Abstract

We have developed a device to measure lightning-channel propagation velocities. It consists of eight solid state silicon photodetectors mounted behind precision horizontal slits in the focal plane of a 50-mm lens on a 35-mm camera body. Each detector has a 0.1° vertical field of view that is separated from adjacent detector slits by 2.8°. The horizontal field-of-view for each detector is 41° and the total vertical field of view for the device is 21°. The signal from each detector is amplified by a circuit with a 10%–90% rise time of 0.6 μs and an equivalent decay time of 400 μs. The eight Photodetector pulses, IRIG-B time, and slow and fast electric field change waveforms are recorded on a 14-track analog tape recorder with an upper frequency response Of 1.0 MHz and a maximum dynamic interchannel timing error of 0.6 μs. To provide images of lightning geometry and permit time-to-thunder measurements, color video and sound are recorded with a standard VHS video cassette recorder. The return stroke velocity (RSV) device, video camera, and microphone are installed and coaxially aimed in an environmental enclosure on a remotely controlled pan-tilt unit atop our mobile laboratory, permitting the recording of lightning signals at remote sites and while mobile. To evaluate the performance of the RSV device, we have analyzed 12 natural return strokes from Alabama, Florida, and Oklahoma and 4 return strokes triggered at the Kennedy Space Center, Florida. The velocities we determined vary from 1.2 to 2.5×10⁸ m s⁻¹, with an average of 1.7×⁸ m s⁻¹±0.8 × ⁸ m s⁻¹. From comparisons of our results to those of a streaking camera, we find no significant differences between the velocities obtained from the same strokes with the two systems. We also find no differences between the characteristics of the pulses or the velocities calculated from them while the RSV device is moving or stationary.

Electric field (E) soundings in the stratiform regions and transition zones of mesoscale convective systems (MCSs) are reported. Most of the E soundings were made during the 1991 Cooperative Oklahoma Profiler Studies (COPS-91). Multiple E soundings were made in several MCSs. All of the E soundings collected here can be grouped into one of two types. Within each type the soundings and the inferred charge structures are remarkably similar from one place in an MCS to another and from one MCS to another. The charge regions inferred from the E soundings are hundreds of meters thick and have charge densities up to 5.3 nC m⁻³. Typically, the maximum E in the soundings is about 100 kV m⁻¹ Here, E soundings from three classes of MCSs are discussed. The bow-echo MCSs have simpler vertical charge structures with four main charge regions, while squall-line MCSs and predominantly stratiform MCSs have five main charge regions. In all of the E soundings there is a substantial region of charge and a large E at or near 0°C.

Abstract

A group of thunderstorms developed in western Oklahoma during the afternoon of 26 April 1984. Two of these storms initially exhibited characteristics of low-precipitation (LP) thunderstorms. Lightning ground flashes produced by these storms were mostly positive. These storms split, with one right-moving component evolving into a tornadic supercell. Ground flashes produced by the supercell, however, were predominantly negative. The highest rate of positive ground flashes (1.5 min⁻¹) occurred during LP storm splitting and merging, when about 84% of ground flashes were positive. The maximum total ground-strike rate was 3.4 min⁻¹and occurred during the tornadic supercell phase and when all but one of 136 ground flashes were negative. Analysis of lightning ground-strike and radar reflectivity data reveals a concentration of positive ground flashes in areas of maximum reflectivity within the LP storms; furthermore, the concentration of positive flashes appeared during storm split. After storm splitting and merging, the number of positive ground flashes in all cells decreased. Recent studies suggested a relationship between high values of wind-shear magnitude within the cloud-bearing layer and the production of positive ground flashes. Analyses of soundings in the environments of the LP and supercell thunderstorms on this day show that the magnitude of the vector-averaged shear vector within the cloud-bearing layer was 4.8 × 10⁻³ and 3.8 × 10⁻³ s⁻¹ respectively. Both are above the previously published thresholds hypothesized for positive flash production. Thus, our analysis suggests that strong shear may be a necessary, but not sufficient, condition for the production of positive ground flashes. Contrary to earlier reports in the literature, our data indicate the height of the −10°C isotherm is not a key parameter for positive flash production in warm-season convection. Finally, a link between positive ground flashes and hail is again suggested.

Abstract

In order to place instruments for measuring meteorological and electrical parameters into thunderstorms, we developed an inexpensive apparatus that allows us to inflate, transport, and launch balloons in high winds. The launching apparatus is a cylinder of “bubble” plastic that is made by joining the sides of the cylinder together with a VELCRO ”rip strip.” We launch a balloon by pulling the rip strip rapidly. This allows the balloon to pop upward into the ambient low-level wind and carry its instrumentation aloft. We construct different-sized launch tubes to accommodate particular sizes of balloons and have successfully launched balloons in winds of about 20 m s⁻¹.

Abstract

A new balloon-borne instrument created by the authors and referred to as the q-d instrument that measures the charge q and size d of precipitation particles is discussed. The instrument measures charge with an induction cylinder size with an optical sensor, and fall speed by the time difference between the two. A second induction cylinder at the top serves as the entry point and detects precipitation that splashes off the entry. In this way, particles contaminated by splashing are removed from the data. It is capable of measuring particle sizes ranging from 0.8 to 8.0 mm in diameter and charges ranging from ±4 to ±400 pC. Since the size is measured optically, one can detect uncharged particles and measure their size. The q-d instrument does not show evidence of corona at its extremities until the electric field is as large as 100 kV m⁻¹ at 700 mb.

