisture using Soundings (ALARMS) is proposed. Gulf surge–induced moisture increases identified by this new set of criteria are compared to other methods in order to determine the usefulness of each. This study demonstrates that the proposed method performs much better than the others that were tested, with the additional advantage of being relatively easy to apply.

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Nyssa Perryman and P. Grady Dixon

Abstract

A better understanding of urban snowfall climatology will help to mitigate winter weather hazards in highly populated cities, such as Minneapolis–St. Paul, Minnesota. Winter road maintenance accounts for roughly 25% of Minnesota Department of Transportation maintenance budgets, and state and local agencies spend more than $2.3 billion on snow and ice control operations annually. Urban snowfall has also been shown to enhance health problems and increase mortality rates. Further research on urban snowfall climatology is needed to more accurately diagnose urban regions susceptible to such winter health risks. The winter urban heat island effect has been suggested to reduce snowfall downwind of city centers because of localized energy and moisture flux variations, but previous research lacks spatial detail since it is primarily based on sparse surface observations. This project utilizes radar data for two studies—a 25-event snowfall composite and an individual-event analysis of 13 snow-only events—occurring from 1995 to 2012 and passing over the Minneapolis–St. Paul urban area to quantify the change in radar reflectivity values as a proxy for snowfall intensity downwind of the city. Results show that for the summed maximum composite event snowfall was significantly decreased downwind of the urban region; however, the highest maximum composite event, as well as 4 of the 13 individual snowfall events evaluated, did not have significantly decreased snowfall downwind of the city center, with the highest maximum composite and two of the three individual events having increased reflectivity values downwind. Analysis of related atmospheric variables for events with increased downwind reflectivity suggests that atmospheric instability and convergence may play a critical role in urban snowfall modification.

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P. Grady Dixon and Thomas L. Mote

Abstract

Because of rapid growth and urbanization of Atlanta, Georgia, over the past few decades, the city has developed a pronounced urban heat island (UHI) that has been shown to enhance and possibly to initiate thunderstorms. This study attempts to find patterns and causes of Atlanta's induced precipitation that might not have been initiated otherwise. Land use maps, radar reflectivity, surface meteorological data, upper-air soundings, and airmass classification (spatial synoptic classification) types are all used to determine when, where, and why precipitation is initiated by Atlanta. Findings illustrate significant spatial and temporal patterns based on a 5-yr climatological description of events. July had the most events, with a diurnal peak just after local midnight. Low-level moisture, rather than UHI intensity, appears to be the most important factor for UHI-induced precipitation. However, UHI intensity also plays an important role. Events tended to occur under atmospheric conditions that were more unstable than those on rain-free days but not unstable enough to produce widespread convection.

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P. Grady Dixon and Andrew E. Mercer

No abstract available.

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Kelsey N. Scheitlin and P. Grady Dixon

Abstract

This study examines the relationship between diurnal temperature range (DTR) and land use/land cover (LULC) in a portion of the Southeast. Temperature data for all synoptically weak days within a 10-yr period are gathered from the National Climatic Data Center for 144 weather stations. Each station is classified as one of the following LULC types: urban, agriculture, evergreen forest, deciduous forest, or mixed forest. A three-way analysis of variance and paired-sample t tests are used to test for significant DTR differences due to LULC, month, and airmass type. The LULC types display two clear groups according to their DTR, with agricultural and urban areas consistently experiencing the smallest DTRs, and the forest types experiencing greater DTRs. The dry air masses seem to enhance the DTR differences between vegetated LULC types by emphasizing the differences in evapotranspiration. Meanwhile, the high moisture content of moist air masses prohibits extensive evapotranspirational cooling in the vegetated areas. This lessens the DTR differences between vegetated LULC types, while enhancing the differences between vegetated land and urban areas. All of the LULC types exhibit an annual bimodal DTR pattern with peaks in April and October. Since both vegetated and nonvegetated areas experience the bimodal pattern, this may conflict with previous research that names seasonal changes in evapotranspiration as the most probable cause for the annual trend. These findings suggest that airmass type has a larger and more consistent influence on the DTR of an area than LULC type and therefore may play a role in causing the bimodal DTR pattern, altering DTR with the seasonal distribution of airmass occurrence.

