Engineering topics

Christian, H. J.

Publications and source records attributed to Christian, H. J..

At least 37 records · Page 2

NASA NTRS2003

Multi-Satellite Observations of Oceanic Lightning

This paper will present several case studies of active oceanic lightning storms. Measurements by instruments on the Tropical Rainfall Measuring Mission (TRMM) and Fast On-orbit Recording of Transient Events (FORTE) platforms demonstrate that the two sets of sensors reinforce and complement one another. There is spatial and temporal coincidence between the optical data sets from Lightning Imaging Sensor (LIS) on TRMM and the photo-diode detector on FORTE. The LIS flash analysis provides a framework to interpret the stroke level data from FORTE. For these cases, the VHF receiver on FORTE is slaved to the optical system to provide stroke level radio frequency (RF) diagnostics. The occasions when TRMM and FORTE simultaneously have a lightning storm in their overlapping fields of view are extremely rare. One case study in the Gulf of Mexico is within range of land based sensor networks. These networks confirm the interpretation of satellite data and well as provide context for the storm conditions.

Boeck, W. L.↗

NASA NTRS2003

Error Analyses of the North Alabama Lightning Mapping Array (LMA)

Two approaches are used to characterize how accurately the North Alabama Lightning Mapping Array (LMA) is able to locate lightning VHF sources in space and in time. The first method uses a Monte Carlo computer simulation to estimate source retrieval errors. The simulation applies a VHF source retrieval algorithm that was recently developed at the NASA-MSFC and that is similar, but not identical to, the standard New Mexico Tech retrieval algorithm. The second method uses a purely theoretical technique (i.e., chi-squared Curvature Matrix theory) to estimate retrieval errors. Both methods assume that the LMA system has an overall rms timing error of 50ns, but all other possible errors (e.g., multiple sources per retrieval attempt) are neglected. The detailed spatial distributions of retrieval errors are provided. Given that the two methods are completely independent of one another, it is shown that they provide remarkably similar results, except that the chi-squared theory produces larger altitude error estimates than the (more realistic) Monte Carlo simulation.

Koshak, W. J.↗

NASA NTRS2003

Error Analyses of the North Alabama Lightning Mapping Array (LMA)

Koshak, W. J.↗

NASA NTRS2003

North Alabama Lightning Mapping Array (LMA): VHF Source Retrieval Algorithm and Error Analyses

Two approaches are used to characterize how accurately the North Alabama Lightning Mapping Array (LMA) is able to locate lightning VHF sources in space and in time. The first method uses a Monte Carlo computer simulation to estimate source retrieval errors. The simulation applies a VHF source retrieval algorithm that was recently developed at the NASA Marshall Space Flight Center (MSFC) and that is similar, but not identical to, the standard New Mexico Tech retrieval algorithm. The second method uses a purely theoretical technique (i.e., chi-squared Curvature Matrix Theory) to estimate retrieval errors. Both methods assume that the LMA system has an overall rms timing error of 50 ns, but all other possible errors (e.g., multiple sources per retrieval attempt) are neglected. The detailed spatial distributions of retrieval errors are provided. Given that the two methods are completely independent of one another, it is shown that they provide remarkably similar results. However, for many source locations, the Curvature Matrix Theory produces larger altitude error estimates than the (more realistic) Monte Carlo simulation.

Koshak, W. J.↗

NASA NTRS2002

The NASA Thunderstorm Observations and Research (ThOR) Mission: Lightning Mapping from Space to Improve the Short-term Forecasting of Severe Storms

The ThOR mission uses a lightning mapping sensor in geostationary Earth orbit to provide continuous observations of thunderstorm activity over the Americas and nearby oceans. The link between lightning activity and cloud updrafts is the basis for total lightning observations indicating the evolving convective intensification and decay of storms. ThOR offers a national operational demonstration of the utility of real-time total lightning mapping for earlier and more reliable identification of potentially severe and hazardous storms. Regional pilot projects have already demonstrated that the dominance in-cloud lightning and increasing in-cloud lash rates are known to precede severe weather at the surface by tens of minutes. ThOR is currently planned for launch in 2005 on a commercial or research satellite. Real-time data will be provided to selected NWS Weather Forecast Offices and National Centers (EMC/AWC/SPC) for evaluation.

Goodman, S. J.↗

NASA NTRS2000

Lightning Activity within a Tornadic Thunderstorm Observed by the Optical Transient Detector (OTD)

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 cloud-to-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, D. E.↗

NASA NTRS2000

Lightning Activity within a Tornadic Thunderstorm Observed by the Optical Transient Detector (OTD)

The first storm-scale, total lightning observations during tornadogenesis from space are presented. The NASA (National Aeronautics and Space Administration) OTD (Optical Transient Detector) overpass of an Oklahoma supercell, just minutes prior to tornado touchdown on 17 April 1995, detected a lightning flash rate of 45 flashes/ min. The total lighting activity was at least 15 times greater than the cloud-to-ground lightning activity detected by the National Lightning Detection Network (NLDN), indicating most of the lightning was intracloud. Total lightning rates were decreasing rapidly prior to tornadic touchdown. These observations are consistent with the limited results from recent ground based measurements of total lightning activity in tornadic storms, suggesting significant intracloud lightning and rapidly decreasing flash rates prior to tornado touchdown.

