Hugh J. Christian

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;

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

Abstract Four years of observations from the NASA Optical Transient Detector and Global Atmospherics National Lightning Detection Network are combined to determine the geographic distribution of the climatological intracloud–cloud-to-ground (CG) lightning ratio, termed Z, over the continental United States. The value of Z 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 8–9 in the central-upper Great Plains. There is some indication that Z covaries with ground elevation, although the relationship is nonunique. Little evidence is found to support a latitudinal covariance. 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 Z anomalies in the Great Plains are coincident with anomalies in the climatological percentage of positive CG occurrence, as well as in the occurrence of...

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.

Comparison of ground‐based 3‐dimensional lightning mapping observations with satellite‐based LIS observations in Oklahoma

3-dimensional lightning mapping observations obtained during the MEaPRS program in central Oklahoma during June, 1998 have been compared with observations of the discharges from space, obtained by NASAs Lightning Imaging Sensor (LIS) on the TRMM satellite. Excellent spatial and temporal correlations were observed between the two sets of observations. Most of the detected optical events were associated with intracloud discharges that developed into the upper part of the storm. Cloud-to-ground discharges that were confined to mid- and lower-altitudes tended not to be detected by LIS. Extensive illumination tended to occur in impulsive bursts toward the end or part way through intracloud flashes and appeared to be produced by energetic K-changes that typically occur at these times.

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.

A survey of thunderstorm flash rates compared to cloud top height using TRMM satellite data

The relationship between cloud height and lightning activity is examined using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. Coincident data from the precipitation radar (PR) and Lightning Imaging Sensor aboard the TRMM satellite are used to examine whether lightning flash rate is proportional to the fifth power of cloud top height. This study is unique in that (1) the relationship between instantaneous rather than maximum storm height and flash rate is obtained and (2) relatively unbiased full data sets for different locations and seasons over the globe are used. The relationship between thunderstorm height and flash rate is nonlinear with large variance. The overall trend shows that flash rate increases exponentially with storm height. Some tall thunderstorms do not have large flash rates, but the reverse situation never occurs. The fifth power dependency that is derived from scaling laws is not inconsistent with, but not necessarily required by, the observed data.

Relationships between lightning activity and various thundercloud parameters: satellite and modelling studies

Abstract The lightning frequency model developed by Baker et al. [Baker, M.B., Christian, H.J., Latham, J., 1995. A computational study of the relationships linking lightning frequency and other thundercloud parameters, Q. J. R. Meteorol. Soc., 121, 1525–1548] has been refined and extended, in an effort to provide a more realistic framework from which to examine computationally the relationships that might exist between lightning frequency f (which is now being routinely measured from a satellite, using the NASA/MSFC Optical Transient Detector (OTD)) and a variety of cloud physical parameters. Specifically, superior or more comprehensive representations were utilised of: (1) glaciation via the Hallett–Mossop (H–M) process; (2) the updraught structure of the model cloud; (3) the liquid-water-content structure of the model cloud; (4) the role of the reversal temperature T rev in influencing lightning characteristics; (5) the critical breakdown field for lightning initiation; and (6) the electrical characteristics of the ice crystal anvil of the model cloud. Although our extended studies yielded some new insights into the problem, the basic pattern of relationships between f and the other parameters was very close to that reported by Baker et al. (1995). The more elaborate treatment of T rev restricted somewhat the range of conditions under which reverse-polarity lightning could be produced if the cloud glaciated via H–M, but confirmed the earlier conclusion that such lightning would not occur if the glaciation was of the Fletcher type. The computations yielded preliminary support for the hypothesis that satellite measurements of f might be used to determine values of the ice-content of cumulonimbus anvils: a parameter of climatological importance. The successful launch and continuing satisfactory functioning of the OTD [Christian, H.J., Goodman, S., 1992. Global observations of lightning from space, Proc. 9th Int. Conf. on Atmospheric Electricity, St. Petersburg, pp. 316–321; Christian, H.J., Blakesee, R.J., Goodman, S.J., 1992. Lightning imaging sensor (LIS) for the earth observing system. NASA Tech. Memorandum, 4350] make it possible—with a high degree of precision—to measure lightning location, occurrence time and frequency f over extensive areas of the Earths surface. Measured global distributions of lightning and associated lightning stroke radiance demonstrate that: lightning activity is particularly pronounced over the tropics, much greater over land than over the oceans, and exhibits great seasonal variability; lightning radiance tends to be greater over the oceans, less when lightning activity is high, and greater in the Northern Hemisphere winter than summer.

