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Dennis E. Buechler and Steven J. Goodman

Abstract

The effects of echo shape and radar viewing angle on detecting small thunderstorms with the NEXRAD storm identification algorithms are examined. The amorphous low level echo shapes are modeled as ellipses with major axes ranging from 5–15 km and minor axes varying between 2–5 km. The model echoes are then used to create a “probability of detection” chart that demonstrates the impact of storm asymmetry on cell identification. Moreover, we examine the algorithm performance on small thunderstorms observed near Huntsville, Alabama and Kennedy Space Center, Florida. The two thunderstorms observed near Huntsville also produced microbursts. The probability of storm detection using the NEXRAD default values for both Huntsville cases is less than 0,5 at the time of the first lightning discharge and less than 0.4 at microbursts onset. The Kennedy Space Center storms were already electrically active when the probability of detection was 0.5 or less. A new algorithm based on the analysis of 15 storms observed in Florida, Alabama and New Mexico is proposed that would identify storms as having Lightning if 40 dBZ reflectivity is present at the −10°C level and the echo top exceeds 9 km. This algorithm would have a 100% probability of detecting lightning producing storms 4–33 min before the first flash, a 7% false alarm rate and a critical success index of 93%.

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Patrick N. Gatlin and Steven J. Goodman

Abstract

An algorithm that provides an early indication of impending severe weather from observed trends in thunderstorm total lightning flash rates has been developed. The algorithm framework has been tested on 20 thunderstorms, including 1 nonsevere storm, which occurred over the course of six separate days during the spring months of 2002 and 2003. The identified surges in lightning rate (or jumps) are compared against 110 documented severe weather events produced by these thunderstorms as they moved across portions of northern Alabama and southern Tennessee. Lightning jumps precede 90% of these severe weather events, with as much as a 27-min advance notification of impending severe weather on the ground. However, 37% of lightning jumps are not followed by severe weather reports. Various configurations of the algorithm are tested, and the highest critical success index attained is 0.49. Results suggest that this lightning jump algorithm may be a useful operational diagnostic tool for severe thunderstorm potential.

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Katrina S. Virts and Steven J. Goodman

Abstract

The Lake Victoria basin of East Africa is home to over 30 million people, over 200 000 of whom are employed in fishing or transportation on the lake. Approximately 3000–5000 individuals are killed by thunderstorms yearly, primarily by outflow winds and resulting large waves. Prolific lightning activity and thunderstorm initiation in the basin are examined using continuous total lightning observations from the Earth Networks Global Lightning Network (ENGLN) for September 2014–August 2018. Seasonal shifts in the intertropical convergence zone produce semiannual lightning maxima over the lake. Diurnally, solar heating and lake and valley breezes produce daytime lightning maxima north and east of the lake, while at night the peak lightning density propagates southwestward across the lake. Cluster analysis reveals terrain-related thunderstorm initiation hot spots northeast of the lake; clusters also initiate over the lake and northern lowlands. The most prolific clusters initiate between 1100 and 1400 LT, about 1–2 h earlier than the average cluster. Most daytime thunderstorms dissipate without reaching Lake Victoria, and annually 85% of clusters producing over 1000 flashes over Lake Victoria initiate in situ. Initiation times of prolific Lake Victoria clusters exhibit a bimodal seasonal cycle: equinox-season thunderstorms initiate most frequently between 2200 and 0400 LT, while solstice-season thunderstorms initiate most frequently from 0500 to 0800 LT, more than 12 h after the afternoon convective peak over land. More extreme clusters are more likely to have formed over land and propagated over the lake, including 36 of the 100 most extreme Lake Victoria thunderstorms. These mesoscale clusters are most common during February–April and October–November.

