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Douglas M. Mach and W. David Rust

Abstract

We have developed a device to measure lightning-channel propagation velocities. It consists of eight solid state silicon photodetectors mounted behind precision horizontal slits in the focal plane of a 50-mm lens on a 35-mm camera body. Each detector has a 0.1° vertical field of view that is separated from adjacent detector slits by 2.8°. The horizontal field-of-view for each detector is 41° and the total vertical field of view for the device is 21°. The signal from each detector is amplified by a circuit with a 10%–90% rise time of 0.6 μs and an equivalent decay time of 400 μs. The eight Photodetector pulses, IRIG-B time, and slow and fast electric field change waveforms are recorded on a 14-track analog tape recorder with an upper frequency response Of 1.0 MHz and a maximum dynamic interchannel timing error of 0.6 μs. To provide images of lightning geometry and permit time-to-thunder measurements, color video and sound are recorded with a standard VHS video cassette recorder. The return stroke velocity (RSV) device, video camera, and microphone are installed and coaxially aimed in an environmental enclosure on a remotely controlled pan-tilt unit atop our mobile laboratory, permitting the recording of lightning signals at remote sites and while mobile. To evaluate the performance of the RSV device, we have analyzed 12 natural return strokes from Alabama, Florida, and Oklahoma and 4 return strokes triggered at the Kennedy Space Center, Florida. The velocities we determined vary from 1.2 to 2.5×108 m s−1, with an average of 1.7×8 m s−1±0.8 × 8 m s−1. From comparisons of our results to those of a streaking camera, we find no significant differences between the velocities obtained from the same strokes with the two systems. We also find no differences between the characteristics of the pulses or the velocities calculated from them while the RSV device is moving or stationary.

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William J. Koshak, Douglas M. Mach, and Phillip M. Bitzer

Abstract

The problem of inferring the location and time of occurrence of a very high frequency (VHF) lightning source emission from Lightning Mapping Array (LMA) network time-of-arrival (TOA) measurements is closely examined in order to clarify the cause of retrieval errors and to determine how best to mitigate these errors. With regard to this inverse problem, the previous literature lacks a comprehensive discussion of the associated forward problem. Hence, the forward problem is analyzed in this study to better clarify why retrieval errors increase with increasing source horizontal range and/or decreasing source altitude. Further insight is obtained by performing carefully designed Monte Carlo inversion simulations that provide specific retrieval error plots, which in turn lead to clear recommendations for mitigating retrieval errors. Based on all of the numerical results, the following strategies are recommended for mitigating retrieval errors (when possible, and without obstructing the line of sight): expand the horizontal extent of the LMA network, maximize the vertical sensor baseline by using mountainous terrain if available, and improve TOA measurement timing accuracy. Adding sensors to the network is relatively ineffective, unless of course the addition of sensors expands the horizontal extent and/or vertical baseline of the network. It is also shown how the standard retrieval method can be generalized by considering, in addition to the regular (unpolarized) point VHF source, the polarized transient very low frequency/low frequency (VLF/LF) electric point dipole source. Multiple observations (i.e., VHF arrival time and power, and VLF/LF arrival time and electric field amplitude) are simultaneously implemented into the new generalized mathematical framework, and the potential benefits are indicated.

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Daniel J. Cecil, Kevin R. Quinlan, and Douglas M. Mach

Abstract

On 17 July, intense convection in the eyewall of Hurricane Emily (2005) was observed by the high-altitude (∼20 km) NASA ER-2 aircraft. Analysis of this convection is undertaken using downward-looking radar, passive microwave radiometer, electric field mills, and Geostationary Operational Environmental Satellite-11 (GOES-11) rapid-scan infrared imagery. Radar data show convection reaching more than 17 km, with reflectivity more than 40 dBZ and estimated updraft speeds greater than 20 m s−1 at ∼14-km altitude. All of the passive microwave frequencies (10, 19, 37, and 85 GHz) experienced scattering by large ice particles. Large electric fields with dozens of lightning flashes were recorded. Because of safety concerns arising from difficulties with the first two transects, the flight plan was modified to avoid passing above the eyewall again. These observations occurred 8–10 h after Emily’s peak 929-hPa intensity, with central pressures from reconnaissance aircraft having risen to 943 hPa immediately before the flight and 946 hPa immediately afterward (no such measurements available during the flight). Rapid-scan infrared imagery reveals that a period of episodic bursts of strong, deep convection was beginning just as the ER-2 arrived. The first leg across the eye coincided with a rapidly growing new cell along the flight track in the western eyewall. This strong convection may have been characteristic of Emily for the ∼24 h leading up to landfall in the Yucatan, but it does not appear to be a continuation of convective trends from the previous rapid intensification or peak intensity periods.

