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Thomas L. Mote

Abstract

This study empirically examines the role of snow depth on the depression of air temperature after controlling for effect of temperature changes above the boundary layer. In addition, this study examines the role of cloud cover, solar elevation angle, and maximum snow-covered albedo on the temperature depression due to snow cover. The work uses a new dataset of daily, gridded snow depth, snowfall, and maximum and minimum temperatures for North America from 1960 to 2000 in conjunction with 850-hPa temperature data for the same period from the NCEP–NCAR reanalysis, version 1. The 850-hPa temperatures are used as a control to remove the effect of temperature changes above the boundary layer on surface air temperatures. Findings from an analysis of variance demonstrate that snow cover can result in daily maximum (minimum) temperature depressions on average of 4.5°C (2.6°C) for snow depths greater than 10 cm over the grasslands of central North America, but temperature depressions average only 1.2°C (1.1°C) overall. The temperature depression of snow cover is shown to be reduced by increased cloud cover and decreased maximum albedo, which is indicative of denser forest cover. The role of snow melting on temperature depression is further explored by comparing days with maximum temperatures above or below freezing.

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Paul W. Miller and Thomas L. Mote

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Isolated, short-lived thunderstorms forming in weakly forced environments are referenced through a surplus of terminology. Further, the language used to describe the strongest, severe-weather-producing subset of these storms is applied inconsistently, posing a communication hurdle for the effective dissemination of hazardous weather risks. The term “pulse thunderstorm” was originally coined to describe an anomalously strong airmass thunderstorm often associated with a larger convective complex. However, recent applications of “pulse” have evolved to also describe nonsevere, single-cell storms, and both uses can currently be observed within research, operational, and educational texts. This paper reviews the history of the term “pulse,” performs a content analysis on nearly 1,500 pulse-referencing Storm Prediction Center (SPC) convective outlooks (CO) and mesoscale discussions (MD), and summarizes the deficiencies with the contemporary disorganized convection nomenclature. The larger CO sample (n = 997) establishes that temporal trends in “pulse” references model traditional expectations whereas the detailed MDs (n = 458) showcase examples of pulse-related terminology. The MD content analysis reveals that 1) the term “pulse” frequently appears in conjunction with severe-weather-related language and 2) that pulse-related words (e.g., brief, isolated) are equally represented in multicell-referencing MDs. In the interest of effective communication and reproducible research, the definition of “pulse” is proposed to be standardized according to the term’s original (i.e., severe, multicellular) meaning. Further, thunderstorms forming within synoptically homogeneous air masses in the absence of large-scale dynamical lift are suggested to be termed “weakly forced thunderstorms.” By corollary, pulse storms represent the subset of weakly forced thunderstorms associated with severe weather.

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Jeremy E. Diem and Thomas L. Mote

Abstract

Through modification of the planetary boundary layer, urbanization has the potential to have a significant impact on precipitation totals locally. Using daily summer-season precipitation data at 30 stations from 1953 to 2002, this study explores the possibility of urban effects as causes of spatial anomalies in precipitation in a zone within 180 km of Atlanta, Georgia. The time period is divided into consecutive epochs (e.g., 1953–77 and 1978–2002), and interepochal differences in precipitation totals, heavy-precipitation days, cumulative heavy precipitation, and atmospheric conditions are explored. The southern stations experienced significant decreases in precipitation, whereas significant precipitation increases occurred at central/west-central stations. The most striking increases occurred at Norcross, Georgia, which is ∼30 km northeast of downtown Atlanta; Norcross had the third smallest number of heavy-precipitation days during 1953–77, but, during 1978–2002, it had the most heavy-precipitation days. Not only did the amount of urban land cover upwind of Norcross increase substantially from the earlier to the later epochs, but regionwide dewpoint temperatures also increased significantly. Therefore, it is suspected that the increased precipitation at Norcross was caused by urban effects, and these effects may have been enhanced by increased atmospheric humidity.

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Mace L. Bentley and Thomas L. Mote

In 1888, Iowa weather researcher Gustavus Hinrichs gave widespread convectively induced windstorms the name “derecho”. Refinements to this definition have evolved after numerous investigations of these systems; however, to date, a derecho climatology has not been conducted.

This investigation examines spatial and temporal aspects of derechos and their associated mesoscale convective systems that occurred from 1986 to 1995. The spatial distribution of derechos revealed four activity corridors during the summer, five during the spring, and two during the cool season. Evidence suggests that the primary warm season derecho corridor is located in the southern Great Plains. During the cool season, derecho activity was found to occur in the southeast states and along the Atlantic seaboard. Temporally, derechos are primarily late evening or overnight events during the warm season and are more evenly distributed throughout the day during the cool season.

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Walker S. Ashley and Thomas L. Mote

Convectively generated windstorms occur over broad temporal and spatial scales; however, the more widespread and longer lived of these windstorms have been given the name “derecho.” Utilizing an integrated derecho database, including 377 events from 1986 to 2003, this investigation reveals the amount of insured property losses, fatalities, and injuries associated with these windstorms in the United States. Individual derechos have been responsible for up to 8 fatalities, 204 injuries, forest blow-downs affecting over 3,000 km2 of timber, and estimated insured losses of nearly a $500 million. Findings illustrate that derecho fatalities occur more frequently in vehicles or while boating, while injuries are more likely to happen in vehicles or mobile homes. Both fatalities and injuries are most common outside the region with the highest derecho frequency. An underlying synthesis of both physical and social vulnerabilities is suggested as the cause of the unexpected casualty distribution. In addition, casualty statistics and damage estimates from hurricanes and tornadoes are contrasted with those from derechos to emphasize that derechos can be as hazardous as many tornadoes and hurricanes.

