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Earle Williams

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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

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Earle Williams and Nilton Renno

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The ice phase is included in thermodynamic calculations of convective available potential energy (CAPE) for a large number of soundings in the tropical atmosphere, at both land and ocean stations. It is found that the positive-buoyancy contribution to CAPE resulting from the latent heat of fusion more than offsets the negative-buoyancy contribution due to water loading in the reversible thermodynamic process. The departure from moist neutrality in much of the tropical atmosphere exhibits a threshold in boundary-layer wet-bulb potential temperature of 22°–23°C. The corresponding sea surface temperature is approximately 26°C, close to the empirical threshold for hurricane formation, which suggests that conditional instability plays an important role in the latter phenomenon. The simultaneous presence of finite CAPE and infrequent deep convection in the tropics is tentatively attributed to the convective inhibition energy (CINE) and to the mixing process that destroys positive buoyancy in incipient cloud parcels.

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Nilton O. Rennó and Earle R. Williams

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

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

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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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Rosana Nieto Ferreira, Thomas Rickenbach, Nick Guy, and Earle Williams

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A radar-based analysis of the structure, motion, and rainfall variability of westward-propagating squall-line mesoscale convective systems (SLMCSs) in Niamey, Niger, during the African Monsoon Multidisciplinary Activities (AMMA) 2006 special observing period is combined with an analysis of 700-mb (hPa) winds and relative vorticity to study the relationship between SLMCSs and African easterly waves (AEWs). Radar results show that SLMCSs were the most important rainmakers in Niamey and accounted for about 90% of the rainfall despite being present less than 17% of the time. Analysis of the 700-mb synoptic-scale flow revealed that during the 2006 West African monsoon season the African easterly jet vacillated between about 10° and 15°N on time scales of 1–2 weeks. AEWs followed the jet as it vacillated north and south, thereby producing two preferred paths for AEWs propagating past Niamey’s longitude, a northern track along 8°–16°N and a southern track along 2°–6°N. It was found that Niamey SLMCSs occurred westward of the trough of AEWs propagating along either track. The properties of SLMCSs must then be placed in the context of their location relative to these two AEW tracks, rather than in the trough and ridge pattern of a single AEW track. Radar analysis further indicated that although the total amounts of rainfall produced by SLMCSs occurring in both African easterly jet latitude regimes were similar, significant structural differences occurred between the two groups of systems. SLMCSs that formed to the west of AEW troughs propagating along the northern track had a significantly larger mean stratiform rain fraction in an environment of lower convective available potential energy when compared with the SLMCSs that occurred to the west of the troughs of AEWs in the southern track. The authors conclude that AEWs that propagated farther north provided a more favorable environment for stratiform rain production in Niamey SLMCSs than those AEWs located farther south. These results may be helpful to studies of the two-way interaction between AEWs and convection in West Africa.

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

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

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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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