AE24A-01 INVITED
VHF Broadband Digital Interferometer for Real-time Operation
Lightning Research Group of Osaka University (LRG-OU) has been developing a VHF lightning mapper, Broadband Digital Interferometer (BDITF), to investigate lightning initiation and progression since 1995. When LRG-OU started the project, a multichannel digital storage oscilloscope was deployed to record VHF waveforms emitted by lightning discharges. VHF broadband antenna and necessary electronics like an amplifier were redesigned. Original VHF BDITF was operated in a rocket-triggered lightning experiment during winter thunderstorm season in Hokuriku, Japan. The first observation by BDITF was a rocket-triggered lightning, which lowered the positive charge to the ground. That meant ascending negative breakdown propagation was recorded, and LRG-OU obtained the lightning channel image by upward triggered lightning. Since LRGOU could validate the function and capability of BDITF through several field campaigns, a project to design and manufacture a special analog to digital converter for BDITF was initiated in 1998. Moreover software for a real-time data processing was developed. The first system of new BDITF was operated during a filed campaign in 2003, and lightning channels in two dimensions (2D), which meant azimuth and elevation format, were able to be reconstructed in a several seconds after occurrence of lightning flash. The BDITF system was considered to be an operational system recently. For three dimensional (3D) imaging, two sites operation of BDITF and post data processing of the triangulation are required. LRG-OU learned the bi- directional leader progression, possible charge distribution related to the leader initiation, and the speed of the leader propagation by the 3D imaging. The achievement of BDITF technique by LRG-OU gives us the chance of deployment and operation of the system around the rocket launching site of JAXA on the Tanegashima island. This operation is expected to contribute the go/no-go judgment of rocket launching by JAXA because of the now casting the location of lightning discharges. LRG-OU also joins in the several satellite projects. BDITF system which can be deployed on the satellite and/or space station is manufactured. The first VHF system is expected to be in the space in early 2009.
AE24A-02 INVITED
Comparison of High-Speed Video and VHF Source Locations for CG Lightning Flashes
We compare high-speed videos of cloud-to-ground (CG) lightning flashes with maps of VHF source locations determined by the Oklahoma Lightning Mapping Array (OKLMA) in order to gain insights into the relationships between visible processes in CG channels and in-cloud processes before, during, and after the CG components of discharges. On May 7, 2008 we obtained simultaneous video images at 10,000 frames per second and VHF source location data for four CG flashes. We present an analysis of the data for two of the flashes. In both cases there was a clearly visible upward-going leader in the last frame (100 microsecond interval) before the first return stroke. One of the flashes was remarkable in that it had a single negative return stroke followed by an extremely long continuing current with numerous M components. Past studies have shown that long continuing currents occur relatively rarely after single or first return strokes and fewer than 1% have durations exceeding 500 ms. The duration of the continuing current in this case was more than 700 milliseconds. During the first 300 milliseconds of the long continuing current interval there were at least 13 M-components visible in the high-speed images. Although there were almost 5000 VHF source locations in the cloud associated with the flash during a period of nearly one second, very few of them were co-located with visible processes within the field of view of the camera. Finally, we investigate the VHF source locations associated with M-components.
AE24A-03
Vector Electric Field Measurements Near an Intra-Cloud Lightning Channel
Balloon-borne vector electric field measurements 200 m from a lightning channel reveal details of the wave fronts of a stepped leader, K-streamers, and other ionizing waves as they pass by. The channel was part of an intra-cloud flash that began at 8 km above sea level, passed by the instrument at 9 km, and stopped propagating at 11 km above sea level. The irregular motion of the stepped leaders is clearly visible, and the electric charge per unit length on the channel after the front passes is constant until the leader stops propagating, except for perturbations from waves that propagate backward from the stepped leader tip. After the leader stops, K-streamers with fast rising wave fronts propagate along the channel in the same direction as did the leader.
AE24A-04
Three Dimensional Current Generator Structure of a Mountain Thunderstorm
Recently, wide band measurements of the electric field in a thunderstorm
have been obtained by a balloon-borne electric field sonde or Esonde.
The data from the Esonde can be combined with simultaneous Lightning Mapping
Array (LMA) measurements of VHF pulses emitted during lightning breakdown
processes to estimate the charge transport associated with lightning.
