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J. O. Yoe and R. Rüster

Abstract

The SOUSY (sounding system) VHF (very high frequency) radar in Germany has been used to make observations of the upper troposphere and lower stratosphere for a number of cases during which the jet stream was overhead, or nearly so. The horizontal and vertical wind components have been calculated from oblique-beam Doppler radial velocities using a velocity-azimuth display (VAD) technique and averaged over periods ranging from 3 h to about 1 day. The vertical wind is found to reverse direction near the height of the maximum horizontal wind in all cases observed, in general agreement with a conceptual model and with other radar measurements. Most commonly, downward vertical motion is observed below the jet-stream wind maximum and upward motion above it for these cases, but in one instance such a circulation has been found to undergo a reversal as the position of the jet shifted. The vertical velocities have magnitudes in excess of 0.5 m s−1. NO minimum critical horizontal wind speed sums to be required in order for the vertical circulation feature to be observed.

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G. D. Nastrom, R. Rüster, and G. Schmidt

Abstract

The perturbations to the static stability (and hence to the radar reflectivity) and to the velocity in a vertically propagating gravity wave are correlated, and the sign of the correlation depends on whether the wave is propagating upward or downward. The wave-induced correlation between radar reflectivity and vertical velocity is the basis of a hypothesis to explain the downward bias in long-term averages of the vertical velocity seen at extratropical sites by wind profiler radars, and for predictions of biases in the horizontal wind speeds and in the vertical momentum flux seen by profiler radars. In this study, the hypothesis that mean vertical velocity is related to the correlation between perturbations to vertical velocity and signal power is tested. Observations with very high time and vertical resolution from the SOUSY VHF radar are used. It is found that the mean vertical velocity in the midtroposphere (2.4–6.3 km) is downward (upward) when the perturbations to vertical velocity and to backscattered power over this height range are negatively (positively) correlated. Similar results are found during summer and winter periods. Results are also similar when the radar was upwind of the nearby Harz Mountains compared to when the radar was downwind of the mountains.

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R. Rüster, G. D. Nastrom, and G. Schmidt

Abstract

Measurements using the Sounding System (SOUSY) VHF radar in the Harz Mountains, Germany, were carried out in the troposphere and lower stratosphere in the summer of 1996. To study the structure and the dynamics with high temporal and spatial resolution, the beam was pointing continuously in the vertical direction, and a 75-m range resolution was used. Three case studies are analyzed in detail: one within a frontal passage, one during a developing ridge, and one during a thunderstorm.

Fine structures in the echo intensity that could not be resolved with the standard resolution of 300 m are observed during the frontal passage and are related to humidity fluctuations. Integrating the high-resolution velocity data with time makes it possible to estimate vertical displacements that are used to indicate vertical transport. The velocities are analyzed harmonically and interpreted as waves with periods of about 6 h and amplitudes of a few centimeters per second.

Stable layers evident in the troposphere and lower stratosphere during the approach of a building ridge of anticyclonic flow have slopes with time reminiscent of the latitudinal pattern of the isentropes of the larger-scale flow field.

Observations during the thunderstorm reveal strong convective cells with periods of about 10–20 min, associated up- and downdrafts, and a series of persistent stable layers in the troposphere ahead of the thunderstorm.

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Tommy R. Shepherd, W. David Rust, and Thomas C. Marshall

Abstract

Earlier studies of mesoscale convective system stratiform regions have shown that large electric fields and charge densities are found near the 0°C level. Here 12 soundings of the electric field were analyzed through the 0°C level in various types of electrified stratiform clouds. For each electric field sounding, the thermodynamic sounding and supporting radar data were also studied. For comparison, five soundings not from stratiform clouds were included. Charge densities were found at or near 0°C in the stratiform clouds of at least 1 nC m−3 in eight of the soundings, and four of those had charge densities of at least 2 nC m−3. Of the stratiform soundings, 11 had an electric field magnitude of greater than 50 kV m−1 near 0°C, and 7 of those had an electric field magnitude of at least 75 kV m−1. The evidence suggests that melting may be the primary cause of the charge density found at and below 0°C in electrified stratiform clouds. In all 12 of the stratiform soundings, positive charge density was found at or near 0°C, and 11 of those had weaker negative charge density below. The evidence further suggests these two features do not exist in the absence of a bright band and (usually) an associated quasi-isothermal layer.

