Lightning optical pulse statistics from storm overflights during the Altus Cumulus Electrification Study (original) (raw)
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Observations of Optical Lighting Emissions from above Thunderstorms Using U-2 Aircraft
Bulletin of the American Meteorological Society, 1983
In order to determine how to achieve orders of magnitude improvement in spatial and temporal resolution and in sensitivity of satellite lightning sensors, better quantitative measurements of the characteristics of the optical emissions from lightning as observed from above tops of thunderclouds are required. A number of sensors have been developed and integrated into an instrument package and flown aboard a NASA U-2 aircraft. The objectives have been to acquire optical lightning data needed for designing the lightning mapper sensor, and to study lightning physics and the correlation of lightning activity with storm characteristics. The instrumentation and observations of the program are reviewed and their significance for future research is discussed.
FORTE observations of simultaneous VHF and optical emissions from lightning: Basic phenomenology
Journal of Geophysical Research, 2000
Preliminary observations of simultaneous VHF and optical emissions from lightning as seen by the Fast on-Orbit Recording of Transient Events (FORTE) spacecraft are presented. VHF/optical waveform pairs are routinely collected both as individual lightning events and as sequences of events associated with cloud-to-ground (CG) and intracloud (IC) flashes. CG pulses can be distinguished from IC pulses on the basis of the properties of the VHF and optical waveforms but mostly on the basis of the associated VHF spectrograms. The VHF spectrograms are very similar to previous ground-based HF and VHF observations of lightning and show signatures associated with return strokes, stepped and dart leaders, attachment processes, and intracloud activity. For a typical IC flash, the FORTE-detected VHF is generally characterized by impulsive broadband bursts of emission, and the associated optical emissions are often highly structured. For a typical initial return stroke, the FORTE-detected VHF is generated by the stepped leader, the attachment process, and the actual return stroke. For a typical subsequent return stroke, the FORTE-detected VHF is mainly generated by dart leader processes. The detected optical signal in both return stroke cases is primarily produced by the in-cloud portion of the discharge and lags the arrival of the corresponding VHF emissions at the satellite by a mean value of 243 s. This delay is composed of a transit time delay (mean of 105 s) as the return stroke current propagates from the attachment point up into the region of in-cloud activity plus an additional delay due to the scattering of light during its traversal through the clouds. The broadening of the light pulse during its propagation through the clouds is measured and used to infer a mean of this scattering delay of about 138 s (41 km additional path length) for CG light. This value for the mean scattering delay is consistent with the Thomason and Krider [1982] model for light propagation through clouds.
Lightning activity within a tornadic thunderstorm observed by the optical transient detector (OTD)
Geophysical Research Letters, 2000
The first storm-scale, total lightning observations from space during tornadogenesis are presented. During the overpass of an Oklahoma supercell, just minutes prior to tornado touchdown on 17 April 1995, the NASA (National Aeronautics and Space Administration) OTD (Optical Transient Detector) detected a total of 143 flashes during approximately 3 minutes of observation time. The estimated total flash rate ranges from 45 (raw counts) to 78 (corrected for detection efficiency) flashes min -1. This total flash rate was at least 17 times greater than the cloudto-ground lightning rate detected by the National Lightning Detection Network (NLDN), indicating most of the lightning was intracloud. Cloud-to-ground lightning at this time was also dominated by positive polarity flashes. In addition, total lightning rates were decreasing rapidly prior to touchdown. These OTD observations are consistent with the limited results from recent ground based measurements of total lightning activity in tornadic storms and corroborate that such storms have unusually high total flash rates, are dominated by intracloud lightning, and that the total flash rates are observed to decrease rapidly in the minutes prior to touchdown. BUECHLER ET AL-LIGHTNING WITHIN A TORNADIC THUNDERSTORM
Thunderstorm and Lightning Studies using the FORTE Optical Lightning System (FORTE/OLS)
1999
Preliminary observations of simultaneous RF and optical emissions from lightning as seen by the FORTE spacecraft are presented. RF/optical pairs of waveforms are routinely collected both as individual lightning events and as sequences of events associated with cloud-to-ground (CG) and intra-cloud (IC) flashes. CG pulses can be distinguished from IC pulses based on the properties of the RF and optical
Journal of Geophysical Research, 2002
1] Lightning type identification of temporally coincident optical-VHF time series event pairs collected by the Fast On-orbit Recording of Transient Events (FORTE) satellite's photodiode detector (PDD) and VHF instrument allows for the investigation of optical properties as a function of lightning type. General trends in the peak optical irradiance and characteristic pulse widths of PDD-VHF coincident events are studied as a function of lightning type, using previously established techniques to identify lightning type based on VHF spectogram-power time series. While lightning type cannot be identified from optical data alone, there are several notable features in the optical record. The distribution of observed characteristic widths of PDD events has a cutoff near 200 ms, which represents a lower limit on the combination of intrinsic optical emission time and pulse broadening due to photon scattering in the intervening clouds. Events with the highest peak optical irradiances observed at FORTE are typically positive initial return strokes. Also, the median value of peak optical irradiance for cloud-to-ground lightning events is more than double that for in-cloud lightning. Citation: Davis, S. M., D. M. Suszcynsky, and T. E. L. Light, FORTE observations of optical emissions from lightning: Optical properties and discrimination capability,
Radio Science, 2002
1] We examine the ratio of energy in the second pulse to energy in the first pulse for 2467 transionospheric pulse pairs (TIPPs). The TIPPs examined here have been attributed to thunderstorms occurring over maritime, continental, and coastal regions near the continental United States. The mean values of the energy ratios are found to be 0.39 ± 0.27 for continental TIPPs and 0.94 ± 0.62 for maritime TIPPs. The energy ratio values for the coastal TIPPs exhibit a bimodal distribution representative of both the continental and the maritime populations. Previous observational evidence has shown that the second pulse of a TIPP is the surface-reflected signal from the same source as the first pulse. We compare the observed pulse energy ratios to the reflection coefficients of soil and seawater given by the Fresnel equations. The average values of the reflection coefficients for sources with vertical plane and horizontal plane polarization are consistent with the data.
