Zen-Ichiro Kawasaki

Wavelet-based data compression of power system disturbances using the minimum description length criterion

This paper introduces a compression technique for power disturbance data via discrete wavelet transform (DWT) and wavelet packet transform (WPT). The data compression leads to a potential application for remote power protection and power quality monitoring. The compression technique is performed through signal decomposition up to a certain level, thresholding of wavelet coefficients, and signal reconstruction. The choice of which wavelet to use for the compression is of critical importance, because the wavelet affects reconstructed signal quality and the design of the system as a whole. The minimum description length (MDL) criterion is proposed for the selection of an appropriate wavelet filter. This criterion permits selection not only of the suitable wavelet filter but also the best number of wavelet retained coefficients for signal reconstruction. The experimental study has been carried out for a single-phase to ground fault event, and the data compression results of using the suitable wavelet filter show that the compression ratios are less than 11 % and are reduced to more than a half of that percentage value by implementing an additional lossless coding.

A survey of thunderstorm flash rates compared to cloud top height using TRMM satellite data

The relationship between cloud height and lightning activity is examined using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. Coincident data from the precipitation radar (PR) and Lightning Imaging Sensor aboard the TRMM satellite are used to examine whether lightning flash rate is proportional to the fifth power of cloud top height. This study is unique in that (1) the relationship between instantaneous rather than maximum storm height and flash rate is obtained and (2) relatively unbiased full data sets for different locations and seasons over the globe are used. The relationship between thunderstorm height and flash rate is nonlinear with large variance. The overall trend shows that flash rate increases exponentially with storm height. Some tall thunderstorms do not have large flash rates, but the reverse situation never occurs. The fifth power dependency that is derived from scaling laws is not inconsistent with, but not necessarily required by, the observed data.

Impact of the 1997–98 El Niño Event on lightning activity over Indonesia

The 1997-98 El Niflo was one of the strongest El Nino Southern Oscillation (ENSO) events of this century. The major impact of the sea surface temperature (SST) change during this El Niflo event was the shift in convection activity from the western to the central and eastern Pacific ocean affecting the response of rain-producing cumulonimbus. As a result, convective rainfalls were suppressed near the Western Pacific regions and the Maritime Continent including Indonesia. On the other hand, the lightning activity during the El Nino period increased in contrast (on the average by 57%). As observed by the Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor and Precipitation Radar, the convective storms during the El Nino were more intense. This was supported by the evidence that the El Nino storms had greater vertical developments and thicker zones containing ice phase precipitation.

Spatial and temporal properties of optical radiation produced by stepped leaders

The relative light intensities as a function of height and time for two negative downward stepped leaders, A and B, recorded by a high-speed digital 16 × 16 photodiode array photographic system, are studied. For leader A it is found that the light waveform for each segment of the leader channel starts with a series of sharp light pulses followed by several slow-rising and longer-lasting light surges, with both the light pulses and surges superimposed on a continuous luminosity slope that has a long rising front followed by an almost constant light level. Analysis indicates that each light pulse involves a step process; it originates at the leader tip and appears to propagate upward, with the pulse amplitude suffering little degradation within the first several tens of meters to 200 m from the leader tip up (bright tip length) but with a severe attenuation above. The light surges are observed to be almost constant in amplitude above the bright tip, and for one of them an upward propagation speed of the order of 108 m/s is inferred. From appearances of the light pulses it is determined that the leader A has an overall velocity of 4.5–11.2×105 m/s, a step interval of 5–50 μs, and a step length of 7.9–19.8 m. For leader B the step light pulses are found to propagate from the leader tip back up at a speed of 0.14–1.7×108 m/s, and the overall leader velocity, the step interval, and the step length are determined to be about 4.9–5.8×105 m/s, 18–21 μs, and 8.5 m, respectively. In addition, on the basis of the light waveforms of the leader A it is inferred that the current of a stepped leader may consist of two parts: an impulsive current within the bright tip and a continuing current above it. After propagating along the bright tip up, because of increasing resistance and capacitance of the leader channel the impulsive current rapidly transforms into part of the continuing current.