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Nicole R. Lund, Donald R. MacGorman, Terry J. Schuur, Michael I. Biggerstaff, and W. David Rust

Abstract

On 19 June 2004, the Thunderstorm Electrification and Lightning Experiment observed electrical, microphysical, and kinematic properties of a small mesoscale convective system (MCS). The primary observing systems were the Oklahoma Lightning Mapping Array, the KOUN S-band polarimetric radar, two mobile C-band Doppler radars, and balloonborne electric field meters. During its mature phase, this MCS had a normal tripolar charge structure (lightning involved a midlevel negative charge between an upper and a lower positive charge), and flash rates fluctuated between 80 and 100 flashes per minute. Most lightning was initiated within one of two altitude ranges (3–6 or 7–10 km MSL) and within the 35-dBZ contours of convective cells embedded within the convective line. The properties of two such cells were investigated in detail, with the first lasting approximately 40 min and producing only 12 flashes and the second lasting over an hour and producing 105 flashes. In both, lightning was initiated in or near regions containing graupel. The upper lightning initiation region (7–10 km MSL) was near 35–47.5-dBZ contours, with graupel inferred below and ice crystals inferred above. The lower lightning initiation region (3–6 km MSL) was in the upper part of melting or freezing layers, often near differential reflectivity columns extending above the 0°C isotherm, which is suggestive of graupel formation. Both lightning initiation regions are consistent with what is expected from the noninductive graupel–ice thunderstorm electrification mechanism, though inductive processes may also have contributed to initiations in the lower region.

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Stephanie A. Weiss, W. David Rust, Donald R. MacGorman, Eric C. Bruning, and Paul R. Krehbiel

Abstract

Data from a three-dimensional lightning mapping array (LMA) and from two soundings by balloon-borne electric field meters (EFMs) were used to analyze the electrical structures of a multicell storm observed on 25 June 2000 during the Severe Thunderstorm Electrification and Precipitation Study (STEPS). This storm had a complex, multicell structure with four sections, each of whose electrical structure differed from that of the others during all or part of the analyzed period. The number of vertically stacked charge regions in any given section of the storm ranged from two to six. The most complex charge and lightning structures occurred in regions with the highest reflectivity values and the deepest reflectivity cores. Intracloud flashes tended to concentrate in areas with large radar reflectivity values, though some propagated across more than one core of high reflectivity or into the low-reflectivity anvil. Intracloud lightning flash rates decreased as the vertical extent and maximum value of reflectivity cores decreased. Cloud-to-ground flash rates increased as cores of high reflectivity descended to low altitudes. Most cloud-to-ground flashes were positive. All observed positive ground flashes initiated between the lowest-altitude negative charge region and a positive charge region just above it. The storm’s complexity makes it hard to classify the vertical polarity of its overall charge structure, but most of the storm had a different vertical polarity than what is typically observed outside the Great Plains. The electrical structure can vary considerably from storm to storm, or even within the same storm, as in the present case.

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W. David Rust, William L. Taylor, Donald R. MacGorman, and Roy T. Arnold

In 1978 we began a coordinated effort to study the electrical behavior of large and severe thunderstorms that form over the Great Plains of the central United States. Methods of approach include the study of characteristics of individual phenomena and storm case studies. Our goal is to understand the interrelationships between electrical phenomena and the dynamics and precipitation of storms. Evidence that interrelationships do exist can be seen in the results to date. In one squall-line storm we have studied, 44% of all observed lightning flashes were cloud-to-ground (CG); the total flashing rate averaged 12 min−1 and coarsely followed the changes in Doppler-derived maximum updraft speed. Most of the intracloud (IC) discharge processes in a supercell severe storm were located predominately around the region of the intense updraft of the mesocyclone and near large gradients in reflectivity and horizontal velocity.