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Steven J. Goodman, Dennis E. Buechler, and Paul J. Meyer

Abstract

A technique is presented for generating convective tendency products by combining satellite images with observations of cloud-to-ground lightning activity. Rapid scan (5-min) infrared satellite images are used to define the areal distribution of convection. Lightning flash rate trends provide diagnostic and predictive information pertaining to the growth and decay of the thunderstorms. A single derived product from these data can show the location of the lightning activity and convective cores, the spatial distribution of convective rainfall, the remaining cloudy and statiform rain areas, and the growing and decaying storms. Examples are given to illustrate how the flash rate trend may produce a much different and more useful portrayal of storm evolution than the time rate-of-change change of cloud-top blackbody temperatures. This difference can be exacerbated in mesoscale convective weather systems where the cirrus debris can mask the life history of the embedded convective elements.

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Dennis J. Boccippio, Steven J. Goodman, and Stan Heckman

Abstract

Observations from the National Aeronautics and Space Administration Optical Transient Detector (OTD) and Tropical Rainfall Measuring Mission (TRMM)-based Lightning Imaging Sensor (LIS) are analyzed for variability between land and ocean, various geographic regions, and different (objectively defined) convective “regimes.” The bulk of the order-of-magnitude differences between land and ocean regional flash rates are accounted for by differences in storm spacing (density) and/or frequency of occurrence, rather than differences in storm instantaneous flash rates, which only vary by a factor of 2 on average. Regional variability in cell density and cell flash rates closely tracks differences in 85-GHz microwave brightness temperatures. Monotonic relationships are found with the gross moist stability of the tropical atmosphere, a large-scale “adjusted state” parameter. This result strongly suggests that it will be possible, using TRMM observations, to objectively test numerical or theoretical predictions of how mesoscale convective organization interacts with the larger-scale environment. Further parameters are suggested for a complete objective definition of tropical convective regimes.

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Steven J. Goodman, Hugh J. Christian, and W. David Rust

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 at cloud top is approximately 7 × 10−3 W m−2 sr−1, and the median energy density is 3 × 10−6 J m−2 sr−1. A simple physical model is used to estimate, for comparative purposes, the radiance and energy density of the original light source within the cloud. First stroke optical pulses are seldom the most radiant or energetic pulses produced by ground discharges as seen from above the clouds. The intracloud components of cloud-to-ground flashes typically produce the optical pulses with the largest peak radiance within a cloud-to-ground flash; however, subsequent strokes are more likely to have the largest energy densities and most complex pulse shapes. On average, intracloud flashes have almost twice as many optical pulses as ground discharges. There is often significant pulse structure variation within and between individual flashes. Because of this variation, multiple stroke cloud-to-ground flashes are difficult to distinguish uniquely from intracloud flashes solely on the basis of their optical signature above cloud. Single stroke cloud-to-ground flashes, however, appear to have a unique single pulse optical signature. The relevance and implications of these pulse characteristics for lightning mapping from satellite-based optical sensors is addressed.

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Kevin R. Knupp, Bart Geerts, and Steven J. Goodman

Abstract

The evolution of a small, vigorous mesoscale convective system (MCS) over northern Alabama is described using Doppler radar, GOES satellite, surface mesonet, lightning, and sounding data. The MCS formed near noon in a relatively unstable environment having weak synoptic forcing and weak shear. The initiation of separate lines and clusters of deep convection occurred in regions exhibiting cumulus cloud streets, horizontal variations in stratocumulus cloud cover, and variations in inferred soil moisture. MCS growth via merger of storms within clusters and lines, and among the clusters, was accomplished largely through intersection of storm-scale and mesoscale outflow boundaries. The MCS maximum anvil area (∼60000 km2 at 220 K) and lifetime (8 h) were about 50% that of the typical Great Plains mesoscale convective complex (MCC).

Despite its smaller size, this MCS displayed many aspects that typify the mostly nocturnal Great Plains MCS. The precipitation output was highly variable due to the transient nature of the intense convective elements, many of which produced microbursts. The radar measurements documented the formation of a stratiform region along the trailing side of an intense convective line. This stratiform region formed as decaying convective cores coalesced, rather than through advection of precipitation particles directly from the convective region. Combined GOES IR imagery and radar reflectivity analyses within the stratiform region show a sinking anvil cloud top in the presence of increases in the vertical radar reflectivity gradient within the cloud during the maturation of the stratiform region.

