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Raùl E. López

Abstract

Law echoes (C-scale in GATE terminology., approximately 100 to 10 000 km2) seem to produce most of the precipitation that occurs in tropical systems. These large echoes are seen to last several hours and comprise many smaller and short-lived cells that grow and die within their bounds or are merged with them. In this paper, quantitative GATE radar data are used to statistically characterize these composite echoes as a whole, and to study their internal structure, formation and development processes. In addition, statistics of the individual cells making up the composite echoes are presented. These are compared to similar data for cells that develop individually away from the aggregates. In this way, some information is obtained about the duration, rain-producing ability and the size of those cells that merge to form larger groups compared to those clouds that form, develop and die relatively independently of one another.

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Raúl E. Lopez and Ronald L. Holle

Abstract

Lightning location data from northeastern Colorado and central Florida for the summer months of 1983 have been studied to ascertain the diurnal development of spatial distributions of flash frequencies. In both locales, a clearly identifiable influence was found of the underlying topography and resulting diurnal circulations on the time and place of occurrence of lightning ground strikes. Although both locations are strongly convective, lightning exhibited a very large day-to-day variability, since most locations had no lightning on half the days. The study also emphasizes the utility of flash data, once carefully corrected, in studying both the characteristics of lightning and the patterns of topographically induced convection.

In Colorado, the total summer flash activity was concentrated just east of the Continental Divide from Longs Peak southward, then eastward along the north side of the much lower Palmer Lake Divide. Two maxima of seven flashes per km2 were located within this peak of activity. First lightning began before noon just east of the Continental Divide, then moved cast during the afternoon; new centers appeared near the Divide again during the afternoon. In the evening thunderstorms moved to the northeast along the north slope of the Palmer Lake Divide. These results verified that lightning activity is in substantial agreement with earlier summer studies of radar and surface winds that define the influence of a low-level circulation in the North Platte River basin.

In central Florida, the total summer lightning activity was highest from Cape Canaveral northward along the cast, and westward from the Cape to Orlando. Peak values were over eight flashes per km2. Significant lightning began along this coastal maximum in midafternoon, followed by the thunderstorms in the maximum toward Orlando. Lightning ended over land by 2200 EST, and some activity occurred offshore during the night.

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Raúl E. López and Ronald L. Holle

Abstract

Long-term fluctuations in the number of lightning deaths and injuries from 1959 to 1990 have been examined for the contiguous United States. After taking into account the population increase, there was an overall trend amounting to a 30% reduction in casualties during the period. It is possible that this trend resulted from improved forecasts and warnings, increased education efforts of the public, and socioeconomic changes.

In addition, there was a 40% reduction in the number of deaths but not of nonfatal injuries. This additional reduction in deaths was probably due to improved medical attention given to lightning victims and a wider knowledge of cardiopulmonary resuscitation techniques among the public. Improved medical care would increase the chances of a person surviving a lightning strike but would not affect the total number of casualties.

Superimposed on the overall downward trend there were fluctuations of one or two decades in duration. From 1959 until 1968 there was a sharp reduction in the number of casualties, but starting in 1969 and continuing until the present, there was an overall increase. These oscillations appear to be climatologically related. The patterns of these fluctuations were parallel to nationwide changes in thunder-day frequencies, cyclone frequencies, and surface temperature values, representing thunderstorm, synoptic, and continental scales.

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Raúl E. López and Ronald L. Holle

Abstract

Long-term changes in the number of lightning deaths from 1900 to 1991 have been examined for the contiguous United States. The population-normalized series revealed an exponential decrease in the number of deaths per million people. This exponential trend is also present in the decrease of the rural U.S. population for the period. The two datasets agree remarkably well and this suggests the downward trend in lightning deaths resulted to a large extent from the reduction of the rural population.

Superimposed on the overall downward trend in lightning deaths were fluctuations of two or three decades in duration. The patterns of these fluctuations are paralleled by nationwide changes in thunder-day frequencies and average surface temperature values. Thus, it appears that the lightning death fluctuations are climatically induced.

