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John H. Helsdon Jr.

Abstract

A two-dimensional, slab-symmetric, time-dependent cloud model has been devised to simulate deep convection in the atmosphere. The dynamics and thermodynamics of deep convection are prescribed and the microphysics of the liquid phase is parameterized. Within this framework, the electrical nature of the atmosphere has been added. Small positive and negative ions as well as the charge associated with rain and cloud particles are included. The electrical properties are allowed full interaction with the hydrodynamic properties of the model. Charge is transported by conduction, convection and turbulent diffusion, and free ions interact with hydrometeors through conduction, diffusion and evaporation. A separation probability is specified for cloud and raindrops colliding in the electric field allowing for charge transfer by the polarization mechanism. Similarly, the charge on cloud droplets is transferred to raindrops during coalescence.

The formulation for the simulation of the chaff seeding process is then developed. An initial distribution of chaff fibers is devised allowing for the introduction of a prescribed amount of chaff at any time and altitude. Transport of the chaff fibers in the wind field is accounted for and ion production by chaff in the electric field is parameterized. Five cases, a control case and four seeding cases, are examined to explore the effect of the chaff ions on the electrical properties of the cloud.

The control case (case 1) is reviewed showing its basic dynamical, microphysical and electrical nature. The chaff seeding experiments are then discussed with the seeding altitudes, times and the amount of chaff dispensed differentiating the four cases.

It is found that chaff seeding at a rate of 4 kg km−1, which yields an initial maximum concentration of 8.66 × 10−8 kg m−3, attains a reduction in the electric field strength within the cloud in two ways. In a direct manner, the chaff ions reduce the charge on the hydrometeors by conduction and diffusion. In an indirect manner, the reduction of charge on the hydrometeors in one region is seen to reduce the electric field in other regions, thereby reducing the efficiency of the polarization mechanism there, and subsequently reducing the amount of charge separated. Comparison of an early and late seeding case reveals that the initial effect of the chaff ions is different, but that after a short time the effects become comparable and the final result of the two cases is nearly identical. This suggests that as long as the chaff fibers penetrate the active portions of the cloud, the exact seeding time is of little consequence. Finally, it is suggested that chaff seeding may be useful in discriminating between inductive and non-inductive charge separation mechanisms.

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J. Doyne Sartor and John H. Helsdon Jr.

Abstract

The rate that vertical vorticity is created or retarded in electrified clouds is calculated from the cross product of the charge gradient and the electrical field and compared with the magnitude of the vertical vorticity produced dynamically. Calculations are made for clouds on the thunderstorm, mesoscale and synoptic scales for midlatitude and tropical conditions. The results show that in moderately electrified clouds with particle charges an order of magnitude less than the observed maxima, the production of vorticity due to electrostatic forces approaches or slightly exceeds the dynamic production in thunderstorm anvil clouds and on the mesoscale in the tropics. Highly electrified clouds with maximum particle charges can produce vorticity at comparable rates to its dynamic production on all scales except the synoptic scale in midlatitudes.

To the extent that cloud charging conditions due to the global electric field and mid-tropospheric conductivity conditions are perturbed by solar events or solar-included electromagnetic disturbances, some solar influence on the electrical conditions could be expected in the mid-troposphere where clouds of the suitable type and extent form as a consequence of the normal meteorological processes. Where the electrical conditions exceed the threshold required for the production of vorticity dynamically, organized circulations produced electrophysically are possible in a solar-disturbed large-scale electrical environment.

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Lance F. Bosart, Cosmo J. Vaudo, and John H. Helsdon Jr.

Abstract

Detailed synoptic analyses are made of mesoscale frontogenesis situations along the southeastern New England coast. Temperature gradients of 5–10C over 5–10 km along a line separating light northerly or northwesterly flow from stronger easterly flow are common. The coastal thermal contrast may follow 6–12 hr after the establishment of a cold anticyclone to the north of New England and persist for 12 hr. Dissipation takes place when in offshore cyclonic circulation reaches the latitude of southern New England such that coastal winds back to northerly. The whole process is called coastal frontogenesis.

