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Earle R. Williams

Abstract

Comparisons are made between the seasonal behavior of the global electrical circuit and the surface air temperature for the Tropics and for the globe. Positive correlations between global circuit parameters and temperature are identified on both semiannual and annual timescales. Lightning is the global circuit quantity found most responsive to temperature, with a sensitivity of the order of 10% per 1°C. These findings lend further validity to the use of global circuit measurements as a diagnostic for global change.

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Earle R. Williams

Abstract

No Abstract available.

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Nilton O. Rennó and Earle R. Williams

Abstract

Measurements were made to determine the level of origin of air parcels participating in natural convection. Lagrangian measurements of conservative variables are ideal for this purpose. A simple remotely piloted vehicle was developed to make in situ measurements of pressure, temperature, and humidity in the convective boundary layer. These quasi-Lagrangian measurements clearly show that convective plumes originate in the superadiabatic surface layer. The observed boundary layer plumes have virtual temperature excesses of about 0.4 K in a tropical region (Orlando, Florida) and of about 1.5 K in a desert region (Albuquerque, New Mexico). The water vapor contribution to parcel buoyancy was appreciable in Orlando but in Albuquerque was insignificant. These observations indicate that convective available potential energy should he determined by adiabatically lifting air parcels from the surface layer, at screen level.

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Steven A. Rutledge, Earle R. Williams, and Thomas D. Keenan

DUNDEE (Down Under Doppler and Electricity Experiment) is described. DUNDEE was carried out in the vicinity of Darwin, Northern Territory, Australia, during the wet seasons of November 1988 through February 1989, and November 1989 through February 1990. The general goal of DUNDEE was to investigate the dynamical and electrical properties of tropical mesoscale convective systems and isolated deep convective storms. Darwin, situated at the southern tip of the “maritime continent,” experiences both monsoon and “break” period conditions during the wet season. We discuss the observational network deployed for DUNDEE and present preliminary scientific results. One particularly interesting observation is a large contrast in the frequency of total lightning between break period convection (high lightning rates) and convection in the monsoon trough (low lightning rates). A relationship between CAPE (convective available potentional energy) and total flash rate is presented and discussed to explain this observation.

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Chuntao Liu, Earle R. Williams, Edward J. Zipser, and Gary Burns

Abstract

The long-standing mainstay of support for C. T. R. Wilson’s global circuit hypothesis is the similarity between the diurnal variation of thunderstorm days in universal time and the Carnegie curve of electrical potential gradient. This rough agreement has sustained the widespread view that thunderstorms are the “batteries” for the global electrical circuit. This study utilizes 10 years of Tropical Rainfall Measuring Mission (TRMM) observations to quantify the global occurrence of thunderstorms with much better accuracy and to validate the comparison by F. J. W. Whipple 80 years ago. The results support Wilson’s original ideas that both thunderstorms and electrified shower clouds contribute to the DC global circuit by virtue of negative charge carried downward by precipitation. First, the precipitation features (PFs) are defined by grouping the pixels with rain using 10 years of TRMM observations. Thunderstorms are identified from these PFs with lightning flashes observed by the Lightning Imaging Sensor. PFs without lightning flashes but with a 30-dBZ radar echo-top temperature lower than −10°C over land and −17°C over ocean are selected as possibly electrified shower clouds. The universal diurnal variation of rainfall, the raining area from the thunderstorms, and possibly electrified shower clouds in different seasons are derived and compared with the diurnal variations of the electric field observed at Vostok, Antarctica. The result shows a substantially better match from the updated diurnal variations of the thunderstorm area to the Carnegie curve than Whipple showed. However, to fully understand and quantify the amount of negative charge carried downward by precipitation in electrified storms, more observations of precipitation current in different types of electrified shower clouds are required.

