Tropical Cyclone Ilsa has been downgraded to a category three cyclone as it moves southeast across Western Australia. The storm first made landfall as a Category Five cyclone, passing near Port Hedland around midnight.
Ilsa plunged into the largely uninhabited region of Pilbara (the country’s most remote region). cyclone-prone region) at record speeds. It delivered Australia’s highest record ten-minute sustained wind speed on landfall: approx 218 kilometers per hour. The previous record of 194 km/h came from Tropical Cyclone George in 2007.
So, does this new speed make Ilsa a particularly imminent disaster? The science of reporting cyclonic wind speeds is very complex – and it can be easy to misinterpret the numbers without any context.
Record-breaking sustained wind speeds
As Ilsa continues to move inland, it looks likely the storm will weaken further before moving into the Northern Territory – and possibly Alice Springs – later today and tomorrow.
Ilsa came ashore about 100 km north of Port Hedland, where they are located the world’s largest iron ore export location. But a red alert caused most of the ships to move further west ahead of time, so it only caused minor destruction.
Bomb
Analysis by James Knight of Aon’s Reinsurance Solutions expects it to generally do only minor damage due to the remoteness of where it hit.
Aside from the ten-minute record mentioned above, Ilsa had a one-minute record of 150 mph and a three-second record of 180 mph.
It is usually the last, more intense wind gusts, that do the most damage in tropical cyclone events. When it comes to assessing potential damage for insurance purposes, companies often model damage associated with a sustained wind speed of three seconds.
But there are several challenges associated with recording and forecasting cyclonic wind speeds.
Read more: Cyclone Freddy was the most energetic storm on record. Is it a harbinger of things to come?
How are tropical cyclone winds recorded?
The Bureau of Meteorology maintains one national wind registration database, which uses instruments called anemometers. These measure wind speeds at locations across the country and are often placed in flat areas, such as near airports.
Their specific placement is very important, as wind can change shape as it moves over and through certain types of terrain.
When we report wind speed, we are generally talking about atmospheric gusts, or wind speeds at least ten meters above the ground, which we also refer to as “open terrain” wind speed.
However, wind that passes closer to the ground, where the topography varies, will often be higher than wind that passes directly above it. For example, wind will speed up when squeezed between two hills.
We know from post-cyclone damage studies that wind speeds can vary significantly from one side of a hill to the other. So aspect and slope are very important.
As far as disaster modeling goes, this isn’t a minor issue, as it can skew recordings. It is quite possible that there have been wind gusts from Ilsa greater than what has been reported so far.
Australia lacks a dense enough network of anemometers set up for long-term testing. If we want to understand the frequency and intensity of extreme cyclonic wind speeds over time, we need a nationwide, quality-controlled network with better spatial coverage.
While the equipment we have is designed to withstand extreme conditions, it can be knocked over and thrown offline, creating data gaps in the time series.
Accurate and consistent data points are crucial if we are to record and predict the types of extreme winds we may experience during future tropical cyclones. And while the efforts of independent storm chasers and university groups are showing some results, measuring different sources can add a lot of uncertainty to the overall process.
The intensity of the cyclone will increase
Since 1975 there been 48 category five tropical cyclones hitting Australia – about one a year on average. Shile Ilsa sets a new record for strongest sustained wind gust at landfall, Category five tropical cyclones have generally occurred with some regularity.
It is worth noting that Ilsa formed quite late in the cyclone season. While the Bureau of Meteorology says cyclones can form any time of year, it’s very rare for it to happen outside of April.
Historical trends and projections of climate change suggest that the number of landfalling cyclones in our region will decrease over time. This is consistent with real world data and puts Australia at odds with other parts of the world where cyclone frequency is on the rise.
However, most climate models also predict that a higher proportion of these cyclones will be stronger. The current scientific consensus is that we will experience these events less often, but when we do, they will be more intense.
Read more: Monster storm anatomy: How Cyclone Ilsa is preparing to wreak havoc on the WA coast