A persistent tendency for coastal fronts to stagnate along a Boston to Providence line is noted. Precipitation appears to be enhanced along and just on the cold side of the frontal zone. Arguments are presented in favor of the importance of surface friction, orography, coastal configuration, and land-sea thermal contrast on the life cycle of coastal fronts. An inspection of eight years of data suggests that coastal frontogenesis is a maximum in late fall and early winter when land-sea temperature contrasts are greatest. Evidence is presented in one case of a coastal front, acting as a channel for a developing secondary wave disturbance, with latent heat release associated with the precipitation maximum proving to be important for development.

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Scot C. Randell, Steven A. Rutledge, Richard D. Farley, and John H. Helsdon Jr.

Abstract

Numerical modeling studies of continental tropical and maritime tropical convection were conducted using the two-dimensional, nonhydrostatic, cloud electrification model developed at the South Dakota School of Mines and Technology. The model contains six classes of water (water vapor, cloud water, cloud ice, rain, snow, and graupel) and a full set of ion equations. All hydrometeors are permitted to exchange charge. Charge transfer between microphysical species is accomplished through a noninductive charging parameterization following Takahashi.

The goal of the numerical experiments was to examine the kinematic and microphysical differences that lead to marked differences in observed electrification between the break (continental) and monsoon (oceanic) convective regimes observed near Darwin, Australia. The break regime is associated with deep, intense convection that forms in high-CAPE (convective available potential energy) environments. Normally, copious amounts of lightning accompany break period convective events. Monsoon conditions are associated with heavy rain and relatively weak convection that forms in moderate to low-CAPE environments. Very little lightning activity is normally observed in the monsoon.

Three numerical simulations ranging from high- to low-CAPE conditions are presented. The results indicate that the electrification of the simulated storm critically depends on the juxtaposition of the level of charge reversal (LCR), which is in turn dependent on temperature and liquid water contents, and the particle interaction region, which is the level where ice particle collisions occur and thus where noninductive charging can take place. In the high-CAPE (break period) case, the LCR is located several kilometers below the interaction region, and strong in-cloud electric fields develop as a consequence. In the low- to moderate-CAPE (monsoon) cases, the LCR and interaction region are closely located in the vertical. As hydrometcors move across the LCR in both directions, the charge on their surfaces continually changes sign, thus preventing the development of a significant in-cloud electric field. It is further hypothesized that in conditions of zero to extremely low CAPE, the particle interaction region would he situated below the LCR, leading to the development of an inverted dipole (positive charge underlying negative charge), such as may occur in the stratiform regions of mesoscale convective systems.

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Tom A. Warner, John H. Helsdon Jr., Matthew J. Bunkers, Marcelo M. F. Saba, and Richard E. Orville
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Bruce A. Boe, Jeffrey L. Stith, Paul L. Smith, John H. Hirsch, John H. Helsdon Jr., Andrew G. Detwiler, Harold D. Orville, Brooks E. Mariner, Roger F. Reinking, Rebecca J. Meitín, and Rodger A. Brown

The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification.

Of primary interest were tracer experiments that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide.

Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunderstorms has resulted.

The objectives of the various project experiments and the specific facilities employed are described. Project highlights and some preliminary results are also presented.

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Timothy J. Lang, L. Jay Miller, Morris Weisman, Steven A. Rutledge, Llyle J. Barker III, V. N. Bringi, V. Chandrasekar, Andrew Detwiler, Nolan Doesken, John Helsdon, Charles Knight, Paul Krehbiel, Walter A. Lyons, Don MacGorman, Erik Rasmussen, William Rison, W. David Rust, and Ronald J. Thomas

During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from supercells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lightning mapping system, an armored research aircraft, electric field meters carried on balloons, mobile mesonet vehicles, instruments to detect and classify transient luminous events (TLEs; e.g., sprites and blue jets) over thunderstorms, and mobile atmospheric sounding equipment. The project featured significant collaboration with the local National Weather Service office in Goodland, Kansas, as well as outreach to the general public. The project gathered data on a number of different cases, including LP storms, supercells, and mesoscale convective systems, among others. Many of the storms produced mostly positive CG lightning during significant portions of their lifetimes and also exhibited unusual electrical structures with opposite polarity to ordinary thunderstorms. The field data from STEPS is expected to bring new advances to understanding of supercells, positive CG lightning, TLEs, and precipitation formation in convective storms.

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