Both 10 cm and 23 cm wavelength radars have been used to detect lightning radar echoes. The lightning echoes from the 10 cm radar generally had peak signals 10–25 dB greater than the largest precipitation echo in the storm, and they usually were observed where precipitation reflectivities were less than maximum. Comparison of lightning echoes and electric field changes shows that abrupt increases in radar reflectivity often are associated with return strokes and K-type field changes.

CG flashes that lower positive charge to earth have been observed to emanate from the wall cloud, high on the main storm tower, and well out in the downwind anvil of severe storms. The percentage of CG flashes that lower positive charge is apparently small.

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Douglas M. Mach, Donald R. MacGorman, W. David Rust, and Roy T. Arnold

Abstract

We have tested a network of magnetic direction-finders (DFs) that locate ground strikes in Oklahoma and surrounding states in order to determine detection efficiency for the network and systematic errors in azimuth (i.e., site errors) for each of four DF sites. Independent data on lightning strike locations were obtained with a television (TV) camera on a mobile laboratory and an all-azimuth TV system at the National Severe Storms Laboratory (NSSL). In two tests using these data, we found a location detection efficiency of about 70% for storms at about 70 and 300 km from the center of the network. Systematic errors in azimuth were determined by comparing locations from the lightning strike locating system with strikes located from the mobile laboratory system; also, for a single DF at NSSL, strike azimuths from the DF were compared with azimuths from the all-azimuth TV system for storms near NSSL. Furthermore, we developed a technique for using redundant DF data to determine systematic errors in azimuth measurements for each DF site. Azimuthal errors found by this analytic technique were consistent with errors found by using the two sets of direct measurements. The azimuthal errors are themselves a function of azimuth, with peak amplitudes ranging from less than 5° for DFs located at favorable sites to about 11° for one DF located at an unfavorable site.

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Eric C. Bruning, W. David Rust, Donald R. MacGorman, Michael I. Biggerstaff, and Terry J. Schuur

Abstract

Lightning mapping, electric field, and radar data from the 26 May 2004 supercell in central Oklahoma are used to examine the storm’s charge structure. An initial arc-shaped maximum in lightning activity on the right flank of the storm’s bounded weak echo region was composed of an elevated normal polarity tripole in the region of precipitation lofted above the storm’s weak echo region. Later in the storm, two charge structures were associated with precipitation that reached the ground. To the left of the weak echo region, six charge regions were inferred, with positive charge carried on hail at the bottom of the stack. Farther forward in the storm’s precipitation region, four charge regions were inferred, with negative charge at the bottom of the stack. There were different charge structures in adjacent regions of the storm at the same time, and regions of opposite polarity charge were horizontally adjacent at the same altitude. Flashes occasionally lowered positive charge to ground from the forward charge region. A conceptual model is presented that ties charge structure in different regions of the storm to storm structure inferred from radar reflectivity.

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Donald R. MacGorman, W. David Rust, Paul Krehbiel, William Rison, Eric Bruning, and Kyle Wiens

Abstract

Balloon soundings were made through two supercell storms during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in summer 2000. Instruments measured the vector electric field, temperature, pressure, relative humidity, and balloon location. For the first time, soundings penetrated both the strong updraft and the rainy downdraft region of the same supercell storm. In both storms, the strong updraft had fewer vertically separated charge regions than found near the rainy downdraft, and the updraft’s lowest charge was elevated higher, its bottom being near the 40-dBZ boundary of the weak-echo vault. The simpler, elevated charge structure is consistent with the noninductive graupel–ice mechanism dominating charge generation in updrafts. In the weak-echo vault, the amount of frozen precipitation and the time for particle interactions are too small for significant charging. Inductive charging mechanisms and lightning may contribute to the additional charge regions found at lower altitudes outside the updraft. Lightning mapping showed that the in-cloud channels of a positive ground flash could be in any one of the three vertically separated positive charge regions found outside the updraft, but were in the middle region, at 6–8 km MSL, for most positive ground flashes. The observations are consistent with the electrical structure of these storms having been inverted in polarity from that of most storms elsewhere. It is hypothesized that the observed inverted-polarity cloud flashes and positive ground flashes were caused by inverted-polarity storm structure, possibly due to a larger than usual rime accretion rate for graupel in a strong updraft.

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