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David O. Blanchard and Raül E. López

Abstract

Although they are a fairly consistent feature, the sea-breeze and lake-breeze convergence lines and the associated convection over south Florida during the summer may vary considerably from one day to the next. Daily radar maps indicate a few basic recurring patterns. Analyses of radiosonde data show significant differences corresponding to the different patterns in the local thermodynamic parameters, most notably the mixing ratio. Changes in the synoptic-scale wind field correspond closely to changes in the observed radar patterns and the local thermodynamic conditions. Explanation of the formation and development of the different patterns of convection is given in terms of the complex interaction between the regional-, synoptic-, peninsular- and local-scale circulations.

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E. Brian Curran, Ronald L. Holle, and Raúl E. López

Abstract

Lightning-caused fatalities, injuries, and damage reports for the United States are listed in the National Oceanic and Atmospheric Administration publication Storm Data. Previously published studies of lightning casualties and damages in the United States covered only portions of the period since Storm Data began publication in 1959, did not weight by population, or did not present complete information with respect to time of year and day. Therefore, an analysis was made of all 3239 deaths, 9818 injuries, and 19 814 property damage reports in Storm Data due to lightning from 1959 to 1994. This paper depicts lightning casualties (deaths and injuries combined) and damage reports stratified by state and region of the United States, decade, population, time of year and day, and all other information in Storm Data.

Florida had the most deaths (345) and injuries (1178) from lightning, and Pennsylvania had the most damage reports (1441). A rate of one fatality per 86 000 cloud-to-ground flashes is estimated from recent lightning detection network information. After population was taken into account, Wyoming and New Mexico had the highest death, injury, and casualty rates. The U.S. rate is 0.42 lightning deaths per million people per year from 1959 to 1994. Highest population-weighted damage rates were on the plains, but the pattern was variable from decade to decade. July had more lightning entries of all types than any other month; damage reports were spread more evenly through the year. Casualties and damages in the northern half of the United States had narrower distributions centered on summer than did the southern half. Two-thirds of the casualties were between noon and 6 p.m.; damage reports were relatively frequent at night in the plains and Midwest. Most lightning incidents involved one person, and males were five times as likely as females to be killed or injured. Storm Data excludes most small losses but includes more expensive and widely known lightning-related losses.

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Abraham Gagin, Daniel Rosenfeld, William L. Woodley, and Raul E. Lopez

Abstract

Volume scan radar studies incorporating the use of an elaborate method of defining and tracking convective rain cells through their lifetime have been used to

a) Explore and verify, in quantitative terms, the basic tenet of the technique of cloud seeding aimed at producing dynamic effects. This technique relates increases in the depth of convective cells, assumed to occur due to this type of seeding, to corresponding increases in the treated cells' rainfall intensity, area and duration of precipitation and, consequently, to the total yield of rainfall volume.

b) Employ the data gathered on the gross properties of rainfall of convective cells, namely their heights, intensities, precipitation areas and their durations and total rain volume, to estimate the effect of seeding, if any, on their properties.

These studies suggest that seeding convective cells for dynamic effects affected the preceding properties of these cells in a manner that resulted in increases in their total rainfall and that the positive changes in these properties could be predicted from the changes in maximum cell height following seeding.

The effect of seeding appears to be strongest for cells treated early in their life cycle with a substantial amount of AgI (i.e., more than 600 g). Seeding effects of 22% increases in cell heights and over 100% increases in cell rain volume are indicated under such seeding conditions. The significance levels of these results are found to be 2.1% and 0.6%, respectively.

The positive effects produced by seeding on the AgI treated cells may have resulted in a compensating negative effect on the smaller untreated clouds forming in the vicinity of these treated cells.

