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Stephanie A. Weiss, Donald R. MacGorman, and Kristin M. Calhoun

Abstract

This study uses data from the Oklahoma Lightning Mapping Array (OK-LMA), the National Lightning Detection Network, and the Norman, Oklahoma (KOUN), prototype Weather Surveillance Radar-1988 Doppler (WSR-88D) radar to examine the evolution and structure of lightning in the anvils of supercell storms as they relate to storm dynamics and microphysics. Several supercell storms within the domain of the OK-LMA were examined to determine whether they had lightning in the anvil region, and if so, the time and location of the initiation of the anvil flashes were determined. Every warm-season supercell storm had some flashes that were initiated in or near the stronger reflectivities of the parent storm and propagated 40–70 km downstream to penetrate well into the anvil. Some supercell storms also had flashes that were initiated within the anvil itself, 40–100 km beyond the closest 30-dBZ contour of the storm. These flashes were typically initiated in one of three locations: 1) coincident with a local reflectivity maximum, 2) between the uppermost storm charge and a screening-layer charge of opposite polarity near the cloud boundary, or 3) in a region in which the anvils from two adjoining storms intersected. In some storms, anvil flashes struck ground beneath a reflectivity maximum in which reflectivity ≥20 dBZ had extended below the 0°C isotherm, possibly leading to the formation of embedded convection. This relationship may be useful for identifying regions in which there is a heightened risk for cloud-to-ground strikes beneath anvil clouds. In one storm, however, anvil lightning struck ground even though this reflectivity signature was absent.

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R. Calhoun, R. Heap, M. Princevac, R. Newsom, H. Fernando, and D. Ligon

Abstract

During the Joint Urban 2003 (JU2003) atmospheric field experiment in Oklahoma City, Oklahoma, of July 2003, lidar teams from Arizona State University and the Army Research Laboratory collaborated to perform intersecting range–height indicator scans. Because a single lidar measures radial winds, that is, the dot product of the wind vector with a unit vector pointing along the lidar beam, the data from two lidars viewing from different directions can be combined to produce horizontal velocity vectors. Analysis programs were written to retrieve horizontal velocity vectors for a series of eight vertical profiles to the southwest (approximately upwind) of the downtown urban core. This technique has the following unique characteristics that make it well suited for urban meteorology studies: 1) continuous vertical profiles from far above the building heights to down into the street canyons can be measured and 2) the profiles can extend to very near the ground without a loss of accuracy (assuming clear lines of site). The period of time analyzed spans from 1400 to 1730 UTC (0900–1230 local time) on 9 July 2003. Both shear and convective heating are important during the development of the boundary layer over this period of time. Differences in 10- and 20-min mean profiles show the effect of the variation of position approaching the urban core; for example, several hundred meters above the ground, velocity magnitudes for profiles separated by less than a kilometer may differ by over 1 m s−1. The effect of the increased roughness associated with the central business district can be seen as a deceleration of the velocity and a turning of the wind direction as the flow approaches the core, up to approximately 10° for some profiles. This effect is evident below 400–500 m both in the wind directions and magnitudes. Recommendations are given for how this type of data can be used in a comparison with model data.

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Kristin M. Calhoun, Edward R. Mansell, Donald R. MacGorman, and David C. Dowell

Abstract

Results from simulations are compared with dual-Doppler and total lightning observations of the 29–30 May 2004 high-precipitation supercell storm from the Thunderstorm Electrification and Lightning Experiment (TELEX). The simulations use two-moment microphysics with six hydrometeor categories and parameterizations for electrification and lightning while employing an ensemble Kalman filter for mobile radar data assimilation. Data assimilation was utilized specifically to produce a storm similar to the observed for ancillary analysis of the electrification and lightning associated with the supercell storm. The simulated reflectivity and wind fields well approximated that of the observed storm. Additionally, the simulated lightning flash rates were very large, as was observed. The simulation reveals details of the charge distribution and dependence of lightning on storm kinematics, characteristics that could not be observed directly. Storm electrification was predominately confined to the updraft core, but the persistence of both positive and negative charging of graupel in this region, combined with the kinematic evolution, limited the extent of charged areas of the same polarity. Thus, the propagation length of lightning flashes in this region was also limited. Away from the updraft core, regions of charge had greater areal extent, allowing flashes to travel farther without termination due to unfavorable charge potential. Finally, while the simulation produced the observed lightning holes and high-altitude lightning seen in the observations, it failed to produce the observed lightning initiations (or even lightning channels) in the distant downstream anvil as seen in the observed storm. Instead, the simulated lightning was confined to the main body of the storm.

