Positive cloud‐to‐ground lightning discharges in Kathmandu thunderstorms

Abstract

The occurrence of positive cloud‐to‐ground lightning discharges in thunderstorms in the vicinity of Kathmandu, Nepal (27.4°N, 85.2°E) has been recorded during the 21‐month period from March 1987 to November 1988 inclusive, using the daily registrations of a CGR3 lightning flash counter that provided records of the approximate numbers of positive ground flashes, negative ground flashes, and cloud flashes, with effective ranges of about 12 km for cloud flashes, 14 km for negative ground flashes, and 16 km for positive ground flashes. The detection of positive ground flashes was based on the presence of a relatively large negative‐going step in the electric field change caused by the positive return stroke in the discharge. It was found that the overall mean proportion of positive ground flashes to all ground flashes was 0.28, and that the overall mean positive ground flash density was 0.6 km−2 yr−1. The uncertainty in these values is about ±50%. The proportions of positive ground flashes in three of the climatic periods in the year were 0.26 in the premonsoon period (February to May inclusive), 0.34 in the monsoon period (June to August inclusive), and 0.38 in the postmonsoon period (September to November inclusive). This apparent increase in the proportion is accompanied by a seasonal decline in electrical activity, the premonsoon period being the most active, and the postmonsoon the least active. The increase also appears to be associated with a decrease in the proportion of cloud flashes to total flashes. The proportion of positive ground flashes is larger than is observed in most low‐altitude tropical and subtropical sites but is smaller than that observed in winter thunderstorms at some higher‐latitude sites. Possible reasons for the relatively high rate of occurrence of positive ground flashes (unsupported by concurrent atmospheric soundings) are the site altitude, thundercloud charge heights, vertical wind shear and the mountainous nature of the terrain.

Thunderstorm and Lightning Climatology of Australia

“…A higher percentage of PGF (28%) was recorded in Kathmandu, Nepal (Mackerras and Darveniza, 1994). These findings were discussed in detail in (Baral and Mackerras, 1993); possible reasons for the relatively high rate of occurrences of PGF were given as the site altitude, thundercloud charge heights, vertical wind shear and the mountainous nature of the terrain.…”

Global lightning: Total, cloud and ground flash estimates

“…The definition and method of calculation of the effective area for flash detection are given by Baral and Mackerras [1993]. The effective areas for flash detection by CGR3 instruments are estimated to be Apg • 830 km 2 for positive ground flashes and Ang • 580 km 2 (for latitudes 25o-60 ø) for negative ground flashes.…”

Occurrence of positive and negative polarity cloud-to-ground lightning flashes: case study of CGR4 and GPATS data for Brisbane, Australia

“…A higher percentage of PGF (28 per cent) was recorded in Kathmandu, Nepal (Mackerras and Darveniza 1994). These findings were discussed in detail in Baral and Mackerras (1993); possible reasons for the relatively high rate of occurrences of PGF were given as the site altitude, thundercloud charge heights, vertical wind shear and the mountainous nature of the terrain.…”