The UNSW Sydney and the Bureau of Meteorology have discovered that the number of days considered ‘hail-prone’ has decreased across much of Australia but has increased by up to approximately 40 percent in some heavily populated areas. The atmospheric conditions required for a hailstorm to form include instability, enough moisture, and wind shear. These findings are important for agricultural, insurance, and city planning sectors to build resilience against future hail events and to protect densely populated areas from damage.

Hailstorms occur when the atmosphere has all the necessary components for hail formation. Dr. Tim Raupach, a researcher in atmospheric science at the UNSW Climate Change Research Centre, explains that hailstorms are measured and modeled, which makes it difficult to understand how they have changed over time or how they are expected to change in the future. To gain a better understanding of the frequency of hail events, the researchers used historical estimates of atmospheric conditions as a "proxy" for hail occurrence over the past forty years to create a continental map of how hail hazard frequency has changed across Australia. The latest study is the first continental-scale analysis of hail hazard frequency trends for Australia. The team of researchers, which included scientists from the Bureau of Meteorology, hopes that this research will help improve our understanding of hail events, which is important for the insurance industry, as well as agricultural and city planning sectors.

Not all thunderstorms are capable of producing hail. Hailstorms require certain atmospheric conditions to develop. One of the essential factors is atmospheric instability, which means that there is a tendency for updrafts to form. Updrafts occur when there is warm air near the ground and cooler air higher up. When a little bit of warm air rises and gets into the cool air, it draws air up leading to updrafts formation, as explained by Dr Raupach.

Another crucial factor is the presence of sufficient moisture in the updraft to create supercooled liquid water and ice, which swirl around in the upper part of the storm. Additionally, hail must be large enough to survive melting as it falls to reach the ground as a block of ice, as explained by Dr Raupach.

Lastly, wind shear, the change in the wind's properties by height, enhances hail formation. Dr Raupach explains that wind shear is the wind changing direction or velocity as you go higher in the atmosphere. When there is a lot of wind shear, the storm tends to be more severe and prone to forming hail.

When all these factors are present, the atmospheric conditions become hail-prone.

Researchers have developed a method to estimate hail-prone days in Australia over the past four decades. They created a "hail proxy" by combining the necessary ingredients for hail formation and applied it to 40 years of reanalysis data. Reanalysis products combine observations with supercomputing weather modeling to estimate the past atmosphere's state. Using this method, the researchers were able to produce a map of how the number of hail-prone days per year has changed across the entire continent, with a resolution of about 30 kilometers per pixel.

The hail proxy enabled the researchers to estimate atmospheric conditions on a grid of points across the country, instead of relying on spotty surface records. The Bureau of Meteorology's long-term weather radar archive was used to compare radar observations with the reanalysis hail proxy. Radar observations were collected from 20 Bureau radar sites across the country, with each site having between 12 and 24 years of records. The radar results corroborated the pattern of results seen through the statistical analysis of historic estimates.

According to Dr. Raupach, there has been a decrease in the number of hail-prone days in most parts of Australia, except for the southeast and southwest regions where large population centers are located. The annual number of hail-prone days has increased by approximately 40% around Sydney and Perth, which is about a 10% increase per decade in the number of hailstorm-causing days. Although there are fewer hail-prone days, the chances of hail occurring are higher when there are more such days. These changes in the data are all relative. Even though not every hail-prone day results in hail, an increase in the number of hail-prone days increases the likelihood of hail.

Although the exact reasons behind the changes in hail patterns are still unknown, the research team has taken into account the possible influence of climate change. Dr Raupach explains that while it is not a climate attribution study, the team considered how hailstorms may behave in a warmer environment.

There is a general expectation that climate change may result in less frequent surface hail due to increased melting. The warmer atmosphere causes more hail to melt before it hits the ground. However, a warmer atmosphere would be more unstable, which means there may be larger hailstones. Large hailstones are more likely to survive increased melting, so hail that does occur could be larger, and therefore more severe.

The team discovered that the changes in hail-prone day patterns are primarily driven by changes in extreme atmospheric instability, which are very complex and regionally dependent. In regions with increased instability, there might be more generation of hail and larger hailstones, which may survive more melting. Conversely, in areas where instability decreases, there may be a dampening effect.

The links between climate change and hailstorms are multifaceted, and more work needs to be done to understand how these patterns will continue to change under warming conditions.

The findings of this study are crucial in our understanding of hail risks. According to Dr. Raupach, the agricultural and insurance industries, as well as city planning, can benefit from this information. Hailstorms can destroy crops and cause significant damage that leads to insured losses, making hail a primary concern for the insurance industry in Australia. The research underscores the importance of building resilient infrastructure that can withstand the potential increase in hail hazards in the future. Dr. Raupach aims to extend his research by using supercomputing and climate models to predict future trends in hailstorms and help plan for the impact of such changes on agriculture, insurance, and densely populated areas.