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Marcos L. Pessoa, Rafael L. Bras, and Earle R. Williams

Abstract

Weather radar, in combination with a distributed rainfall-runoff model, promises to significantly improve real-time flood forecasting. This paper investigates the value of radar-derived precipitation in forecasting streamflow in the Sieve River basin, near Florence, Italy. The basin is modeled with a distributed rainfall-runoff model that exploits topographic information available from digital elevation maps. The sensitivity of the flood forecast to various properties of the radar-derived rainfall is studied. It is found that use of the proper radar reflectivity-rainfall intensity (Z-R) relationship is the most crucial factor in obtaining correct food hydrographs. Errors resulting from spatially averaging radar rainfall are acceptable, but the use of discrete point information (i.e., raingage) can lead to serious problems. Reducing the resolution of the 5-min radar signal by temporally averaging over 15 and 30 min does not lead to major errors. Using 3-bit radar data (rather than the usual 8-bit data) to represent intensifies results in significant operational savings without serious problems in hydrograph accuracy.

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Spiros G. Geotis, Earle R. Williams, and Chester Liu

Abstract

No abstract available.

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Earle R. Williams, Spiros G. Geotis, and A. B. Bhattacharya

Abstract

Radar measurements and model studies are combined to investigate the plasma condition and the physical structure of lightning in thunderclouds. The lightning radar target is inferred to be an arclike plasma whose temperature exceeds 5000 K, thereby implying overdense plasma at all meteorological wavelengths. Lightning echoes are treated as volume targets and are modeled as treelike assemblages of conductive channels which are each long and thin compared to the radar wavelength. The channel lengths per unit volume deduced from more than one thousand reflectivity measurements at 11 cm wavelength range from 10−3 to 102 km km−3. Comparisons with more than 200 measurements at 5 cm wavelength show that the wavelength dependence is highly variable. On the average, the apparent dependence is λ−2 but this is unreliable because of the masking effects of precipitation. The infrequent detection of lightning at short wavelengths (λ ≥ 5 cm) is also attributed to masking rather than to an underdense plasma condition.

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Cynthia D. Engholm, Earle R. Williams, and Randall M. Dole

Abstract

Meteorological and electrical conditions associated with the occurrence of positive cloud-to-ground (CG) lightning (i.e., lightning that lowers positive charge to ground) are examined. Results from case studies in winter and summer storms reveal common features and lend support to the tilted dipole hypothesis. Lightning bipoles, whose lengths range from the convective scale to the mesoscale, are aligned with the vertical wind shear, with a predominance of negative locations in proximity to the deepest convection and a mixture of positive and negative locations displaced downshear from the deepest convection. Comparisons with radar data show that all lightning events am located within a distance of 10–20 km of precipitation extending from the surface to several kilometers above the O°C isotherm. Electrostatic field measurements beneath precipitation removed from the deepest convection indicate a positive dipole structure and a tilting deformation by vertical wind shear. These observations suggest that the principal contributor to positive lightning downshear of the deepest convection is mesoscale charge separation by differential particle motions rather than mesoscale advection over distances of 100 km or more.

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Walter A. Lyons, Thomas E. Nelson, Earle R. Williams, Steven A. Cummer, and Mark A. Stanley

Abstract

During the summer of 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program deployed a three-dimensional Lightning Mapping Array (LMA) near Goodland, Kansas. Video confirmation of sprites triggered by lightning within storms traversing the LMA domain were coordinated with extremely low frequency (ELF) transient measurements in Rhode Island and North Carolina. Two techniques of estimating changes in vertical charge moment (M q) yielded averages of ∼800 and ∼950 C km for 13 sprite-parent positive polarity cloud-to-ground strokes (+CGs). Analyses of the LMA's very high frequency (VHF) lightning emissions within the two mesoscale convective systems (MCSs) show that +CGs did not produce sprites until the centroid of the maximum density of lightning radiation emissions dropped from the upper part of the storm (7–11.5 km AGL) to much lower altitudes (2–5 km AGL). The average height of charge removal (Z q) from 15 sprite-parent +CGs during the late mature phase of one MCS was 4.1 km AGL. Thus, the total charges lowered by sprite-parent +CGs were on the order of 200 C. The regional 0°C isotherm was located at about 4.0 km AGL. This suggests a possible linkage between sprite-parent CGs and melting-layer/brightband charge production mechanisms in MCS stratiform precipitation regions. These cases are supportive of the conceptual MCS sprite-production models previously proposed by two of the authors (Lyons and Williams).

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