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Francis J. Merceret, Jennifer G. Ward, Douglas M. Mach, Monte G. Bateman, and James E. Dye

Abstract

Electric-field measurements made in and near clouds during two airborne field programs are presented. Aircraft equipped with multiple electric-field mills and cloud physics sensors were flown near active convection and into thunderstorm anvil and debris clouds. The magnitude of the electric field was measured as a function of position with respect to the cloud edge to provide an observational basis for modifications to the lightning launch commit criteria (LLCC) used by the U.S. space program. These LLCC are used to reduce the risk that an ascending launch vehicle will trigger a lightning strike that could cause the loss of the mission or vehicle. Even with fields of tens of kV m−1 inside electrically active convective clouds, the fields external to these clouds decay to less than 3 kV m−1 within 15 km of cloud edge. Fields that exceed 3 kV m−1 were not found external to anvil and debris clouds.

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Douglas M. Mach, Donald R. MacGorman, W. David Rust, and Roy T. Arnold

Abstract

We have tested a network of magnetic direction-finders (DFs) that locate ground strikes in Oklahoma and surrounding states in order to determine detection efficiency for the network and systematic errors in azimuth (i.e., site errors) for each of four DF sites. Independent data on lightning strike locations were obtained with a television (TV) camera on a mobile laboratory and an all-azimuth TV system at the National Severe Storms Laboratory (NSSL). In two tests using these data, we found a location detection efficiency of about 70% for storms at about 70 and 300 km from the center of the network. Systematic errors in azimuth were determined by comparing locations from the lightning strike locating system with strikes located from the mobile laboratory system; also, for a single DF at NSSL, strike azimuths from the DF were compared with azimuths from the all-azimuth TV system for storms near NSSL. Furthermore, we developed a technique for using redundant DF data to determine systematic errors in azimuth measurements for each DF site. Azimuthal errors found by this analytic technique were consistent with errors found by using the two sets of direct measurements. The azimuthal errors are themselves a function of azimuth, with peak amplitudes ranging from less than 5° for DFs located at favorable sites to about 11° for one DF located at an unfavorable site.

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Robbie E. Hood, Daniel J. Cecil, Frank J. LaFontaine, Richard J. Blakeslee, Douglas M. Mach, Gerald M. Heymsfield, Frank D. Marks Jr., Edward J. Zipser, and Michael Goodman

Abstract

During the 1998 and 2001 hurricane seasons of the western Atlantic Ocean and Gulf of Mexico, the Advanced Microwave Precipitation Radiometer (AMPR), the ER-2 Doppler (EDOP) radar, and the Lightning Instrument Package (LIP) were flown aboard the NASA ER-2 high-altitude aircraft as part of the Third Convection and Moisture Experiment (CAMEX-3) and the Fourth Convection and Moisture Experiment (CAMEX-4). Several hurricanes, tropical storms, and other precipitation systems were sampled during these experiments. An oceanic rainfall screening technique has been developed using AMPR passive microwave observations of these systems collected at frequencies of 10.7, 19.35, 37.1, and 85.5 GHz. This technique combines the information content of the four AMPR frequencies regarding the gross vertical structure of hydrometeors into an intuitive and easily executable precipitation mapping format. The results have been verified using vertical profiles of EDOP reflectivity and lower-altitude horizontal reflectivity scans collected by the NOAA WP-3D Orion radar. Matching the rainfall classification results with coincident electric field information collected by the LIP readily identifies convective rain regions within the precipitation fields. This technique shows promise as a real-time research and analysis tool for monitoring vertical updraft strength and convective intensity from airborne platforms such as remotely operated or uninhabited aerial vehicles. The technique is analyzed and discussed for a wide variety of precipitation types using the 26 August 1998 observations of Hurricane Bonnie near landfall.

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