Martin J. Murphy

A Combined TOA/MDF Technology Upgrade of the U.S. National Lightning Detection Network

The U.S. National Lightning Detection Network TM (NLDN) has provided lightning data covering the continental United States since 1989. Using information gathered from more than 100 sensors, the NLDN provides both real-time and historical lightning data to the electric utility industry, the National Weather Service, and other government and commercial users. It is also the primary source of lightning data for use in research and climatological studies in the United States. In this paper we discuss the design, implementation, and data from the time-of-arrival/magnetic direction finder (TOA/MDF) network following a recent system-wide upgrade. The location accuracy (the maximum dimension of a confidence region around the stroke location) has been improved by a factor of 4 to 8 since 1991, resulting in a median accuracy of 500 m. The expected flash detection efficiency ranges from 80% to 90% for those events with peak currents above 5 kA, varying slightly by region. Subsequent strokes and strokes with peak currents less than 5 kA can now be detected and located; however, the detection efficiency for these events is not quantified in this study because their peak current distribution is not well known.

An Overview of Lightning Locating Systems: History, Techniques, and Data Uses, With an In-Depth Look at the U.S. NLDN

Lightning in all corners of the world is monitored by one or more land- or space-based lightning locating systems (LLSs). The applications that have driven these developments are numerous and varied. This paper describes the history leading to modern LLSs that sense lightning radiation fields at multiple remote sensors, focusing on the interactions between enabling technology, scientific discovery, technical development, and uses of the data. An overview of all widely used detection and location methods is provided, including a general discussion of their relative strengths and weaknesses for various applications. The U.S. National Lightning Detection Network (NLDN) is presented as a case study, since this LLS has been providing real-time lightning information since the early 1980s, and has provided continental-scale (U.S.) information to research and operational users since 1989. This network has also undergone a series of improvements during its >20-year life in response to evolving detection technologies and expanding requirements for applications. Recent analyses of modeled and actual performance of the current NLDN are also summarized. The paper concludes with a view of the short- and long-term requirements for improved lightning measurements that are needed to address some open scientific questions and fill the needs of emerging applications.

Lightning locating systems: Characteristics and validation techniques

Ground-based or satellite-based lightning locating systems are the most common way to geolocate lightning. Depending upon the frequency range of operation, such systems can also report a variety of characteristics associated with lightning events (channel formation processes, leader pulses, cloud-to-ground return strokes, M-components, ICC pulses, and cloud lightning pulses). In this paper, we summarize the various methods to geolocate lightning, both ground-based and satellite-based, and discuss the characteristics of lightning data available from various sources. The performance characteristics of lightning locating systems are determined by their ability to geolocate lightning events accurately and report various features such as lightning type and peak current. We examine the various methods used to validate the performance characteristics of different types of lightning locating systems.

Deep excavations in water-bearing gravels in Cork

Historically only shallow basements were included in developments in Cork city owing to the underlying layers of soft alluvium and high-permeability glaciofluvial gravels. In recent times several large schemes, with basements up to 10 m deep, have been completed. In this paper the design and performance of these developments is summarized. Particular emphasis is placed on two schemes at Eglinton St. and Half Moon St. Although dewatering by deep well systems is feasible, flow rates can be substantial. Careful consideration needs to be given to the site-specific properties of the glaciofluvial gravels to permit the level of the cut-off to be efficiently chosen so as to minimize inflows and external groundwater lowering and to mitigate against the possible risk of surface settlements. A ‘soft gel’ grout blanket was used successfully as an alternative at one site. Lateral wall movements were low and it seems possible that more efficient designs are possible, especially if the small strain stiffness and dilation properties of the material are taken into account.

Lightning locating systems: Insights on characteristics and validation techniques: LLS Characteristics and Validation

Ground-based and satellite-based lightning locating systems are the most common ways to detect and geolocate lightning. Depending upon the frequency range of operation, LLSs may report a variety of processes and characteristics associated with lightning flashes including channel formation, leader pulses, cloud-to-ground return strokes, M-components, ICC pulses, cloud lightning pulses, location, duration, peak current, peak radiated power and energy, and full spatial extent of channels. Lightning data from different types of LLSs often provide complementary information about thunderstorms. For all the applications of lightning data, it is critical to understand the information that is provided by various lightning locating systems in order to interpret it correctly and make the best use of it. In this study, we summarize the various methods to geolocate lightning, both ground-based and satellite-based, and discuss the characteristics of lightning data available from various sources. The performance characteristics of lightning locating systems are determined by their ability to geolocate lightning events accurately with high detection efficiency and with low false detections and report various features of lightning correctly. Different methods or a combination of methods may be used to validate the performance characteristics of different types of lightning locating systems. We examine these methods and their applicability in validating the performance characteristics of different LLS types.

Measurement of preliminary breakdown pulse trains in cloud-to-ground lightning using lightning locating systems

The first stroke leader in a cloud-to-ground lightning discharge is thought to be preceded by the initial or preliminary breakdown process which produces a train of relatively large microsecond-scale electric field pulses. In addition to producing the information used to geolocate various types of lightning events (cloud-to-ground return strokes, M-components, ICC pulses, and cloud lightning pulses) and report their parameters such as estimated peak currents, and field waveform characteristics such as risetimes and peak-to-zero times, the sensors in the U.S. National Lightning Detection Network (NLDN) can record the magnetic field waveforms of lightning events. This makes them a valuable tool in studying the phenomenology and physics of lightning discharges. In this study, we use multiple-station measurements of preliminary breakdown pulse trains obtained using NLDN sensors at various distances from the lightning event. Assuming that the preliminary breakdown pulse train in a negative cloud-to-ground discharge is generated when a negatively-charged channel extends downward from the main negative charge region and interacts with the lower positive charge region, we use a modified transmission line model to reproduce the bipolar pulse train electric-field signature associated with preliminary breakdown. The model-predicted waveforms are compared with those measured at different distances by the NLDN sensors.