Douglas M. Mach

Global frequency and distribution of lightning as observed from space by the Optical Transient Detector

of uncertainty for the OTD global totals represents primarily the uncertainty (and variability) in the flash detection efficiency of the instrument. The OTD measurements have been used to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe. An analysis of this annual lightning distribution confirms that lightning occurs mainly over land areas, with an average land/ocean ratio of 10:1. The Congo basin, which stands out year-round, shows a peak mean annual flash density of 80 fl km 2 yr 1 in Rwanda, and includes an area of over 3 million km 2 exhibiting flash densities greater than 30 fl km 2 yr 1 (the flash density of central Florida). Lightning is predominant in the northern Atlantic and western Pacific Ocean basins year-round where instability is produced from cold air passing over warm ocean water. Lightning is less frequent in the eastern tropical Pacific and Indian Ocean basins where the air mass is warmer. A dominant Northern Hemisphere summer peak occurs in the annual cycle, and evidence is found for a tropically driven semiannual cycle. INDEX TERMS: 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3324 Meteorology and Atmospheric Dynamics: Lightning; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques;

The Optical Transient Detector (OTD): Instrument Characteristics and Cross-Sensor Validation

Abstract Lightning data from the U.S. National Lightning Detection Network (NLDN) are 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. The sensor is estimated to have, on average, about 20–40-km spatial and better than 100-ms temporal accuracy. The detection efficiency for cloud-to-ground lightning is about 46%–69%. It is most likely slightly higher for intracloud lightning. There are only marginal day/night biases in the dataset, although 55- or 110-day averaging is required to remove the sampling-based diurnal lightning cycle bias.

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 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., anomalous VHF noise sources) are neglected. The detailed spatial distributions of retrieval errors are provided. Even though the two methods are independent of one another, they nevertheless provide remarkably similar results. However, altitude error estimates derived from the two methods differ (the Monte Carlo result being taken as more accurate). Additionally, this study clarifies the mathematical retrieval process. In particular, the mathematical difference between the first-guess linear solution and the Marquardt-iterated solution is rigorously established thereby explaining why Marquardt iterations improve upon the linear solution.

Site Errors and Detection Efficiency in a Magnetic Direction-Finder Network for Locating Lightning Strikes to Ground

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 ...

Two-dimensional velocity, optical risetime, and peak current estimates for natural positive lightning return strokes

We report velocities, optical risetimes, and transmission line model peak currents for seven natural positive return strokes. The data were taken with our return stroke velocity device mounted on the National Severe Storms Laboratory mobile laboratory. The average two-dimensional positive return stroke velocity for channel segments of 500 m starting near the base of the channel is 0.9 ± 0.4 × 108 m s−1. The corresponding average velocity for our natural negative first strokes is 1.2 ± 0.6 × 108 m s−1. We find no significant velocity change with height for positive return strokes. The average 10–90% optical risetime from channel segments with an average length of 3.2 ± 1.7 m for positive return strokes within 100 m of the base of the channel is 9.4 ± 3.0 μs; the corresponding measurement for negative strokes is 3.5 ± 1.7 μs. We detected no optical leaders from the positive flashes. This limits the relative brightness of positive stroke leaders to approximately 0.08 times the optical output of the return stroke. The logarithmic average peak current from the transmission line model for positive return strokes is 59.8 kA, approximately 5 times the logarithmic average peak current we calculated for natural negative first return strokes. All differences between positive and negative natural strokes noted above are statistically different at the 99% level.

Electric Fields, Cloud Microphysics, and Reflectivity in Anvils of Florida Thunderstorms

into the interior, electric fields very frequently increased abruptly from � 1 to >10 kV m 1 even though the particle concentrations and radar reflectivity increased smoothly. The abrupt increase in field usually occurred when the aircraft entered regions with a reflectivity of 10–15 dBZ. We suggest that the abrupt increase in electric field was because the charge advection from the convective core did not occur across the entire breadth of the anvil and because the advection of charge was not constant in time. Also, some long-lived anvils showed enhancement of electric field and reflectivity far downwind of the convective core. Screening layers were not detected near the edges of the anvils. Comparisons of electric field magnitude with particle concentration or reflectivity for a combined data set that included all anvil measurements showed a threshold behavior. When the average reflectivity, such as in a 3-km cube, was less than approximately 5 dBZ, the electric field magnitude was <3 kV m 1 . Based on these

