My name is Annette Hirsch and I’m a climate scientist at the ARC Center of Excellence for Climate Extremes, and my research focuses on understanding how surface conditions influence the atmosphere, and what that means for different weather phenomena and climate extremes.
Our research was motivated by how urban areas are often hotter than their surroundings, a phenomenon known as the urban heat island effect. If we want to understand how urban heat islands are affected by coastal breezes, air flows coming from further inland, or weather phenomena like heatwaves, and the interaction between these different processes, we need to use atmospheric models that are fit for purpose and can see the complexity of our cities. Our research aimed to do just that and uses Sydney, Australia’s largest city, as a test case for our new configuration of the weather and research forecasting model run at a very high resolution of 800 meters with a new urban classification scheme that describes the complexity of Sydney’s built environment.
Our results show that heatwaves often start with a hot continental flow over the Blue Mountains descending into the Greater Sydney region and not to get stuck. This can lead to temperature differences across the city exceeding 15 degrees. The urban environment on average adds about one degree of heat into the lower atmosphere. But over heatwave periods this can exceed 10 degrees, and also impact the Blue Mountains area as well when sea breezes act to push the heat further inland. The simulations cover a sequence of five heat waves over the summer of 2017, allowing us to understand what happens over the buildup, duration and dissipation of each of these events as they occur.
Starting from the 7th of January there is a change in the wind direction from an easterly flow to a southward flow over the ocean, and over the land there is a change in the wind direction bringing warming conditions over the area from the west. Here we can see the interaction of these different flows and how variations in their strength influences how the heat is distributed over the coastal land areas. On the ninth of January, a warm flow pushes over the land, followed by some convective activity later in the day, where we see what looks like small explosive pulses in the temperature field. The situation cools down overnight with the heat returning the following day, which marks the first day of the first heat wave event where the temperature increases, and the bottom panel where we can see the atmospheric profile, we see a pulse of heat moving over the region and out over the adjacent coastal ocean area. The heat really kicks in on the 11th of January, where a strong westerly flow brings hot conditions that extend out over the adjacent ocean areas. Later in the day a strong southerly pushes through, cooling the area down by dispersing the heat. Hot conditions returned on the 13th of January with another strong hot westerly flow heating up the area again and the temperatures are very hot and red over Sydney.
So this first heatwave wasn’t a continuous pulse of heat. It built up over the preceding days before peaking on the first day, with the southerly providing temporary relief before the temperatures dramatically rise on the final day, before a southeasterly flow arrives around the 14th of January, where we see the contrast of the opposing southeasterly and westerly flows converging close to the coast, before the southeasterly flow pushes through, and we see the whole temperature profile cooled off by the 15th of January.
This kind of behaviour repeats itself for the other heatwave events, however the buildup can occur over a very short time period, which is certainly the case for the second heatwave event, which closely follows the first. Here, relatively cooler conditions remain until the 16th of January, where it starts to warm up again over the west and sweeps over the land areas. There is a temporary relief where the temperatures dropped slightly before the heat returns on the 17th of January, where a strong hot westerly flow returns over the land, and over the ocean the flow is mostly southward. On the 18th of January another strong southerly arrives to flush the heat out where the area looks much cooler as well as through the atmospheric profile.
Over the 20th of January we see the heat mostly localised over Sydney starting from the north before travelling south over the city and extending towards Wollongong. This time round, the wind field limits heat being transported west over the Blue Mountains region, and later in the day a strong suddenly arrives to disperse the heat, and this is sustained until the 22nd of January helping to cool things down.
The level of detail we can see from these high resolution simulations is so high that we can really interrogate the role of local breezes to either enhance or dissipate heat, and this capability is a first step towards building an understanding of how our cities will need to adapt to climate change.
- Paper: Hirsch, Annette L., Jason P. Evans, Christopher Thomas, Brooke Conroy, Melissa A. Hart, Mathew Lipson, and William Ertler. “Resolving the Influence of Local Flows on Urban Heat Amplification during Heatwaves.” Environmental Research Letters 16, no. 6 (June 2021): 064066. https://doi.org/10.1088/1748-9326/ac0377.