The synoptic decomposition of cool-season rainfall in the Southeastern Australian cropping region

Daily rainfall during the April-October growing season in a major cropping region of southeastern Australia has been related to particular types of synoptic weather systems over a period of 33 yr. The analysis reveals that cutoff lows were responsible for at least 50% of all growing-season rainfall and accounted for 80% of daily rainfall events exceeding 25 mm per station. The proportion of rainfall contributed by cutoff lows varies throughout the growing season. It is highest in austral autumn and spring (55% and 57%, respectively) and falls to a minimum in July (42%). By way of contrast, the total contribution of all types of frontal systems to growing-season rainfall is about 32%, although the monthly value reaches a maximum of 41% in July when mean cutoff rainfall reaches a minimum. Rainfall associated with fronts is strongly concentrated in the lower range of daily falls ( less than 10 mm per station). Frontal rainfall is found to be more consistent from year to year than is cutoff rainfall. The number of cutoff lows per season is highly variable, and there is a significant correlation between the number of cutoff days and atmospheric blocking in the region south of Australia in each month of the growing season. The mean amount of rainfall per cutoff day is also variable and has declined by approximately 0.8 mm over the analysis period. An understanding of the mechanisms controlling year-to-year variability of cutoff rainfall is therefore an important step in improving seasonal forecasts for agriculture in southeastern Australia.