Understanding the relationships between lightning, cloud microphysics, and airborne radar-derived storm structure during Hurricane Karl (2010)

This study explores relationships between lightning, cloud microphysics, and tropical cyclone (TC) storm structure in Hurricane Karl (16 September 2010) using data collected by the NASA DC-8 and Global Hawk (GH) aircraft during NASA’s Genesis and Rapid Intensification Processes (GRIP) experiment. The research capitalizes on the unique opportunity provided by GRIP to synthesize multiple datasets from two aircraft and analyze the microphysical and kinematic properties of an electrified TC. Five coordinated flight legs through Karl by the DC-8 and GH are investigated, focusing on the inner-core region (within 50km of the storm center) where the lightning was concentrated and the aircraft were well coordinated. GRIP datasets are used to compare properties of electrified and nonelectrified inner-core regions that are related to the noninductive charging mechanism, which is widely accepted to explain the observed electric fields within thunderstorms. Three common characteristics of Karl’s electrified regions are identified: 1) strong updrafts of 10‐20ms 21 , 2) deep mixed-phase layers indicated by reflectivities .30dBZ extending several kilometers above the freezing level, and 3) microphysical environments consisting of graupel, very small ice particles, and the inferred presence of supercooled water. These characteristics describe an environment favorable for in situ noninductive charging and, hence, TC electrification. The electrified regions in Karl’s inner core are attributable to a microphysical environment that was conducive to electrification because of occasional, strong convective updrafts in the eyewall.

A Low-Noise, Microprocessor-Controlled, Internally Digitizing Rotating-Vane Electric Field Mill for Airborne Platforms

This paper reports on a new generation of aircraft-based rotating-vane-style electric field mills designed and built at NASA’s Marshall Space Flight Center. The mills have individual microprocessors that digitize the electric field signal at the mill and respond to commands from the data system computer. The mills are very sensitive (1 V m 1 bit 1 ), have a wide dynamic range (115 dB), and are very low noise (1 LSB). Mounted on an aircraft, these mills can measure fields from 1Vm 1 to 500 kV m 1 . Once-per-second commanding from the data collection computer to each mill allows for precise timing and synchronization. The mills can also be commanded to execute a self-calibration in flight, which is done periodically to monitor the status and health of each mill.

Classification of tropical oceanic precipitation using high-altitude aircraft microwave and electric field measurements

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 reflectivit...

Two-dimensional speed and optical risetime estimates for natural and triggered dart leaders

We report velocities, risetimes, and other optical measurements of a set of 35 natural and 26 triggered dart leaders. All of the dart leaders are from negative strokes. The data were taken with our return stroke velocity device mounted on the National Severe Storms Laboratory mobile laboratory. The average two-dimensional (2-D) speed for the natural leaders is 1.9 ± 0.2 x 10 7 m s -1 , while the triggered dart leader average 2-D speed is 1.3 ± 0.1 x 10 7 m s -1 . These two averages are significantly different. We find no significant change in the dart leader 2-D speed with height. The mean 10-90% optical risetime for the natural dart leaders is 2.6 ± 0.4 μs. The optical risetime is evaluated from short channel segments (length of 3.2 ± 1.7 m) located within 100 m of the ground. The corresponding risetime for triggered leaders is 1.4 ± 0.4 μs. These averages are significantly different. We calculated an average dart leader head length of 35 ± 5.4 m. We find no significant difference between natural and triggered dart leader head lengths. The channel traversed by the dart leader and before the return stroke is often bright enough to be detected by our instrument. We find that the ratio of the channel brightness after the dart leader head to the dart leader head is closer to unity for triggered dart leaders. We see no clear relationship between dart leader optical risetimes and subsequent return stroke peak transmission line model (TLM) current. We find that natural dart leader speeds are correlated with the return stroke peak TLM currents: the peak TLM current increases with increasing natural dart leader 2-D speed. We see no correlation between triggered dart leader speeds and subsequent return stroke peak TLM currents. At a given dart leader speed, natural return strokes will have smaller peak TLM currents. Overall, these results indicate that there are significant differences between natural and triggered dart leaders.