The techniques we have developed
to process Esonde data are further enhanced
by taking better account of instrument rotation,
and by computing the local horizontal electric field, not just the
lightning induced electric field change.
Using these techniques, we analyze lightning charge transport
for a thunderstorm which occurred on August 18, 2004, near
Langmuir Laboratory, New Mexico.
The analysis yields the three dimensional
current generator structure of the thunderstorm.
We present the structure and analyze it characteristics.
http://www.math.ufl.edu/~hager
AE24A-05
Mechanism of Lightning Associated Infrasonic Pulses from Thunderclouds
The infrasonic waves correspond to the region of frequencies of acoustic sound waves 0.02-10 Hz, higher than the acoustic cut-off frequency but lower than the audible frequencies [e.g., Blanc, Ann. Geophys., 3, 673, 1985]. There is a strong experimental evidence that thunderstorms represent significant sources of infrasonic wave activity spanning a broad altitude range from the troposphere and up to the thermosphere [e.g., Blanc, 1985; Few, in Handbook of Atmospheric Electrodynamics, Vol. 2, edited by H. Volland, pp. 1-31, CRC Press, 1995; Drob et al., JGR, 108, 4680, 2003]. This evidence includes electrostatic production of 0.1- 1 Hz infrasonic waves from thunderclouds [Few, 1995] and recent discovery of infrasound from lightning induced transient luminous events in the mesosphere called sprites [Liszka, J. Low Freq. Noise Vibr. Active Control, 23, 85, 2004; Farges et al., GRL, 32, L13824, 2005; Liszka and Hobara, JASTP, 68, 1179, 2006]. The understanding and classification of different infrasonic waves and their sources is of great current interest from a Comprehensive Nuclear-Test-Ban Treaty (CTBT) verification perspective [e.g., Assink et al., GRL, 35, L15802, 2008]. It has been pointed many decades ago by C. T. R. Wilson [Phil. Trans. R. Soc. London A, 221, 73, 1920] that sudden reduction of the electric field inside a thundercloud immediately following a lightning discharge should produce an infrasound signature. Wilson [1920] noted that the pressure within a charged cloud must be less than the pressure outside, similarly to that within a charged soap bubble. In contrast to the sudden expansion of the air along the track of a lightning flash, the sudden contraction of a large volume of air must furnish a measurable rarefaction pulse [Wilson, 1920]. Many experimental and theoretical contributions followed these predictions by C. T. R. Wilson (see [Few, JGR, 90, 6175, 1985] and extensive list of references therein). Modeling investigation of related scenarios leading to emission of infrasound pulses documented in the existing literature, with particular emphasis on the initial compression phase of the observed infrasonic waveforms, represents a goal of the present work. The model employed in the present study utilizes linearized equations of acoustics with classical viscosity and atmospheric gravitational stratification effects. It is demonstrated that a growth of charge density in thundercloud prior to lightning discharge on time scales on the order of 2 to 6 seconds, comparable to typical documented time scales of generation of charge in thunderclouds, is fully sufficient for explanation of the initial compression waves in observed infrasonic pulses generated in accordance with the electrostatic mechanism proposed in [Wilson, 1920; Dessler, JGR, 78, 1889, 1973]. The arguments advanced in the present study agree with the ideas by Bohannon et al. [GRL, 4, 49, 1977] indicating a rapid intensification of the field prior to the lightning discharge. However, we provide a quantitative demonstration that the intensification does not need to be as fast as 0.5 seconds proposed in [Bohannon et al., 1977].