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Timothy A. Coleman and P. Grady Dixon

Abstract

In this paper, an objective analysis of spatial tornado risk in the United States is performed, using a somewhat different dataset than in some previous tornado climatologies. The focus is on significant tornadoes because their reporting frequency has remained fairly stable for several decades. Also, data before 1973 are excluded, since those tornadoes were rated after the fact and were often overrated. Tornado pathlength within the vicinity of a grid point is used to show tornado risk, as opposed to tornado days or the total number of reported tornadoes. The possibility that many tornadoes in the Great Plains were underrated due to the lack of damage indicators, causing a low bias in the number of significant tornadoes there, is mostly discounted through several analyses. The kernel density analysis of 1973–2011 significant tornadoes performed herein shows that the area of highest risk for tornadoes in the United States extends roughly from Oklahoma to Tennessee and northwestern Georgia, with the highest risk in the southeastern United States, from central Arkansas across most of Mississippi and northern Alabama.

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P. Grady Dixon, Gregory B. Goodrich, and William H. Cooke

Abstract

Previous wildfire research in the United States has been focused primarily on the western states. Much of this research has discovered relationships between wildfire variability and atmospheric teleconnections. Thus far, few published projects have addressed the effects of various teleconnections on wildfire in the southeastern United States. Index values for the El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Pacific decadal oscillation (PDO), and Pacific–North American (PNA) pattern are all tested for relationships with fire variables in the state of Mississippi. Each of the indices displays significant correlations with wildfire occurrence and/or size in Mississippi. The findings of this research suggest that it might be feasible to create predictive fire-risk models for the southeastern United States based on the combination of these teleconnection indices.

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Scott C. Sheridan, P. Grady Dixon, Adam J. Kalkstein, and Michael J. Allen

Abstract

Much research has shown a general decrease in the negative health response to extreme heat events in recent decades. With a society that is growing older, and a climate that is warming, whether this trend can continue is an open question. Using eight additional years of mortality data, we extend our previous research to explore trends in heat-related mortality across the United States. For the period 1975–2018, we examined the mortality associated with extreme-heat-event days across the 107 largest metropolitan areas. Mortality response was assessed over a cumulative 10-day lag period following events that were defined using thresholds of the excess heat factor, using a distributed-lag nonlinear model. We analyzed total mortality and subsets of age and sex. Our results show that in the past decade there is heterogeneity in the trends of heat-related human mortality. The decrease in heat vulnerability continues among those 65 and older across most of the country, which may be associated with improved messaging and increased awareness. These decreases are offset in many locations by an increase in mortality among men 45–64 (+1.3 deaths per year), particularly across parts of the southern and southwestern United States. As heat-warning messaging broadly identifies the elderly as the most vulnerable group, the results here suggest that differences in risk perception may play a role. Further, an increase in the number of heat events over the past decade across the United States may have contributed to the end of a decades-long downward trend in the estimated number of heat-related fatalities.

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Adam J. Kalkstein, Miloslav Belorid, P. Grady Dixon, Kyu Rang Kim, and Keith A. Bremer

Abstract

South Korea has among the highest rates of suicide in the world, and previous research suggests that suicide frequency increases with anomalously high temperatures, possibly as a result of increased sunshine. However, it is unclear whether this temperature–suicide association exists throughout the entire year. Using distributed lag nonlinear modeling, which effectively controls for nonlinear and delayed effects, we examine temperature–suicide associations for both a warm season (April–September) and a cool season (October–March) for three cities across South Korea: Seoul, Daegu, and Busan. We find consistent, statistically significant, mostly linear relationships between relative risk of suicide and daily temperature in the cool season but few associations in the warm season. This seasonal signal of statistically significant temperature–suicide associations only in the cool season exists among all age segments, but especially for those 35 and older, along with both males and females. We further use distributed lag nonlinear modeling to examine cloud cover–suicide associations and find few significant relationships. This result suggests that that high daily temperatures in the cool season, and not exposure to sun, are responsible for the strong temperature–suicide associations found in South Korea.

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P. Grady Dixon, Andrew E. Mercer, Jinmu Choi, and Jared S. Allen

The term “Tornado Alley” is a gross approximation of the most tornado-prone region in the United States. Depending on calculation methods, Tornado Alley can vary dramatically across the area between the Rocky and Appalachian Mountains. There is some evidence that multiple alleys of peak tornado activity exist around the country, including “Dixie Alley” in the Southeast. Therefore, we assess the spatial tornado risk and seek any regions of elevated tornado risk that are distinctly separate from the traditional Tornado Alley of the Great Plains. Results show there are no tornado risk areas statistically separate from Tornado Alley, but there are large portions of the Southeast that experience more tornadoes than the rest of the country. It appears that Tornado Alley and Dixie Alley are part of a single large region of high tornado risk with a relative minimum near the middle due to the Ozark and Ouachita Mountains. Placement of the maximum tornado density in Mississippi, along with other regions of relative maxima across the Southeast, may warrant modification of the traditional tornado risk map that focuses only on the Great Plains. Understanding such patterns is important for preparing the public and mitigating tornado hazards.

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