Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Steven E. Nelson x
  • All content x
Clear All Modify Search
Marshall Shepherd, Thomas Mote, John Dowd, Mike Roden, Pamela Knox, Steven C. McCutcheon, and Steven E. Nelson

No Abstract available.

Full access
Walter A. Lyons, Thomas E. Nelson, Earle R. Williams, Steven A. Cummer, and Mark A. Stanley

Abstract

During the summer of 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program deployed a three-dimensional Lightning Mapping Array (LMA) near Goodland, Kansas. Video confirmation of sprites triggered by lightning within storms traversing the LMA domain were coordinated with extremely low frequency (ELF) transient measurements in Rhode Island and North Carolina. Two techniques of estimating changes in vertical charge moment (M q) yielded averages of ∼800 and ∼950 C km for 13 sprite-parent positive polarity cloud-to-ground strokes (+CGs). Analyses of the LMA's very high frequency (VHF) lightning emissions within the two mesoscale convective systems (MCSs) show that +CGs did not produce sprites until the centroid of the maximum density of lightning radiation emissions dropped from the upper part of the storm (7–11.5 km AGL) to much lower altitudes (2–5 km AGL). The average height of charge removal (Z q) from 15 sprite-parent +CGs during the late mature phase of one MCS was 4.1 km AGL. Thus, the total charges lowered by sprite-parent +CGs were on the order of 200 C. The regional 0°C isotherm was located at about 4.0 km AGL. This suggests a possible linkage between sprite-parent CGs and melting-layer/brightband charge production mechanisms in MCS stratiform precipitation regions. These cases are supportive of the conceptual MCS sprite-production models previously proposed by two of the authors (Lyons and Williams).

Full access
Jerald A. Brotzge, Steven E. Nelson, Richard L. Thompson, and Bryan T. Smith

Abstract

The ability to provide advanced warning on tornadoes can be impacted by variations in storm mode. This research evaluates 2 yr of National Weather Service (NWS) tornado warnings, verification reports, and radar-derived convective modes to appraise the ability of the NWS to warn across a variety of convective modes and environmental conditions. Several specific hypotheses are considered: (i) supercell morphologies are the easiest convective modes to warn for tornadoes and yield the greatest lead times, while tornadoes from more linear, nonsupercell convective modes, such as quasi-linear convective systems, are more difficult to warn for; (ii) parameters such as tornado distance from radar, population density, and tornado intensity (F scale) introduce significant and complex variability into warning statistics as a function of storm mode; and (iii) tornadoes from stronger storms, as measured by their mesocyclone strength (when present), convective available potential energy (CAPE), vertical wind shear, and significant tornado parameter (STP) are easier to warn for than tornadoes from weaker systems. Results confirmed these hypotheses. Supercell morphologies caused 97% of tornado fatalities, 96% of injuries, and 92% of damage during the study period. Tornado warnings for supercells had a statistically higher probability of detection (POD) and lead time than tornado warnings for nonsupercells; among supercell storms, tornadoes from supercells in lines were slightly more difficult to warn for than tornadoes from discrete or clusters of supercells. F-scale intensity and distance from radar had some impact on POD, with less impact on lead times. Higher mesocyclone strength (when applicable), CAPE, wind shear, and STP values were associated with greater tornado POD and lead times.

Full access
Walter A. Lyons, Steven A. Cummer, Mark. A. Stanley, Gary R. Huffines, Kyle C. Wiens, and Thomas E. Nelson

Over a decade of monitoring mesospheric transient luminous events (TLEs) above U.S. high plains storms confirmed sprites are almost exclusively associated with positive polarity cloud-to-ground lightning (+CGs). Following C. T. R. Wilson's theory proposed in 1925, only those +CGs lowering large amounts of charge to ground should induce sprites. The key metric, the charge moment change, generally must exceed ~600 C km to initiate the electric breakdown at 75 km, which evolves into the sprite. High plains storms generate the highest percentage, the largest average peak current, and highest density of +CGs in the nation. Various storm types generate +CGs, and especially supercells are often dominated by positive strokes. Few sprites observations above supercells have been obtained (and usually during their decaying phase), while thousands of sprites have been imaged above mesoscale convective system (MCS) stratiform regions and some squall lines. During the 2000 Severe Thunderstorm Electrification and Precipitation Study (STEPS), two supercells were examined. One storm contained >90% +CGs, but none exceeded the sprite charge moment change threshold. A second nocturnal supercell did produce sprites from the last two +CGs of the storm as a stratiform region developed, more favorable for significant continuing currents to follow the +CG return stroke. Unexpectedly, three sprites occurring during the most intense phase of the storm were triggered by unusually intense and impulsive +CGs, which lowered sufficient charge in the return stroke alone for sprite initiation. Such +CGs, and thus sprites, are probably relatively rare events during the supercell mature stage.

Full access

NPOESS

Next-Generation Operational Global Earth Observations

Thomas F. Lee, Craig S. Nelson, Patrick Dills, Lars Peter Riishojgaard, Andy Jones, Li Li, Steven Miller, Lawrence E. Flynn, Gary Jedlovec, William McCarty, Carl Hoffman, and Gary McWilliams

The United States is merging its two polar-orbiting operational environmental satellite programs operated by the Department of Commerce and the Department of Defense into a single system, which is called the National Polar-orbiting Operational Environmental Satellite System (NPOESS). During the next decade, NPOESS will provide global operational data to meet many of the needs of weather forecasters, climate researchers, and global decision makers for remotely sensed Earth science data and global environmental monitoring. The NPOESS Preparatory Project (NPP) will be launched in 2011 as a precursor to NPOESS to reduce final development risks for NPOESS and to provide continuity of global imaging and atmospheric sounding data from the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) missions. Beginning in 2014, NPOESS spacecraft will be launched into an afternoon orbit and in 2016 into an early-morning orbit to provide significantly improved operational capabilities and benefits to satisfy critical civil and national security requirements for space-based, remotely sensed environmental data. The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Operation (MetOp) spacecraft will complement NPOESS in a midmorning orbit. The joint constellation will provide global coverage with a data refresh rate of approximately four hours. NPOESS will observe more phenomena simultaneously from space and deliver a data volume significantly greater than its operational predecessors with substantially improved data delivery to users. Higher-resolution (spatial and spectral) and more accurate imaging and atmospheric sounding data will enable improvements in short- to medium-range weather forecasts. Multispectral and hyperspectral instruments on NPOESS will provide global imagery and sounding products useful to the forecaster that are complementary to those available from geostationary satellites. NPOESS will support the operational needs of meteorological, oceanographic, environmental, climatic, and space environmental remote sensing programs and provide continuity of data for climate researchers. This article that describes NPOESS was completed and accepted for publication prior to the White House decision in February 2010 ordering a major restructuring of the NPOESS program. The Department of Commerce will now assume primary responsibility for the afternoon polar-orbiting operational environmental satellite orbit and the Department of Defense will take primary responsibility for the early morning orbit. However, NPP, as described in this article, is still scheduled to be launched in 2011. Several of the instruments and program elements described in this article are also likely to be carried forward into future U.S. polar-orbiting operational environmental satellite missions.

Full access