Buechler, D. E.↗

NASA NTRS1999

Investigation of a Frequency Spectrum of Discharges Generated by a Charged Aerosol Cloud

Charged aerosol clouds allow to create strong electric fields which generate discharges. A character of appearance and development of the discharges is similar to thundercloud processes. That is why the charged aerosol clouds are used for experiments on studying of discharges in air. Experimental data on electromagnetic fields investigation produced by discharges between charged cloud and ground are considered.

Christian, H. J.↗

NASA NTRS1999

Optical Detection of Lightning from Space

Two primary detection techniques (optical and RF) have a proven capability for detecting lightning from low earth orbit. However, the lightning processes that generate the optical and RF signals are vastly different providing significantly different information content from each sensor type. Because of the intervening ionosphere, low frequency RF components do not reach satellite altitudes. As a consequence, many of the processes associated with the major energy release of a lightning event (i.e. return strokes, k-changes, recoil streamers, etc), in all likelihood contribute little to the RF signal arriving at the satellite. The optical output from lighting, on the other hand, has been shown to be highly correlated with the energetic, charge-transferring processes mentioned above. On the down side, the optical energy, while essentially unaffected by the atmosphere once it emerges from the cloud, is heavily scattered within the cloud. While there is little absorption by the cloud, the great optical depth makes the total light energy emerging from the cloud to be dependent on where in the cloud the lightning occurred. Analyses suggest that when lightning is confined to the lowest regions of the cloud, the light is strongly attenuated and detection becomes problematic. Fortunately, the vast majority of lightning flashes are comprised of channels that propagate through the middle of the cloud and higher. These flashes produce bright signals at the top of a cloud and are readily detectable. Presently, we have two optical instruments in orbit. The Optical Transient Detector (OTD) has been orbiting the earth since April, 1995, while the Lightning Imaging Sensor (LIS) was launched on the Tropical Rainfall Measuring Mission (TRMM) in November of 1997. Both instruments are relatively small, solid state optical imagers, designed specifically to detect and locate lightning activity from low earth orbit with high detection efficiency and location accuracy.

Christian, H. J.↗

NASA NTRS1999

Optical Transient Detector (OTD) Observations of a Tornadic Thunderstorm

Total lightning observations made by the Optical Transient Detector (OTD) of a tornadic thunderstorm that occurred over Oklahoma on 17 April 1995 are presented. The average flash rate of the tornadic storm during the 3.2 min observation period was 45 flashes/min, with a flash rate density of 1.16 x 10(exp -4)/s sq km. The total flash rate was almost 18 times higher than the cloud-to-ground rate measured by the National Lightning Detection Network (NLDN). In addition, total lightning rates were observed to decrease prior to tornadic development.

Buechler, D. E.↗

NASA NTRS1999

Land-Ocean Differences in LIS and OTD Tropical Lightning Observations

Significant differences are known to exist on a global scale between continental and oceanic total lightning regional flash rates, suggesting differences in the properties of convective storms in these regimes. Lightning properties observed by the Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) over land and ocean are compared, limited to analysis over the tropics in order to simplify physical interpretation. We find that the mean flash rates of individual storms over tropical land only exceed those over ocean by a factor of 2 (far less than the observed differences in regional flash rates). However, the average nearest neighbor distance of continental thunderstorms is half that over oceans. Cloud-top lightning optical radiance in oceanic storms is also twice as large as over land, suggesting either more energetic flashes over the oceans or less intervening cloud particles.

Boccippio, D. J.↗

NASA NTRS1999

Optical Transient Detector (OTD) Observations of a Tornadic Thunderstorm

Total lightning observations made by the Optical Transient Detector (OTD) of a tornadic thunderstorm that occurred over Oklahoma 17 April 1995 are presented. The average flash rate of the tornadic storm during the 3.2 min observation period was 45 flashes min.with a flash rate density of 1.16 x 10(exp -4)/s sq km. The total flash rate was almost 18 times higher than the cloud-to-ground rate measured by the National Lightning Detection Network (NLDN). In addition. total lightning rates were observed to decrease prior To tornadic development.

