A climatology of atmospheric pressure jumps over southeastern Australia

Abstract

Black Saturday provided the first evidence of an atmospheric bore affecting the behaviour of a bushfire. As the bore passed, the fire unexpectedly strengthened. This behaviour highlighted the lack of understanding of how common bores are in the southeastern part of Australia, a region of relatively high bushfire risk. The present study addresses that lack of understanding. Pressure jumps are identified in the 1 min records at four automatic weather stations in southeastern Australia by correlating the pressure time series with a large-amplitude step function. These jumps are then separated into two classes: bores and frontal pressure jumps. Bores are defined as pressure jumps without a change in relative humidity whereas frontal pressure jumps are defined by jumps with an accompanying decrease in temperature greater than 3 °C. About 15 pressure jumps per station per year are found. Most jumps are found in the spring and summer and fewest in winter. Bores are found most frequently in the early morning and late evening at most stations, whereas frontal pressure jumps are most frequently found in the late afternoon or early evening. Following their passage, frontal pressure jumps are associated with higher 30 min mean wind speeds than bores (9.0 and 6.1 m s−1 respectively), both of which are higher than climatology (4.7 m s−1).