Tropical–extratropical cloudbands and Australian rainfall: I. climatology

This paper examines the contribution of tropical–extratropical cloudbands, and of interactions between tropical cloud and mid-latitude systems, to cool season (April–October) rainfall in agriculturally marginal areas of Australia. A following paper describes inter- and intra-annual variability of these features. A classification scheme for these tropical influences based on GMS satellite imagery is described, and used to compile a 15-year archive of events. It is shown that cloudbands extending from the tropical oceans bordering Australia (‘Oceanic’ Cloudbands) are most frequent and influential between April and July, but decrease sharply after August, at which time bands originating over the continental interior (‘Continental’ Cloudbands) increase. The contribution of these systems to rainfall at stations representing agriculturally marginal areas is assessed. Oceanic Cloudbands originating west of 120°E contribute 70–90 per cent of cool-season rain in north-western Australia, with the contribution decreasing to the south and east. North-eastern Australia receives a significant portion of its rain from Cloudbands originating east of 120°E. Tropical–mid-latitude interactions are more important over eastern than western Australia, and produce some 30–40 per cent of rain over much of inland eastern Australia. The overall tropical influence (Cloudbands plus interactions) on rainfall is least in South Australia and western Victoria, but still amounts to some 35–40 per cent of cool-season rain in those areas. The proportion of events producing significant rainfall (>10 mm) is also examined: almost two-thirds of the Oceanic Cloudbands to affect western Australia produce significant rain, and about half of those affecting eastern Australia. © 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 807–829 (1997) (No. of Figures: 10. No. of Tables: 4. No. of References: 33.)