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D. Zajic, H. J. S. Fernando, R. Calhoun, M. Princevac, M. J. Brown, and E. R. Pardyjak

Abstract

A better understanding of the interaction between the built environment and the atmosphere is required to more effectively manage urban airsheds. This paper reports an analysis of data from an atmospheric measurement campaign in Oklahoma City, Oklahoma, during the summer of 2003 that shows wind flow patterns, turbulence, and thermal effects in the downtown area. Experimental measurements within a street canyon yielded airflow patterns, stability conditions, and turbulence properties as a function of the incoming wind direction and time of the day. Air and surface temperatures at two different sites, one within the downtown urban canyon and the other in a nearby park, were measured. A study of the stability conditions within the urban canyon during the campaign indicates that dynamically stable conditions did not occur within the canyon. This provides evidence that the built environment can strongly influence the thermal characteristics in cities. Mean flow patterns close to the street level are analyzed for two different ranges of incoming wind directions and are compared with those obtained from a previous field experiment featuring idealized building configurations. This paper presents an approach allowing the estimation of wind direction in an urban canyon, given inflow conditions, that shows good agreement with wind patterns in the Oklahoma City street canyon. Turbulence statistics were calculated and normalized using different velocity scales to investigate the efficacy of the latter in specifying turbulence levels in urban canopies. The dependence of turbulence quantities on incoming wind direction and time of the day was investigated.

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Kristin M. Calhoun, Donald R. MacGorman, Conrad L. Ziegler, and Michael I. Biggerstaff

Abstract

A high-precipitation tornadic supercell storm was observed on 29–30 May 2004 during the Thunderstorm Electrification and Lightning Experiment. Observational systems included the Oklahoma Lightning Mapping Array, mobile balloon-borne soundings, and two mobile C-band radars. The spatial distribution and evolution of lightning are related to storm kinematics and microphysics, specifically through regions of microphysical charging and the location and geometry of those charge regions. Lightning flashes near the core of this storm were extraordinarily frequent, but tended to be of shorter duration and smaller horizontal extent than typical flashes elsewhere. This is hypothesized to be due to the charge being in many small pockets, with opposite polarities of charge close together in adjoining pockets. Thus, each polarity of lightning leader could propagate only a relatively short distance before reaching regions of unfavorable electric potential. In the anvil, however, lightning extended tens of kilometers from the reflectivity cores in roughly horizontal layers, consistent with the charge spreading through the anvil in broad sheets. The strong, consistent updraft of this high-precipitation supercell storm combined with the large hydrometeor concentrations to produce the extremely high flash rates observed during the analysis period. The strength and size of the updraft also contributed to unique lightning characteristics such as the transient hole of reduced lightning density and discharges in the overshooting top.

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Alexandre O. Fierro, Jidong Gao, Conrad L. Ziegler, Kristin M. Calhoun, Edward R. Mansell, and Donald R. MacGorman

Abstract

This work evaluates the performance of the assimilation of total lightning data within a three-dimensional variational (3DVAR) framework for the analysis and short-term forecast of the 24 May 2011 tornado outbreak using the Weather Research and Forecasting (WRF) Model at convection-allowing scales. Between the lifted condensation level and a fixed upper height, pseudo-observations for water vapor mass first are created based on either the flash extent densities derived from Oklahoma Lightning Mapping Array data or the lightning source densities derived from the Earth Networks pulse data, and then assimilated by the 3DVAR system. Assimilation of radar data with 3DVAR and a cloud analysis algorithm (RAD) also are performed as a baseline for comparison and in tandem with lightning to evaluate the added value of this lightning data assimilation (LDA) method.