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Andrew I. Watson, Ronald L. Holle, and Raúl E. López

Abstract

Convective bursts and breaks in the southwest U.S. monsoon are investigated in a lightning context because cloud-to-ground (CG) lightning is an excellent indicator of deep convection. Bursts and breaks are identified using six years of Bureau of Land Management CG lightning information. Composited upper-air analyses for 12 bursts and 10 breaks are developed to examine the synoptic-scale differences between these two regimes. Anomaly patterns are investigated, and average burst and break regimes am presented.

This investigation shows the importance of moisture, the location of the subtropical ridge axis, and the high-plateau thermal low. For the burst, die ridge axis is displaced northward across Arizona and New Mexico and moisture is usually abundant in the southwestern United States. During the break, the ridge retreats southward into northern Mexico, giving way to dry westerly winds across Arizona. The high-plateau thermal low is firmly in place during July and August, and it pulls low-level moist air upslope into the Great Basin from the Gulf of California through the only opening available, which is the lower desert of Arizona.

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Andrew I. Watson, Raúl E. López, and Ronald L. Holle

Abstract

Cloud-to-ground (CG) lightning shows great variability across Arizona from one year to the next as well as from one day to the next. Availability of moisture, location of the subtropical ridge axis, transitory troughs in both the westerlies and easterlies, and low-level moisture surges from the Gulf of California can affect thunderstorm occurrence, which, in turn, will affect lightning production. Diurnal CG lightning patterns in Arizona are also determined by daily heating cycles and topography. Six years of Bureau of Land Management CG flash data are used in this investigation.

In Arizona, lightning usually starts first, on a daily basis, in the plateau region and extends in an arc from the White Mountains of eastern Arizona westward across the Mogollon Rim and then northward onto the Kaibab Plateau of northern Arizona. Flash activity moves in a more or less continuous fashion off the plateau, south and westward down the topography gradient, and enters the lower desert by early evening. At the same time, flash activity develops in the highlands of southeast Arizona and moves west-northwestward, reaching the lower desert by late afternoon. Cloud-to-ground activity across Arizona is at a minimum at 1000 MST and rapidly reaches a peak at 1600 MST in central and southeast Arizona. Maximum CG activity in the Phoenix vicinity is at 2200 MST and appears to be the result of the intersection of both plateau-generated and southeast highlands-generated convection. Precipitation and lightning are well correlated, except that precipitation seems to linger longer than lightning, probably due to the occasional development of mesoscale convective systems, which product light stratiform precipitation during their dissipation stage.

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Andrew I. Watson, Ronald L. Holle, and RaúL. E. López

Abstract

Two national cloud-to-ground (CG) lightning-detection systems were operating across the contiguous United States during 1993. These two networks are compared to each other and to WSR-88D radar information collected by the Twin Lakes, Oklahoma, radar on 9 June 1993 as a squall line moved through central Oklahoma. Next Generation Weather Radar Information Dissemination Service data are used to investigate the relationship of CG lightning to WSR-88D echo tops and vertically integrated liquid (VIL). Such relationships appear not to have been investigated in such fine time and space resolution to date.

When lightning is normalized by the frequency of occurrence of 4 km × 4 km resolution echo-top areas, the greatest percentage of echoes with lightning occurs when echo-top heights exceed 50 000 ft (15.2 km). The percentage of echoes with lightning drops significantly as echo tops decrease. The relationship of VIL with lightning is not as clearly defined. The frequency of echoes with lightning increases gradually with 4 km × 4 km resolution VIL values from 15 kg m−2 to about 40–45 kg m−2. Then a drop in the frequency occurs with higher values of VIL. However, a maximum in the frequency of echoes with lightning was observed at very high values of VIL (>65 kg m−2) by both lightning-detection systems.

The Atmospheric Research Systems, Inc., (ARSI) time-of-arrival lightning detection system recorded nearly twice the number of positive flashes when compared with the GeoMet Data Services (GDS) direction-finder (DF) system. It appears that intraclound (IC) flashes contaminated the ARSI positive dataset to some extent. ARSI also recorded 24% fewer negative strokes probably due to communications saturation or signal attenuation.

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