During its intense developing stages, the MCS generated a peak cloud-to-ground (CG) flash rate of 2400 h−1, comparable to rates produced by larger MCCs. Early on, positive CG flashes were most prevalent around intense convective core regions exhibiting strong divergence at anvil level. During the latter stages, the emergence of positive CG was coincident with the formation of a prominent radar bright band within the stratiform region. Thus, a bipole was established, but its length was quite short at approximately 50 km, 25%–50% of the distance documented in other MCSs.

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Daniel J. Cecil, Steven J. Goodman, Dennis J. Boccippio, Edward J. Zipser, and Stephen W. Nesbitt

Abstract

During its first three years, the Tropical Rainfall Measuring Mission (TRMM) satellite observed nearly six million precipitation features. The population of precipitation features is sorted by lightning flash rate, minimum brightness temperature, maximum radar reflectivity, areal extent, and volumetric rainfall. For each of these characteristics, essentially describing the convective intensity or the size of the features, the population is broken into categories consisting of the top 0.001%, top 0.01%, top 0.1%, top 1%, top 2.4%, and remaining 97.6%. The set of “weakest/smallest” features composes 97.6% of the population because that fraction does not have detected lightning, with a minimum detectable flash rate of 0.7 flashes (fl) min−1. The greatest observed flash rate is 1351 fl min−1; the lowest brightness temperatures are 42 K (85 GHz) and 69 K (37 GHz). The largest precipitation feature covers 335 000 km2, and the greatest rainfall from an individual precipitation feature exceeds 2 × 1012 kg h−1 of water. There is considerable overlap between the greatest storms according to different measures of convective intensity. The largest storms are mostly independent of the most intense storms. The set of storms producing the most rainfall is a convolution of the largest and the most intense storms.

This analysis is a composite of the global Tropics and subtropics. Significant variability is known to exist between locations, seasons, and meteorological regimes. Such variability will be examined in Part II. In Part I, only a crude land–ocean separation is made. The known differences in bulk lightning flash rates over land and ocean result from at least two differences in the precipitation feature population: the frequency of occurrence of intense storms and the magnitude of those intense storms that do occur. Even when restricted to storms with the same brightness temperature, same size, or same radar reflectivity aloft, the storms over water are considerably less likely to produce lightning than are comparable storms over land.

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Rachel I. Albrecht, Steven J. Goodman, Dennis E. Buechler, Richard J. Blakeslee, and Hugh J. Christian

Abstract

Previous total lightning climatology studies using Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) observations were reported at coarse resolution (0.5°) and employed significant spatial and temporal smoothing to account for sampling limitations of TRMM’s tropical to subtropical low-Earth-orbit coverage. The analysis reported here uses a 16-yr reprocessed dataset to create a very high-resolution (0.1°) climatology with no further spatial averaging. This analysis reveals that Earth’s principal lightning hotspot occurs over Lake Maracaibo in Venezuela, while the highest flash rate density hotspot previously found at the lower 0.5°-resolution sampling was found in the Congo basin in Africa. Lake Maracaibo’s pattern of convergent windflow (mountain–valley, lake, and sea breezes) occurs over the warm lake waters nearly year-round and contributes to nocturnal thunderstorm development 297 days per year on average. These thunderstorms are very localized, and their persistent development anchored in one location accounts for the high flash rate density. Several other inland lakes with similar conditions, that is, deep nocturnal convection driven by locally forced convergent flow over a warm lake surface, are also revealed.

Africa is the continent with the most lightning hotspots, followed by Asia, South America, North America, and Australia. A climatological map of the local hour of maximum flash rate density reveals that most oceanic total lightning maxima are related to nocturnal thunderstorms, while continental lightning tends to occur during the afternoon. Most of the principal continental maxima are located near major mountain ranges, revealing the importance of local topography in thunderstorm development.

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Dennis J. Boccippio, Kenneth L. Cummins, Hugh J. Christian, and Steven J. Goodman

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 large positive CGs characteristic of organized or severe storms. This suggests that storm type, morphology, and level of organization may dominate over environmental cofactors in the local determination of this ratio.

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