Broad band interferometric measurement of rocket triggered lightning in Japan

A broad band interferometer to investigate the location of radio sources due to lightning flashes in two dimension has been manufactured and evaluated during the Rocket Triggered Lightning Experiment in Japan. Having high digitization rate, a broad band interferometer is not able to record the entire radiation process emitted by a lightning discharge. Therefore, we introduced a method of sequential triggering for each electromagnetic pulse that allows to record as long periods of radiation as we need. We obtained several data sets of upward stepped leaders for both negative polarity and positive one during the experiment. The apparent speed of an upward negative leader is estimated at 6 × 105 m/sec. The time intervals between successive electromagnetic pulses are from 11 to 93 microseconds with a mean of 22 microseconds. A triggered lightning to an experimental power transmission tower, normally it is called an altitude triggered lightning, is also conducted. One of the altitude triggered lightning lowered negative charges to the tower. In this case both the negative downward leader and the positive upward one can be generated. Some isolated pulses emitted by the downward negative stepped leader which attaches to the transmission tower are observed and located. Pulses emitted by the positive upward leader penetrated into a negatively charged region of a thundercloud are also observed. The location of radiation sources are distributed within a few degrees of azimuth. The time intervals between successive pulses are from 14 to 188 microseconds with a mean of 78 microseconds.

Simultaneous measurement of lightning parameters for strokes to the Toronto Canadian National Tower

Successful simultaneous measurements of significant parameters for lightning strikes to the Canadian National (CN) Tower in Toronto have been performed since the summer of 1991. Three 10-bit 10-ns computer-controlled double-channel digitizers, with long segmented memory, have simultaneously captured the current derivative at the CN Tower and the corresponding electric and magnetic fields 2.0 km north of the tower. Lightning flashes to the tower were videotaped from two mutually perpendicular directions for the purpose of constructing a three-dimensional image of the lightning path. Furthermore, the return stroke velocity, a parameter also needed for the analysis of lightning radiation models, has been measured by a computer-controlled photodiode system. In this paper all relevant parameters for a CN Tower lightning stroke, observed on August 17, 1991, are shown and analyzed.

MMSE Beam Forming on Fast-Scanning Phased Array Weather Radar

A fast-scanning phased array weather radar (PAWR) with a digital beam forming receiver is under development. It is important in beam forming for weather radar observation with temporally high resolution to form a stable and robust main lobe and adaptively suppress sidelobes with a small number of pulses in order to accurately estimate precipitation profiles (reflectivity, mean Doppler velocity, and spectral width). A minimum mean square error (MMSE) formulation with a power constraint, proposed in this paper, gives us adaptively formed beams that satisfy these demands. The MMSE beam-forming method is compared in various precipitation radar signal simulations with traditional beam-forming methods, Fourier and Capon methods, which have been applied in atmospheric research to observe distributed targets such as precipitation, and it is shown that the MMSE method is appropriate to this fast-scanning PAWR concept.

Sprite observations in the Northern Territory of Australia

Sprites, a form of brief luminous discharge in the upper atmosphere above a thunderstorm, were observed and imaged on two video cameras in Australias Northern Territory. These were the first such ground-based observations made outside the United States. Sprite discharges typically took place between the altitudes of 50 km and 80 km and spanned an average width of 44 km. Many of the sprite events were of long duration, with an average of 145 ms. These spatial and temporal features were similar to those observed from the ground and the air in the United States. During the longer events, some luminous discharge elements were observed to decay as other new elements formed. As the new elements were often laterally displaced from the old, the sprites sometimes appeared to dance across the sky. This phenomenon has been observed in Colorado and named “dancing sprites.” The lateral progression of sprite elements observed in the Northern Territory was overwhelmingly in one direction and covered distances of up to 90 km.

Electric fields of initial breakdown in positive ground flash

Pulse trains superimposed on the initial part of the electric field change of positive ground flashes are investigated. The characteristics of these pulse trains are found to be different from those of negative ground flashes and cloud flashes. Each pulse included in a pulse train appears as a bipolar waveform with pulse width from 5 to 52 μs and a mean of 18 μs. The time intervals between successive pulses are 10 to 180 μs with a mean of 54 μs. For each bipolar pulse, the ratio of the peak amplitude of the initial polarity to the maximum overshoot amplitude ranges from 0.4 to 4.7 with a mean value of 1.3. The ratio of maximum peak amplitude of successive pulses to the amplitude of the following return-stroke peak ranges from 0.02 to 1.9 with a mean of 0.27.

Common physical processes in natural and triggered lightning in winter storms in Japan

An analysis of measurements of electric, magnetic, and radiation field variations produced by rocket-triggered discharges in winter storms in Japan shows that a discharge initiated by a rocket with trailing and grounded wire is a negative leader that consists of continuous current and a pulse series associated with streamer development at the tip. The negative leader is not followed by a process analogous to a return stroke in cloud-to-ground flashes. Application of the electrostatic model of lightning as a bidirectional and uncharged leader in an ambient electric field to analysis of rocket-initiated discharges and positive cloud-to-ground flashes uncovers the commonality of processes occurring in both types of discharges.