Proceedings of the Technical Session of Institute of Physics, Sri Lanka 16 (2000) 1-13

Absolute PulseAmplitudes of Isolated Cloud Flashes
Chandima Gomes and Vernon Cooray
Institute of high Voltage Research, Uppsala University, Sweden

Sixty isolated cloud flashes pertinent to two Swedish thunder storms of which 30 flasheswere distance located with time-to-thunder measurements, have been analysed. The main objectives of this analysis were to estimate the temporal pulse characteristics, to determine the absolute pulse amplitudes and to investigate the time of occurrence of the most active region of the pulse train. The long duration of the cloud flashes, mainly distinguish them from other known pulse trains. In pulse characteristics, they are considerably different from the preliminary breakdown pulse trains of negative return strokes and somewhat similar to those pulse trains of positive return strokes,observed in Sweden. There are two types of pulses, multiple peaked pulses, with less pronounced positive overshoot, and single peaked pulses, with pronounced positive overshoot. The mean pulse characteristics, width of the initial half cycle, width of the second half cycle, full width of the pulses and the pulse separation, have values of 21 ms, 23 ms, 43 ms, and 147 ms, respectively. The mean pulse train duration is 55 ms. The pulse amplitude, during the first 10 ms, is 3 times greater than that ofpulses occur, during the following 30 ms. The pulse occurrence density, during the first 10 ms, is about 4 times greater than that of the following 30 ms. The emission of the 3 MHz radiation starts with the onset of the pulse train and the strongest emission is observed during the first 10 ms. The mean amplitude of the largest pulse, normalised to 15 km, is 20.3 V/m, thus, similar to that of subsequent strokes.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 16 (2000) 14-33

Lightning Electromagnetic Environment
Chandima Gomes
Department of Physics, University of Colombo, Colombo 03,Sri Lanka

Lightning current and electromagnetic field parameters are of prime importance in designing lightning protection devices and in many other physics and engineering applications. Several reviews on lightning currents in both natural and artificial lightning are published, however a review on lightning generated electromagnetic fields is not available in either local or international literature. This study provides a concise review on up-to-date information of the lightning generatedelectromagnetic fields measured at various distances. The discussion is mainly confined into the lightning return stroke fields. The fields due to both negative and positive return strokes are treated.

Another important aspect of lightning, discussed in this work, is the repetitive nature of the main discharge event, the return stroke. This includes the number of strokes per flash, the fraction of strokes with amplitudes greater than that of the first stroke, the inter-stroke interval etc. These parameters are significant in the withstanding ability and the long-term durability of the lightning protection devices.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 16 (2000) 34-41

A Study of LLPSystems and their Performance in Locating Lightning Ground Flashes inSri Lanka (1998 - 1999)
D. U. J. Sonnadara for LLP group
Department of Physics,University of Colombo, Colombo 3

A study of the performance of wideband magnetic direction finding systems used in locating lightning ground flashes in Sri Lanka is presented. The studies carried out with these systems show that they are highly sensitive to the geometric placement with respect to each other as well as to the induced magnetic fields produce by the external objects in the vicinity of the antennas (site errors). The results of two studies carried out by placing two of these stations in the same vicinity of each other and also in two remote locations (Colombo6.90° N, 79.86° E and Ratnapura 6.68° N, 80.40° E)are presented.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 16 (2000) 42-49

Preliminary Results of Long-Term and Short-Term Rainfall Forecasting
H. K. W. I. Perera and D. U. J. Sonnadara
Department of Physics, University of Colombo

Possibility of using statistical methods for long-term and short-term rainfall forecasting was investigated. Daily rainfall data from 8 meteorology stations namely, Colombo, Ratnapura, Kandy, Galle, Hambanthota, Batticoloa, Anuradhapura and Trincomalee were utilised in this study. A time series model was used for long-term forecasting and a Markov Chainmodel was used for short-term forecasting. The preliminary results show that the time series model with exponential smoothing fitted the data best and seasonal variations can be predicted with this model from weekly and monthly averages. The Markov chain model, applied by considering only two states, wet or dry, was successful to the levelof 70% in predicting the status of a given day.