Given a scenario wherein the control experiment without radar or lightning data assimilation fails to accurately initiate and forecast the observed storms, the LDA and RAD yield comparable short-term forecast improvements. The RAD alone produces storms of similar strength to the observations during the first 30 min of forecast more rapidly than the LDA alone; however, the LDA is able to better depict individual supercellular features at 1-h forecast. When both the lightning and radar data are assimilated, the 30-min forecast showed noteworthy improvements over RAD in terms of the model’s ability to better resolve individual supercell structures and still maintained a 1-h forecast similar to that from the LDA. The results chiefly illustrate the potential value of assimilating total lightning data along with radar data.

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Alexandre O. Fierro, Jidong Gao, Conrad L. Ziegler, Kristin M. Calhoun, Edward R. Mansell, and Donald R. Macgorman
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R. J. Barthelmie, P. Crippa, H. Wang, C. M. Smith, R. Krishnamurthy, A. Choukulkar, R. Calhoun, D. Valyou, P. Marzocca, D. Matthiesen, G. Brown, and S. C. Pryor

The 3D wind and turbulence characteristics of the atmospheric boundary layer experiment (3D Wind) was conducted to evaluate innovative remote sensing and in situ platforms for measurements of wind and turbulence regimes. The experiment is part of a planned series that focuses on quantifying wind and turbulence characteristics at the scales of modern wind turbines and wind farms and was conducted in northern Indiana in May 2012. 3D Wind had the following specific objectives: (i) intercomparison experiments evaluating wind speed profiles across the wind turbine rotor plane from traditional cup anemometers and wind vanes on a meteorological mast and from a tethered balloon, sonic anemometers (mast mounted and on an unmanned aerial vehicle), three vertical-pointing (continuous wave) lidars and a pulsed scanning lidar, and (ii) integrate these measurements and output from 3-km-resolution (over the inner domain) simulations with the Weather Research and Forecasting Model to develop a detailed depiction of the atmospheric flow, upwind, within, and downwind of a large, irregularly spaced wind farm. This paper provides an overview of the measurement techniques, their advantages and disadvantages focusing on the integration of wind and turbulence characteristics that are necessary for wind farm development and operation. Analyses of the measurements are summarized to characterize instrument cross comparison, wind profiles, and spatial gradients and wind turbine wakes.

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Christopher D. Karstens, James Correia Jr., Daphne S. LaDue, Jonathan Wolfe, Tiffany C. Meyer, David R. Harrison, John L. Cintineo, Kristin M. Calhoun, Travis M. Smith, Alan E. Gerard, and Lans P. Rothfusz

Abstract

Providing advance warning for impending severe convective weather events (i.e., tornadoes, hail, wind) fundamentally requires an ability to predict and/or detect these hazards and subsequently communicate their potential threat in real time. The National Weather Service (NWS) provides advance warning for severe convective weather through the issuance of tornado and severe thunderstorm warnings, a system that has remained relatively unchanged for approximately the past 65 years. Forecasting a Continuum of Environmental Threats (FACETs) proposes a reinvention of this system, transitioning from a deterministic product-centric paradigm to one based on probabilistic hazard information (PHI) for hazardous weather events. Four years of iterative development and rapid prototyping in the National Oceanic and Atmospheric Administration (NOAA) Hazardous Weather Testbed (HWT) with NWS forecasters and partners has yielded insights into this new paradigm by discovering efficient ways to generate, inform, and utilize a continuous flow of information through the development of a human–machine mix. Forecasters conditionally used automated object-based guidance within four levels of automation to issue deterministic products containing PHI. Forecasters accomplished this task in a timely manner while focusing on communication and conveying forecast confidence, elements considered necessary by emergency managers. Observed annual increases in the usage of first-guess probabilistic guidance by forecasters were related to improvements made to the prototyped software, guidance, and techniques. However, increasing usage of automation requires improvements in guidance, data integration, and data visualization to garner trust more effectively. Additional opportunities exist to address limitations in procedures for motion derivation and geospatial mapping of subjective probability.

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