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John D. Frye and Thomas L. Mote

Abstract

Changes in low-level moisture alter the convective parameters [e.g., convective available potential energy (CAPE), lifted index (LI), and convective inhibition (CIN)] as a result of alterations in the latent and sensible heat energy exchange. Two sources for low-level moisture exist in the southern Great Plains: 1) moisture advection by the low-level jet (LLJ) from the Gulf of Mexico and 2) evaporation and transpiration from the soils and vegetation in the region. The primary focus of this study is to examine the spatial distribution of soil moisture on a daily basis and to determine the effect it has on the convective parameters. The secondary objective is to investigate how the relationship between soil moisture and convective parameters is altered by the presence of an LLJ. The soil moisture data were obtained through newly developed procedures and advances in technology aboard the Tropical Rainfall Measuring Mission Microwave Imager. The convective parameter data were obtained through the North American Regional Reanalysis dataset. The study examined seven warm seasons (April–September) from 1998 to 2004 and found that the convective environment is more unstable (CAPE > 900 J kg−1, LI < −2°C) but more strongly capped (CIN > 70 J kg−1) on days with an LLJ present. Spearman’s rank correlation analysis showed a less stable atmosphere with increased soil moisture, after soil moisture reached 5%, on most days. Additional analysis determined that on all synoptic-type days the probability of reaching various thresholds of convective intensity increased as soil moisture values increased. The probabilities were even greater on days with an LLJ present than on the days without an LLJ present. An examination of four days representing each synoptic-type day indicates that on the daily scale the intensity of the convective environment is closely related to the high soil moisture and the presence of an LLJ.

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P. Grady Dixon and Thomas L. Mote

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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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Paul W. Miller and Thomas L. Mote

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Weakly forced thunderstorms (WFTs), convection forming in the absence of a synoptic forcing mechanism and its associated shear regime, are the dominant convective mode during the warm season in the southeast United States. This study uses 15 yr (2001–15) of warm-season (May–September) composite reflectivity images from 30 WSR-88D sites in the southeastern United States to detect WFTs and pulse thunderstorms, defined as WFTs associated with a severe weather event. Thunderstorms were identified as regions of contiguous reflectivities greater than or equal to 40 dBZ using connected neighborhoods labeling. Ward’s clustering was then performed upon the duration, size, strength, initiation time, and solidity of the approximately 1 900 000 thunderstorms. Of the 10 clusters of morphologically similar storms, five groups, containing 885 496 thunderstorms, were designated as WFTs. In line with previous work, WFT development mirrors landscape features, such as the Appalachian Mountains and Mississippi Delta. However, the large sample size also reveals more subtle nuances to the spatial distribution, such as decreases over river valleys and increases along the Atlantic fall line. The most active pulse thunderstorm region, the Blue Ridge Mountains, was displaced from the overall WFT maximum: the Florida Peninsula and Gulf Coast. Most pulse thunderstorms were associated with larger moisture values, particularly in the midlevels, which supported larger and longer-lasting WFT complexes. Synoptically, two distinct modes of variability yielded WFT-favorable environments: the intrusion of the Bermuda high from the east and the expansion of high pressure over the southern Great Plains from the west.

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Alan W. Black and Thomas L. Mote

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Winter precipitation can be very disruptive to travel by aircraft and by motor vehicles. Vehicle fatalities due to winter precipitation are considered “indirect” and are not counted in Storm Data, the publication commonly used to evaluate losses from meteorological hazards. The goal of this study is to examine the spatial and temporal characteristics of these indirect transportation fatalities that involve winter precipitation for the period 1975–2011. Motor vehicle fatalities were gathered from the National Highway Traffic Safety Administration’s (NHTSA) Fatality Analysis Reporting System (FARS) database, while aviation fatalities were collected from the National Transportation Safety Board’s (NTSB) Aviation Accident database. Statistical analysis and geographic information systems (GIS) were used to assess the spatial and temporal characteristics of these deaths. Most winter-precipitation-related motor vehicle fatalities occur during the daylight hours. Fatal motor vehicle accident rates are higher than expected in the Northeast and Great Lakes regions, while winter-precipitation-related aviation fatalities are most common in the western United States. Vehicle fatality counts due to winter weather are compared to fatality counts for various hazards from Storm Data to highlight the differences between the datasets. Because of the exclusion of vehicle fatalities, Storm Data underestimates by an order of magnitude the number of fatalities that involve winter weather each year. It is hoped that a better understanding of winter precipitation mortality can be applied in order to reduce fatalities in the future.

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John D. Frye and Thomas L. Mote

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Boundaries between two dissimilar air masses have been shown to be the focus region for convection initiation. One feature that has been shown to create these boundaries, as well as mesoscale circulation patterns conducive for convection, is soil moisture heterogeneities. These relationships have been validated in modeling studies, short-term field campaigns, and reanalysis of severe weather events. This study examines the role of soil moisture on convection initiation by using observational data over 7 yr (1998–2004) in the southern Great Plains. A key component to this research is the recently developed daily soil moisture product from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). The locations of convection initiation, based on the Weather Surveillance Radar-1988 Doppler (WSR-88D) data, were compared to volumetric soil moisture values and volumetric soil moisture gradient values. The locations of convection initiation were also examined based on synoptic-type day. On synoptically benign days, increased soil moisture and soil moisture gradient values were associated with decreased convection initiation, to a point. After soil moisture reached 15% (25%) on days with (without) a low-level jet, the likelihood of convection initiation increased. On synoptically primed days, the probabilities of convection initiation were more variable throughout the range of soil moisture values, indicating that the synoptically primed conditions may reduce the influence of soil moisture heterogeneities. Results indicate that a critical value in soil moisture and soil moisture gradient may exist that alters the mesoscale effect of changes in soil moisture on convection initiation, particularly on days that would be classified as synoptically benign.

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