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.

Abstract

Meteorological radiosondes that use navigation systems to determine winds (and horizontal location) can be susceptible to data loss in thunderstorm environments. This paper reports on tests of a radiosonde that uses the Global Positioning System (GPS) for windfinding. Tests were made by flying the GPS radiosonde into three thunderstorms on free balloons that also carried an electric field meter and a long-range navigation (loran) radiosonde of a type previously tested. The GPS radiosonde performed without any significant loss of wind or thermodynamic data in in-storm maximum electric fields of up to −104 kV m⁻¹. Also, no obvious deleterious effect on radiosonde data was found from the presence of nearby lightning. The radiosonde was further tested in a laboratory-produced electric field in an ambient atmospheric pressure of about 70 kPa, in which the radiosonde functioned normally in a vertical electric field up to 160 kV m⁻¹ and in a horizontal electric field up to 100 kV m⁻¹, the respective maximum applied. Radiosondes that were sprayed with water to simulate flight in rain performed correctly in an electric field of 135 kV m⁻¹—the maximum that could be applied safely. The hypothesized reason for the excellent windfinding performance in high electric fields is partly the very short antenna length needed for GPS reception. Other factors, which could not be assessed in this study, may include the inherent low-noise susceptibility of the GPS signals and the processing circuitry. The tests showed that the GPS radiosonde obtains wind data in larger electric fields than does the loran radiosonde. It is concluded that GPS radiosondes will acquire windfinding data in most, if not all, thunderstorm and nonthunderstorm clouds that contain high electric fields. The thermodynamic data were also very good in the large electric fields.

Abstract

Remote sensing instruments have the ability to collect data over extensive temporal periods and spatial regions. A common thread between all these sensors is the need to relate the measured quantity to a meaningful observation of a system property. If the relationship between each measurement and the set of atmospheric quantities that influence that measurement is known, the problem can be reduced to a set of linear equations. Solving for the unknown atmospheric quantities then becomes a linear algebra problem, where the solution vector is equal to the inverse of the kernel matrix multiplied by the set of independent measurements. However, in most remote sensing applications, inversion of the kernel matrix is unstable, resulting in the amplification of measurement and computational uncertainties. Techniques to circumvent this error amplification have focused on methods of constraining the solution. In this paper, the authors adapt an existing technique to do such an inversion. Noise reduction is accomplished by the addition of double-sided inequality constraints for each unknown variable. The advantage of such a technique is the ability to individually adjust the solution space of each individual unknown, depending on a priori knowledge.

The inversion algorithm is applied to the problem of retrieving radar Doppler spectra, which have been artificially broadened by turbulent air motions. First, to test the algorithm, radar Doppler spectra were simulated using known drop size and vertical air motion distributions. The simulated spectra were used as input to the retrieval algorithm, and the results were compared to the initial quiet-air spectrum. Results indicate that accurate retrievals can be performed despite the addition of moderate amounts of noise to the simulated spectra. Then, to demonstrate the practical retrieval of quiet-air Doppler spectra, the algorithm was used to process radar observations collected from continental stratocumulus. From these retrievals, a two-dimensional map of the large-scale vertical motions within the cloud was constructed as well as a profile of vertical velocity variance. In addition, a drop size distribution was also derived from an updraft region of the cloud.

Abstract

The time-resolved optical waveforms at 777.4 nm and electric-field changes produced by intracloud and cloud-to-ground lightning flashes were measured above clouds from a U2 airplane (flying at a height of 20 km) at the same time that ground-based measurements of lightning were obtained from a mobile laboratory and a regional lightning location network. The U2 optical pulse trains are examined for variability both within and between flashes. The optical pulse characteristics of cloud-to-ground flashes are further subdivided into first strokes, subsequent strokes, and intracloud components (k-changes). Descriptive statistics on these pulse categories have been compiled for 25 visually confirmed cloud-to-ground flashes (229 optical pulses) and 232 intracloud flashes (3126 optical pulses). The pulse shapes and intensities of intracloud and cloud-to-ground flashes as viewed from above cloud are shown to exhibit remarkably similar waveshapes, radiances, and radiant energy densities. The median radiance at cloud top is approximately 7 × 10⁻³ W m⁻² sr⁻¹, and the median energy density is 3 × 10⁻⁶ J m⁻² sr⁻¹. A simple physical model is used to estimate, for comparative purposes, the radiance and energy density of the original light source within the cloud. First stroke optical pulses are seldom the most radiant or energetic pulses produced by ground discharges as seen from above the clouds. The intracloud components of cloud-to-ground flashes typically produce the optical pulses with the largest peak radiance within a cloud-to-ground flash; however, subsequent strokes are more likely to have the largest energy densities and most complex pulse shapes. On average, intracloud flashes have almost twice as many optical pulses as ground discharges. There is often significant pulse structure variation within and between individual flashes. Because of this variation, multiple stroke cloud-to-ground flashes are difficult to distinguish uniquely from intracloud flashes solely on the basis of their optical signature above cloud. Single stroke cloud-to-ground flashes, however, appear to have a unique single pulse optical signature. The relevance and implications of these pulse characteristics for lightning mapping from satellite-based optical sensors is addressed.

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.