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Joachim Kuettner

Abstract

It has been generally accepted that segregation of charges in thunderstorms is accomplished by gravitational sedimentation. In contrast to the much-studied phenomena of charge generation and charge distribution, the mechanism of charge segregation has commanded little attention so far although it is the connecting link between the generation and distribution of charges. As a consequence, conclusions as to the “proper” polarity of charging effects have been based on relatively weak reasoning.

A more thorough investigation of the sedimentation process reveals that the mechanism is more complicated and tends to “mask” rather than to segregate charges. Only the net space charge is perceptible through the electric field gradient.

The masking process is illustrated by studying the development of charges on solid precipitation in a supercooled water cloud. During the charging process, a volume of cloud air obviously loses the charge withdrawn by precipitation particles falling through it. The theory shows that it takes only about one minute from the onset of precipitation until the opposite charges on precipitation and cloud particles compensate each other completely in a given space. After this “compensation period”, the cloud charge begins to predominate over the masked precipitation charge and determines the polarity of the (net) space charge. However, it does not grow indefinitely.

Subsequently air conductivity enforces an equilibrium in which the two opposite charges mask each other in a fixed ratio expressed by the “masking factor”. The rate at which the equilibrium is approached is controlled by the ionic relaxation period. The masking factor is given simply by the ratio of compensation period to relaxation period.

It is further shown that the result is different if updrafts exceed the falling speed of precipitation. The precipitation charge then predominates over the cloud charge. In this way, the distribution of vertical motions in the mature thunderstorm cell tends to favor the development of a charge dipole. A positive ice phase is tentatively concluded.

At certain “stop levels” (for example near the freezing level), an “unmasking process” sets in which is capable of exposing the hidden precipitation charge.

Quantitative estimates of the masking process are based on precipitation mechanisms. The resulting field gradients seem to bear out the essential properties of observed thunderstorm fields. Especially the discontinuity in the freezing level is confirmed. It is pointed out that the masking process works in every type of charge segregation, regardless of whether or not precipitation is an essential participant.

Some implications of the masking process indicate a need for a critical review of generally accepted concepts in the field of thunderstorm electricity.

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Joachim Kuettner

Abstract

An investigation of 125 storms (67 thunderstorms and 58 shower clouds with high electric fields) completed during 1945–1948 at the Zugspitze Observatory in Germany (10,000 ft) is presented. The results are discussed with regard to electrical conditions, precipitation, and some other meteorological details. Approximately 80 per cent of the observations were made inside clouds. The more important aspects of the results concern the following points:

1. Average position of space charges inside thunderclouds. A local positive charge is found precisely at the freezing level; it is located in the center of precipitation and lightning and has a restricted horizontal depth (order of magnitude: 1 km). In the same vertical air column, but at lower temperatures (around −8C), the center of the negative main charge is evidenced. The center of the positive main charge appears to be outside this air column, following at a distance somewhat behind the negative charge and at a higher level.

2. Role of precipitation in thunderclouds. Solid precipitation prevails for the greatest part inside the clouds. In the precipitation center, graupel (especially in the form of snow pellets), consisting of frozen cloud droplets, is the basic type of hydrometeor. Hail is relatively rare. During the later periods of a storm, snow crystals (formed from water vapor by sublimation) prevail in the area of more steady and moderate precipitation. Ice particles inside the clouds are usually oppositely charged to the simultaneous potential gradient.

3. Relation between space charges and precipitation. There is some evidence that in mature thunderstorm cells two vertical dipoles follow each other: (a) a “graupel dipole” with a more concentrated upper charge of negative sign and (b) a more elevated “snow dipole” with a concentrated upper positive charge. The lightning exchange between these upper space-charges gives the illusion of one slanted dipole.

The distribution of charges indicates that the two upper space-charges are attached to the cloud droplets, the weaker lower charges to the precipitation. This indicates a preferred positive sign of the graupel particles and a preferred negative sign of snow crystals.

The possibility is considered that the graupel dipole is formed by a primary process and gives rise to an electric field in which the snow dipole develops by a secondary process.

4. Cell structure and parcel structure. Meteorological and electrical records indicate that the individual cells in a thunderstorm are further sub-divided into units which are approximately one order of magnitude smaller in linear dimension. These parcels may be related to turbulence systems and may account for the local character of high electric fields and for the complicated structure of thunderstorms.

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Joachim P. Kuettner
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Joachim P. Kuettner and Joshua Holland

This article, written a few days before the beginning of the field operations, summarizes the scientific and operational plans for BOMEX, the “Barbados Oceanographic Meteorological Experiment.” The basic concept of BOMEX has been described earlier by its former director, Ben Davidson (1968). His untimely death created a temporary crisis in the scientific direction of the project; however, his experiment design and his underlying thoughts have proved sound and no major changes in the layout or scientific objectives have been necessary. In the opinion of the project staff, Ben Davidson remains the man behind BOMEX. Continuity has been preserved throughout the BOMEX preparations by the Project Manager, William Barney, and his multi-agency staff.

At this time, ships, airplanes and scientists are converging on Barbados. Therefore, the plans described here are by necessity final. Only facts of life can change them.

