by Demetrios Parascos (ELP 2017) | Chair, Environmental Institute of Australia
As a student at the Australian National University studying environmental science, a course I decided to take last semester was focused entirely on weather. In this course, we would cover the fundamentals of severe weather events with lectures from an expert in that field. One of these severe weather events covered was tornadoes, which was covered by Owen Shieh, Head of Training at the Joint Typhoon Warning Centre. Elaborating on what we learned with Owen about cyclonic activity and high wind events, I wanted to introduce another severe weather event that also originates from thunderstorms – downbursts. Much like how a tornado forms by extending downwards from the base of a thunderstorm, a downburst is a powerful downdraught produced from a falling precipitation column. The air rapidly sinks until it hits the ground and shoots out in all directions creating a ‘burst’ of air, which can be as intensely damaging as a hurricane. I personally found this quite interesting as it is something completely new to me.
I also found that there are different types of downbursts. There are: macrobursts, and wet/dry microbursts. Macrobursts form from large storms and affect areas greater than 4km across, and can produce winds up to 215km/h. Microbursts are smaller, affecting areas less than 4km across, and are more intense. Microbursts can produce winds up to 270km/h. The unsurprising difference between wet and dry microbursts is the presence of precipitation at the ground level. Dry microbursts occur when the rain evaporates before reaching the ground level, this results in intense cooling which increases the density of the downdraught causing even stronger winds. Wet microbursts deliver large amounts of precipitation and are often known as ‘rain bursts’, or ‘rain bombs’. A video of such an event is available below:
https://www.youtube.com/watch?v=a_G2KRzha7o
The video was taken on the 8th of August in Tucson, Arizona. This video is particularly good as you can actually see the outflow front (the outward flowing winds from the microburst), and the horizontal vortex (the upwards curling motion) after it reaches the ground. This severe weather event is particularly dangerous to aircrafts, especially those that are taking off or landing, as they appear so suddenly giving very little warning time. There have been many fatal air crashes due to this weather event. The final connection I would like to make is with the compound effects of downbursts with other severe weather events, with bushfires in particular. As Geoff Cary, a lecturer at the Australian National University, explained to us earlier in the semester, storms can have compounding effects on bushfires, mostly with changing or intensifying winds or with lightning starting multiple fire events at one time. I found that thunderstorms over a fire put firefighters at even greater risk as there is the chance of a downburst occurring which will accelerate the fire front and send it in multiple directions, which as we all know can be potentially fatal to firefighters if they are not ready for it. The importance of this is known as I also found information on it in the NSW bushfire bulletin (below).
http://www.rfs.nsw.gov.au/__data/assets/pdf_file/0020/23537/Bush-Fire-BulletinLIFTOUT-WeatherandBushFireBehaviour.pdf
Questions that I would like to further explore are: 1. What other severe weather events could be negatively compounded with downbursts? And 2. What might be other cool ‘new’ weather events to learn about?