Determination of ice precipitation rates and thunderstorm anvil ice contents from satellite observations of lightning

Abstract The continuous satisfactory functioning of satellite-borne devices for the detection of global lightning offers the opportunity to explore relationships between lightning frequency f and other thundercloud parameters, notably, in this paper, the precipitating and non-precipitating contents and fluxes of ice. Simple calculations predict that the lightning frequency f is proportional to the product of the downward flux of solid precipitation through the body of the thundercloud and the upward flux of ice crystals into its anvil. This prediction is reinforced by computations performed using the multiple lightning model of Baker et al. [Q. J. R. Meteorol. Soc. 121 (1995) 1525; Atmos. Res. 51 (1999) 221]. Calculations indicate that the separation of charge and associated field development in thunderclouds are not significantly limited by charge saturation of the interacting hydrometeors: and that the mutual interactions of graupel pellets in the charging zones of thunderstorms can significantly enhance electric field development, culminating in lightning. An examination of data from the satellite-borne Lightning Imaging Sensor (LIS) and TRMM Microwave Imager (TMI) suggests that thunderstorms with the highest frequency of total lightning also possess the most pronounced microwave scattering signatures at 37 and 85 GHz. A total of 292 individual thunderstorms were examined, and a log-linear relationship was shown to exist (one for each frequency) between the number of optical lightning pulses produced by each storm and the corresponding microwave brightness temperatures. These relationships are consistent throughout the seasons in a variety of regimes (12 sites encompassing five continents, as well as oceanic measurements), suggesting that global relationships may be found to exist between lightning activity and cloud ice content.

A Comparison of the Optical Pulse Characteristics of Intracloud and Cloud-to-Ground Lightning as Observed above Clouds

Abstract The time-resolved optical waveforms at 777.4 nm and electric-field changes produced by intracloud and cloud-to-ground lightning flashes were measured above clouds from a U2 airplane (flying at a height of 20 km) at the same time that ground-based measurements of lightning were obtained from a mobile laboratory and a regional lightning location network. The U2 optical pulse trains are examined for variability both within and between flashes. The optical pulse characteristics of cloud-to-ground flashes are further subdivided into first strokes, subsequent strokes, and intracloud components (k-changes). Descriptive statistics on these pulse categories have been compiled for 25 visually confirmed cloud-to-ground flashes (229 optical pulses) and 232 intracloud flashes (3126 optical pulses). The pulse shapes and intensities of intracloud and cloud-to-ground flashes as viewed from above cloud are shown to exhibit remarkably similar waveshapes, radiances, and radiant energy densities. The median radiance...

Lightning-induced energetic electron flux enhancements in the drift loss cone

With its relatively low altitude (520 670 km) orbit, SAMPEX is mostly below the stable trapping region where charged particles repeatedly drift around the Earth, especially at midlatitudes. Recent analyses of SAMPEX data have revealed a surprisingly common set of observations of enhanced energetic (150 keV) electron fluxes at L 3, during times when SAMPEX was located such that any electrons that it observed were in the drift loss cone (and were thus destined to be precipitated upon reaching the longitude of the South Atlantic Magnetic Anomaly in the course of their eastward drift). The data were acquired with the Heavy Ion Large Telescope on board SAMPEX, which provides high time resolution measurements (30 ms sample rate) of electrons above energy thresholds of 150 keV and 1 MeV. Preliminary examination of 1995 SAMPEX data (when the 150 keV detector was available) revealed hundreds of cases of newly enhanced drift loss cone fluxes in localized L shell regions often associated with individual thunderstorms. In one case, SAMPEX data from three consecutive days (95/196, 95/197, and 95/198) were analyzed as the satellite proceeded northbound over the same ground track from west of New Zealand and Hawaii toward Alaska, with the underlying lightning activity documented by the Optical Transient Detector on board the OrbView-1 satellite (750 km, 70 inclination circular orbit). Enhanced fluxes observed on SAMPEX during day 95/197 were directly associated with an oceanic storm just to the west of the SAMPEX ground track, which was well placed to generate the observed drift loss cone flux enhancements. The drift loss cone electron flux enhancements were also observed 20 min later as SAMPEX crossed the same L shells in the north and in subsequent orbits, indicating that lightning-induced precipitation of electrons into the drift loss cone persisted at least for a few hours. Data from UARS satellite, passing through the same region within the same hour, also confirmed the presence of L-dependent structure and further allowed the determination of the electron energy spectra, which exhibited a general shape and range strikingly similar to previously documented spectral characteristics of lightning-induced electron precipitation (LEP) events in the bounce loss cone (Voss et al., 1998). This similarity lends support to the argument that the observed drift loss cone features are produced by the LEP process. In summary, SAMPEX data indicate that globally distributed thunderstorms may continually precipitate energetic electrons from the radiation belts, producing transient enhancements in the drift loss cone that are detected within the few hour periods as they drift around the Earth and precipitate in the South Atlantic Magnetic Anomaly.