Daily Bulletin

The Conversation

  • Written by The Conversation
imageRecent extreme rains such as those that hit Sydney recently are actually decreasing, but extreme rain in summer is going up.AAP Image/NEWZULU/LISA HOSKING

The extreme rainfall that hit New South Wales over the past week caused flash-flooding and resulted in deaths and homes destroyed. While analysis shows that these events are not unprecedented, are they becoming more common?

In research published recently in Nature Climate Change, we show that extreme rainfall in Sydney is increasing, but mostly during the summer. Longer periods of extreme rainfall, such as the event we’ve just seen, haven’t changed or have actually decreased.

Warmer means wetter

One of the major concerns of climate change is that flood risk might increase. However, estimating the size of future change has proven to be challenging, in part because historical flood records are not only affected by changes in climate but also by changes to land use (such as urbanisation, dams, forestry, agriculture).

This means that it is difficult to use historical flood records as the basis for inferring future changes to flood risk.

An alternative approach relies on looking at historical changes to extreme rainfall patterns. The basic story of how extreme rainfall is expected to change under future climate change is linked to how much moisture the atmosphere can hold. For every degree of temperature increase, the atmosphere can hold 7% more water.

At the global scale, this has been found to be approximately true in both historical datasets and future model projections. However, changes at the local and regional scale are much more uncertain.

Types of rain

The type of rainfall is important. Convective events such as summer thunderstorms are driven by the instability of moist parcels of air due to temperature gradients, and can lead to localised, highly intense bursts of rainfall.

Stratiform rainfall (such as the kind that hit NSW over the past week) is linked to large-scale circulation patterns of the atmosphere such as fronts and east coast lows, and is often more widespread.

Understanding rainfall is not just a question of studying amounts and patterns, but appreciating the climate mechanisms that gave rise to the rainfall.

In our new paper, we show that summer rainfall – characterised by short, intense bursts – is becoming more intense. But intense rainfall in other seasons showed little change, or even decreased.

More research on the causes of these changes is certainly required, however our results are consistent with future climate projections of intensifying summer rainfall, and a reduced frequency of east coast lows.

What does this mean for flooding?

Whether rainfall causes a flood depends on where you live. In larger catchments, floods are mostly caused by long periods of extreme rainfall. In smaller, steeper and more urban catchments, floods are caused by short bursts.

This can lead to unexpected results for future flood risk. For example, a large catchment might have smaller floods because of less extreme winter events, yet a smaller upstream region of the same catchment may be subject to more intense flash flooding because of the increased intensity of summer extremes.

Assessing the magnitude of floods is a complex science with many interactions between rainfall and the catchment. The timing of rainfall, the size, steepness and shape of the catchment, the type of land use and the wetness of the catchment from prior rainfall events all act to modify the size of a flood.

Even if these factors were held constant, the fact that a single region such as the Greater Sydney basin can have multiple trends across timescales demonstrates the need for care in assessing the impact of future floods.

These issues are critically important for understanding future floods, and work is currently under way to better account for climate change as part of Australia’s national flood guidelines.

However, more work needs to be done if we are to understand the full picture of future flood risk across Australia’s diverse climates. Improving our ability to account for climate change will contribute to better design and security of our water infrastructure (dams, levees, bridges and storm-water drainage networks), the management of our floodplains, emergency response and community safety.

Read more:

Michael Leonard has received funding from Engineers Australia.

Feifei Zheng receives funding from Geoscience Australia as part of the revision of the Australian Rainfall and Runoff flood guidelines

Seth Westra receives funding from Geoscience Australia as part of the revision of the Australian Rainfall and Runoff flood guidelines, as well as from the Australian Research Council.

Authors: The Conversation

Read more http://theconversation.com/sydney-storm-are-extreme-rains-and-flash-floods-increasing-40696

Writers Wanted

An Indigenous 'Voice' must be enshrined in our Constitution. Here's why


Biden's economic centrism isn't exciting, but right for these divisive times


Why are Japan's leaders clinging to their Olympic hopes? Their political fortunes depend on it


The Conversation


Ray Hadley's interview with Scott Morrison

RAY HADLEY: Prime Minister, good morning.    PRIME MINISTER: G’day Ray.   HADLEY: I was just referring to this story from the Courier Mail, which you’ve probably caught up with today about t...

Ray Hadley & Scott Morrison - avatar Ray Hadley & Scott Morrison

Prime Minister's Remarks to Joint Party Room

PRIME MINISTER: Well, it is great to be back in the party room, the joint party room. It’s great to have everybody back here. It’s great to officially welcome Garth who joins us. Welcome, Garth...

Scott Morrison - avatar Scott Morrison

Prime Minister Interview with Ben Fordham, 2GB

BEN FORDHAM: Scott Morrison, good morning to you.    PRIME MINISTER: Good morning, Ben. How are you?    FORDHAM: Good. How many days have you got to go?   PRIME MINISTER: I've got another we...

Scott Morrison - avatar Scott Morrison

Business News

Tips to find the best plastic manufacturing supplier for your needs

Plastics are very much an important part of all of our lives, but they’re particularly valuable to a wide variety of industries that rely on their production for their operations. The industries, ...

News Co - avatar News Co

7 foolproof tips for bidding successfully at a property auction

Auctions can be beneficial for prospective buyers, as they are transparent and fair. If you reach the limit you are willing to pay, you can simply walk away. Another benefit of an auction is tha...

Dominique Grubisa - avatar Dominique Grubisa

Getting Ready to Code? These Popular and Easy Programming Languages Can Get You Started

According to HOLP (History Encyclopedia of Programing Languages), there are more than 8,000 programming languages, some dating as far back as the 18th century. Although there might be as many pr...

News Co - avatar News Co

News Co Media Group

Content & Technology Connecting Global Audiences

More Information - Less Opinion