Buechler, D. E.↗

NASA NTRS1999

Cross-Sensor Validation of the Optical Transient Detector (OTD)

Lightning data from the U.S. National Lightning Detection Network (NLDN) is used to perform preliminary validation of the satellite-based Optical Transient Detector (OTD). Sensor precision, accuracy, detection efficiency and biases of the deployed instrument are considered. We estimate the sensor to have, on average, better than 30 km spatial and 100 ms temporal accuracy. The detection efficiency for cloud-to-ground lightning is about 45-70%, slightly higher for intracloud lightning. There are only marginal day/night biases in the dataset, although 55 day averaging is required to remove sampling-based diurnal lightning cycle biases. The sensor detects statistically significant differences in the optical characteristics of intracloud, negative cloud-to-ground and positive cloud-to-ground lightning, although it cannot a priori determine flash types.

Boccippio, D. J.↗

NASA NTRS1999

Combined Satellite and Surface-Based Estimation of the Intracloud/ Cloud-to-Ground Lightning Ratio Over the Continental United States

Four years of observations from the NASA Optical Transient Detector (OTD) and Global Atmospherics National Lightning Detection Network (NLDN) are combined to determine the geographic distribution of the intracloud/cloud to ground lightning ratio over the continental United States. The mean ratio over this region is 2.64-2.94, with a standard deviation of 1.1-1.3 and anomalies as low as 1.0 or less over the Rocky and Appalachian mountains and as high as 8-10 in the central-upper midwest. There is some indication that the ratio covaries with ground elevation, although the relationship is nonunique. Little evidence is found to support a latitudinal covariance, despite significant variation in the climatological mean tropopause pressure over the latitudes considered. The dynamic range of local variability is comparable to the range of values cited by previous studies for latitudinal variation from the deep tropics to midlatitudes. Local high anomalies of this ratio in the midwest are coincident with anomalies in the climatological percentage of positive CG occurrence, as well as in the occurrence of large positive CGs characteristic of MCS convective and trailing stratiform regions. This suggests that storm type, morphology and level of organization may dominate over environmental cofactors in the local determination of this ratio.

Boccippio, D. J.↗

NASA NTRS1998

Global Validation of Single-Station Schumann Resonance Lightning Location

Global measurements of large, optically bright lightning events from the Optical Transient Detector (OTD) satellite are used to validate estimates of lightning location from single-station Schumann resonance (SR) data. Bearing estimates are obtained through conventional magnetic direction-finding techniques, while source range is estimated from the range-dependent impedance spectrum of individual SR transients. An analysis of 40 such transients suggests that single-station techniques can locate lightning globally with an accuracy of 1-2mm. This is confirmed by further validation at close ranges from flashes detected by the National Lightning Detection-Network (NLDN). Observations with both OTD and SR systems may be useful for globally locating lightning with necessary, if not sufficient, characteristics to trigger mesospheric sprites.

Boccippio, Dennis↗

NASA NTRS1997

Results from Two Years of Global Lightning Observation with the Optical Transient Detector

The Optical Transient Detector (OTD) is an instrument designed specifically to detect and locate lightning during either daytime or nighttime. A predecessor of the Lightning Imaging Sensor (LIS), it has been in orbit for over two and half years. During this time global lightning activity has been observed on a quasi continuous basis. During this time an extensive effort has been under way to quantify and verify the OTD performance. OTD data has been compared with both the National Lightning Detection Network (NLDN) and the Lightning Detection and Ranging (LDAR) system at KSC. Results of these intercomparisons will be presented as well as estimates of OTD overall performance parameters. In addition, one year of OTD results will be presented showing the strong influence of the sun on global lightning activity. A month by month collage of global lightning maps will be used to demonstrate both the first order forcing and more subtle influences. Finally, initial results form the Lightning Imaging Sensor, an instrument on the Tropical Rainfall Measuring Mission (TRMM) will be presented.

Christian, H. J.↗

NASA NTRS1994

Discovery of Intense Gamma-Ray Flashes of Atmospheric Origin

Detectors aboard the Compton Gamma Ray Observatory have observed an unexplained terrestrial phenomenon: brief, intense flashes of gamma rays. These flashes must originate in the atmosphere at altitudes above at least 30 kilometers in order to escape atmospheric absorption and reach the orbiting detectors. At least a dozen such events have been detected over the past 2 years. The photon spectra from the events are very hard (peaking in the high-energy portion of the spectrum) and are consistent with bremsstrahlung emission from energetic (million-electron volt) electrons. The most likely origin of these high- energy electrons, although speculative at this time, is a rare type of high-altitude electrical discharge above thunderstorm regions.

Fishman, G. J.↗

NASA NTRS1989

The Atlas/Centaur lightning strike incident

Consideration is given to an incident on March 26, 1987, in which an Atlas/Centaur 67 vehicle carrying a naval communication satellite was struck by a triggered, cloud-to-ground lightning flash about 48 s into the flight. The lightning current coupled a transient signal into the wiring of the Centaur digital computer unit, ultimately causing the breakup of the vehicle. The meteorological conditions at the time of the launch and the lightning flash triggered by the Atlas/Centaur 67 vehicle are analyzed on the basis of studies of the incident by the NASA Investigation Board.

Christian, H. J.↗