Observations of lightning in the stratosphere
Journal of Geophysical Research, 1995
An examination and analysis of video images of lightning, captured by the payload bay TV cameras of the space shuttle, provided a variety of examples of lightning in the stratosphere above thunderstorms. These images were obtained on several recent shuttle flights while conducting the Mesoscale Lightning Experiment (MLE). The images of stratospheric lightning illustrate the variety of filamentary and broad vertical discharges in the stratosphere that may accompany a lightning flash. A typical event is imaged as a single or multiple filament extending 30 to 40 km above a thunderstorm that is illuminated by a series of lightning strokes. Examples are found in temperate and tropical areas, over the oceans, and over the land. 1465 ning. These include the detection of gamma-ray burst of atmospheric origin [Fishman et al., 1994], lightning-induced brightening of the airglow layer [Boeck et al., 1992], and unusual tran-ionospheric pulse pair radio signals detected by the Blackbeard experiment on the ALEXIS satellite [Holden et al.,
A demonstration of the capabilities of multisatellite observations of oceanic lightning
Journal of Geophysical Research, 2004
1] We have examined lightning flashes in five nighttime, oceanic thunderstorms, which were jointly observed by Tropical Rainfall Measuring Mission (TRMM) and Fast On-Orbit Recording of Transient Events (FORTE). The multiplicity of instruments on board these satellites presents a multiphenomenological snapshot view of oceanic nighttime convection. Data are available for five oceanic storms with a total of 40 flashes. The independent optical imagers on each satellite establish the flash locations. The relative fraction of Lightning Imaging Sensor (LIS)-detected optical pulses that were also observed by the FORTE/photodiode detector varied from 0% for LIS range less than 10 4 J sr À1 m À2 mm À1 to 100% for LIS range greater than 10 6 mJ À1 sr À1 m À2 mm À1 . The FORTE/VHF data sometimes allow estimation of the VHF source emissions heights and identification of individual discharge processes as positive or negative, in-cloud or cloud-to-ground. These observations reinforce the concepts that the VHF pulses are produced by a breakdown process, while the optical pulses are the result of current flow.
Journal of Geophysical Research, 2002
1] The FORTE satellite's radiofreqency receiver/recorder system has been used to study extremely narrow ($100 ns width) radiofrequency pulses accompanying the initiation of negative cloud-to-ground strokes. These pulses have been observed from the ground for over two decades. The FORTE space-based observations are substantially consistent with the prior ground-based results, at least in regard to pulsewidth, distance-scaled pulse amplitude, and the pulses' basic association with negative cloud-to-ground strokes, relative to either positive cloud-to-ground or intracloud strokes. New results from the FORTE observations include (1) information on the radiation pattern (versus elevation angle), (2) the tendency of the underlying fast-pulse radiation process to occur preferentially in marine, rather than dry-land, locations, and (3) the high degree of linear polarization of the radiated signal. These three new results were not accessible to ground-based measurements, which do not sample elevation angles other than zero, whose signal distortion with overland propagation paths (at zero elevation angle) tends to confuse the issue of the (land versus sea) location affinity of the pulse source itself, and whose received signal is a fortiori linearly polarized because of the adjacent ground plane. The narrow radiofrequency pulse that accompanies negative cloud-to-ground strokes provides a useful identifier, in the satellite's radiofrequency datastream, of the occurrence of this particular kind of stroke. Additionally, the observations reported here indicate that the radiating element is a single, vertical current stalk, rather than a collection of randomly oriented, mutually incoherent dipoles, such as are believed to be responsible for most very high frequency signals from lightning.
Characteristics of impulsive VHF lightning signals observed by the FORTE satellite
Journal of Geophysical Research Atmospheres, 2002
We study very high frequency (VHF) and optical emissions from lightning, observed by the FORTE satellite, differentiating between impulsive (transionospheric pulse pairs (TIPPs)) and nonimpulsive events. TIPPs are seen to constitute 47% of the FORTE VHF data but only 32% of the optically coincident data. The median peak optical irradiance of the optical emission associated with TIPPs is 916 μW/m2 at FORTE and for non-TIPPs is 195 μW/m2. The median effective pulse width of the optical signal from TIPPs is 658 μs, and it is 548 μs for non-TIPPs. In the VHF, both event types have similar observed peak powers (0.086 mV2/m2 and 0.089 mV2/m2, for TIPPs and non-TIPPs, respectively). The optically coincident lightning (of either type) is weaker in peak VHF emission than is the lightning that lacks coincident optical signals, although for non-TIPPs, the stronger the VHF peak, the more likely the event is to have a coincident optical signal. For TIPPs, however, this is true only for events with peak E2 < 0.1 mV2/m2. Above that threshold, TIPPs are increasingly less likely to show coincident optical emission with increasing VHF peak E2. For both TIPPs and non-TIPPs, the peak current reported by the U.S. National Lightning Detection Network™ and peak VHF power reported by FORTE are statistically proportional. The nature of the proportionality appears to depend upon the polarity of the discharge but not upon the event type. We also find that only 11% of TIPPs are associated with negative-polarity discharges, compared to 75% of non-TIPPs. Finally, we find that TIPPs arise from events with altitudes of 6-15 km, although we see optical coincidence only for those TIPPs occurring above ˜10 km.