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Joachim P. Kuettner and Stanley D. Soules
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Joachim P. Kuettner and Thomas H. R. O'Neill

The problem of airflow over and around mountains, as originally proposed by J. Charney, R. Hide, F. Mesinger, and G. Goetz, was approved in 1978 as a subprogram of the Global Atmospheric Research Program (GARP) by the Joint Organizing Committee (JOC) of the International Council of Scientific Unions (ICSU) and the World Meteorological Organization (WMO). ALPEX will be the field project of this subprogram and, as the name indicates, the general area of the Alps has been selected as its site. The primary observing period will be during March and April 1982. ALPEX will complete the series of large international field projects of GARP (UCAR, 1980; ICSU/WMO, 1980e).

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Joachim P. Kuettner, J. Doyne Sartor, and Zev Levin

Abstract

Most of the precipitation related theories on charge generation in thunderstorms fall into one of two categories: the inductive or polarization mechanism initiated by the ambient fair-weather field, and the non-inductive mechanism connected with certain electrochemical or thermoelectric particle characteristics. Our numerical study addresses the question of which mechanism gives more realistic results with regard to charge distribution and hold strength and what effect a combination of the two processes produces. The investigation is a first attempt using a simplified model.

In this model the microphysical processes of particle growth and simultaneous electrification are embedded in a steady state two-dimensional vortex circulation with and without vertical wind shear. The net space charge and potential are obtained everywhere in the cloud and the resulting electric fields are calculated. Computations are made for the collisions of growing solid precipitation (graupel) particles with either supercooled droplets (ice-water case) or with ice crystals (ice-ice case).

The results indicate that the non-inductive mechanism produces a rapid growth of the electric field in the early stages but tends to level out at a stable value considerably below the breakdown field strength. The inductive mechanism in turn shows a slow initial field growth with quickly varying charge distributions of often inverted polarity; however, it will reach breakdown field strength eventually due to its quasi-exponential growth character. Only the combination of the two processes achieves realistic thunderstorm conditions. It appears that the non-inductive mechanism controls the charge distribution and its polarity, and the inductive mechanism the field strength. both ice-water and ice-ice collisions give similar results, the only difference being a higher elevation of the charge dipole. in the ice-ice case. The opposite precipitation and cloud charges are always strongly masked.

The results permit some interesting conclusions on the origin of the fair-weather field.

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GATE

final international scientific plans

International and Scientific Management Group for GATE, Joachim P. Kuettner, David E. Parker, David R. Rodenhuis, Heinrich Hoeber, Helmut Kraus, and G. Philander
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Randolph H. Ware, David W. Fulker, Seth A. Stein, David N. Anderson, Susan K. Avery, Richard D. Clark, Kelvin K. Droegemeier, Joachim P. Kuettner, J. Bernard Minster, and Soroosh Sorooshian

“SuomiNet,” a university-based, real-time, national Global Positioning System (GPS) network, is being developed for atmospheric research and education with funding from the National Science Foundation and with cost share from collaborating universities. The network, named to honor meteorological satellite pioneer Verner Suomi, will exploit the recently shown ability of ground-based GPS receivers to make thousands of accurate upper- and lower-atmospheric measurements per day. Phase delays induced in GPS signals by the ionosphere and neutral atmosphere can be measured with high precision simultaneously along a dozen or so GPS ray paths in the field of view. These delays can be converted into integrated water vapor (if surface pressure data or estimates are available) and total electron content (TEC), along each GPS ray path. The resulting continuous, accurate, all-weather, real-time GPS moisture data will help advance university research in mesoscale modeling and data assimilation, severe weather, precipitation, cloud dynamics, regional climate, and hydrology. Similarly, continuous, accurate, all-weather, real-time TEC data have applications in modeling and prediction of severe terrestrial and space weather, detection and forecasting of low-altitude ionospheric scintillation activity and geomagnetic storm effects at ionospheric midlatitudes, and detection of ionospheric effects induced by a variety of geophysical events. SuomiNet data also have potential applications in coastal meteorology, providing ground truth for satellite radiometry, and detection of scintillation associated with atmospheric turbulence in the lower troposphere. The goal of SuomiNet is to make large amounts of spatially and temporally dense GPS-sensed atmospheric data widely available in real time, for academic research and education. Information on participation in SuomiNet is available via www.unidata.ucar.edu/suominet.

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THE TERRAIN-INDUCED ROTOR EXPERIMENT

A Field Campaign Overview Including Observational Highlights

Vanda Grubišić, James D. Doyle, Joachim Kuettner, Stephen Mobbs, Ronald B. Smith, C. David Whiteman, Richard Dirks, Stanley Czyzyk, Stephen A. Cohn, Simon Vosper, Martin Weissmann, Samuel Haimov, Stephan F. J. De Wekker, Laura L. Pan, and Fotini Katopodes Chow

The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotemporal characteristics and internal structure of a tightly coupled system consisting of an atmospheric rotor, terrain-induced internal gravity waves, and a complex terrain boundary layer. In addition, T-REX had several ancillary objectives including the studies of UTLS chemical distribution in the presence of mountain waves and complex-terrain boundary layer in the absence of waves and rotors. This overview provides a background of the project including the information on its science objectives, experimental design, and observational systems, along with highlights of key observations obtained during the field campaign.

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