AE24A-06
Flash Rate, Electrical, Microphysical, and Dynamical Relationships Across a Simulated Storm Spectrum
Many studies have suggested relationships between intracloud flash rates and microphyiscal variables, both from an observational perspective and for specific storm intensity and morphology. These relationships are necessary for any analytic approach to forecasting lightning activity or for inferring storm properties from lightning observations. This study provides an analysis of the relationships between flash rates and several microphysical quantities across a wide spectrum of simulated storms. Some of these quantities include electric field, graupel volume, updraft mass flux, rain mass, ice crystal mass flux, updraft volume, maximum updraft, and cloud ice mass. Eleven unique storms were simulated using the Collaborative Model for Multiscale Atmospheric Simulation (COMMAS) to maximize the temporal and spatial resolution of the analysis. Modifications to surface moisture and bulk shear depth and magnitude yielded a wide range of storm intensity and morphology, from weak, unicell storms, to strong squall lines and supercells. Each case was run with two different noninductive graupel-ice charge separation schemes, for a total of 22 simulations. Results show that the relationships of total flash rates with rain mass, ice crystal mass flux, and graupel volume are significant, while the relationships are weak with maximum electric field and maximum updraft. For cases in which convection remained isolated (i.e., one cell in the model domain for most of the 120 minutes), the correlations between detrended total flash rate and graupel volume were also found to be significant. Furthermore, by translating flash rate time series backwards in time, the correlation coefficients between flash rates and some of the microphysical variables were found to increase. Understanding these relationships can provide the foundation for future work in predicting flash rates across a wide range of storms based on observational information, including radar data.
AE24A-07 INVITED
Summary of Colorado State University's LMA-Related Observations from STEPS
The Severe Thunderstorm Electrification and Precipitation Study (STEPS) combined in situ measurements with Doppler-polarimetric radar and lightning mapping, resulting in a deep set of detailed observations of thunderstorms in the Great Plains. In this presentation, we will summarize some of Colorado State University's key scientific results from STEPS, with emphasis on insights provided by observations from the very high frequency lightning mapping array (LMA). We will illustrate (1) how comparisons between LMA and radar have further refined our understanding of the strong, consistent relationships among total lightning flash rate, precipitation ice, and overall convective vigor, (2) how the LMA may indicate convective surges via features such as bursts of vertically elevated lightning activity and lightning "holes" associated with strong updrafts and bounded weak echo regions, and (3) how the LMA has been used to infer the 3-D, time- evolving charge structure of storms within the context of the radar observations. This summary includes results from a wide variety of storm types (single cell, multicell, supercell, and mesoscale convective systems). The electrical structure and lightning activity of these storms ranges from simple dipole and tripole structures that produce mostly negative cloud-to-ground flashes to more complex structures such as inverted tripoles that produce mostly positive cloud-to-ground flashes.
AE24A-08 INVITED
Total Lightning Observations of Extreme Weather Events over the Contiguous United States
The overall objective is to investigate total lightning characteristics of extreme weather events over the contiguous United States (CONUS) using TRMM (Tropical Rainfall Measuring Mission) LIS (Lightning Image Sensor) and OTD (Optical Transient Detector) satellite observations. A large LIS (10+ years) and OTD (5 years) data base is available to study the instantaneous total or cloud-to-ground (CG) plus intracloud (IC) lightning characteristics of extreme weather events. More specifically, the LIS and OTD data are combined with National Lightning Detection Network (NLDN) observations to examine the total and CG lightning flash rate and density, the IC:CG ratio, and positive CG percentage. These instantaneous lightning characteristics can be used for basic science studies to better understand the physical and dynamical linkages between lightning and precipitation and their environmental controls. They can also provide a first-look of extreme weather events leading up to future satellite observations (e.g., NOAA GOES-R Geostationary Lightning Mapper [GLM]) for use in climate studies and the short-term prediction and warning process. Extreme weather events are defined by the NOAA Storm Data reports of tornadoes, large hail (at least 0.75 inch) and strong straight-line winds (at least 50 kts). Over CONUS, there are over 70,000 severe storm reports in the TRMM spatial domain (< 35 degrees N) from 1998-2007 and over 100,000 storm reports in the OTD spatial domain (5/1995-4/2000). Temporal co-location is on the order of 1% (i.e., 1000's of coincident overpasses), providing a statistically significant sample of instantaneous total lightning properties. This instantaneous behavior of lightning in extreme weather is then compared to that of typical thunderstorm events, or randomly sampled LIS/OTD events in which the extreme events have been eliminated from the population. Results describing the instantaneous behavior of total lightning within a large sample of extreme and typical thunderstorms over CONUS will be presented. When possible, coincident VHF lightning observations from the ground-based Northern Alabama Lightning Mapping Array (LMA) are compared to the LIS/OTD optical lightning signatures, providing a validation source for instantaneous space-based optical lightning properties and a means to extend lightning inferences over the life-cycle of extreme weather.