Image: Storm over Melbourne. Credit: Fábio Hanashiro (Unsplash).

It has long been suggested in the literature, and discussed casually by meteorologists, that rainfall in Melbourne often occurs as lines of precipitation. However, this had yet to be quantified. We analysed 15 years of radar data from the Australian Radar Archive, using an objective method to identify and track these ‘linear systems’ based on radar reflectivity, size, and shape characteristics. Specifically, we were interested in large systems (at least 100 km long), with radar reflectivity of greater than 30dBz (more than 2.5 mm/hr).

We found that one linear system occurs somewhere near Melbourne once every 6-7 days on average. A typical linear system near Melbourne is oriented north-northwest to south-southeast and moves towards the east at speeds between 35 and 70 km/hr. They don’t necessarily occur in the same place each time and are more common to the east of the city than to the west, which is similar to the pattern of overall rainfall for this area.

This type of storm can produce severe wind, hail, tornadoes, and extreme rainfall depending on their structures. In this work, we explored just one of these, extreme rainfall. There are many different ways to define extreme, but for our purposes, “heavy” events are the top 5% and “extreme” events are the top 1% of average rainfall days.

On these heavy/extreme days, those that had linear systems contributed to 70-85% of the rainfall. We found that the linear systems on days with heavy rainfall had more N-S orientation, and were larger, slower, and longer-lived than more ordinary linear systems. Those on extreme rainfall days were even larger, slower, and longer-lived, but also had a greater degree of southward movement and were more likely to be associated with taller, more intense storms.

  • Paper: Hitchcock, S. M., Lane, T. P., Warren, R. A., & Soderholm, J. S. (2021). Linear rainfall features and their association with rainfall extremes near Melbourne, Australia, Monthly Weather Review (published online ahead of print 2021).