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New research led by the 91ɫ��Ƭ Institute for Climate Risk & Response suggests a meaningful proportion of hailstorms detected by satellites over tropical Australia could produce damaging hail at ground level.

Hail can be quite difficult to measure in the tropics. It does occur in the tropics, but rarely. While satellites can identify the signatures associated with hail high inside tropical thunderstorms, scientists can’t always tell whether that ice survives or melts during the long journey down from inside the storm, through warm air and to the ground.

Now, led by Dr Timothy Raupach from the 91ɫ��Ƭ Institute for Climate Risk & Response (ICRR) has found some of those satellite detections could indeed represent damaging hail at the surface.

Published in Atmospheric Science Letters, the study examined 61 satellite-detected hailstorms across northern Australia. Using high-resolution weather simulations, researchers investigated whether these storms could have produced hailstones of at least 20 millimetres in diameter – a size considered potentially damaging.

Across 244 simulations using four different representations of cloud and precipitation processes, 27% of hailstorms produced damaging surface hail according to a hail-growth model.

Dr Raupach says hail occurrence in the tropics remains highly uncertain because of its rarity and because ground observations and radar coverage are sparse.

“Satellites often see signatures that look like hail in tropical storms, but that doesn’t necessarily mean large hail is reaching the ground,” he says.

“The tropical atmosphere is deep, warm and humid – so hailstones form high and have a long distance to fall, which comes with a greater opportunity to melt.

“What our simulations show is that at least a subset of those satellite detections could still be producing damaging hail at the surface.”

How do tropical hailstones survive?

The researchers also investigated what distinguished the simulations producing damaging hail from those in which the ice melted before reaching the ground.

They found the amount of atmospheric instability –the energy available to fuel thunderstorms – was not significantly different between the two groups.

Instead, damaging-hail storms tended to occur where there was greater vertical wind shear and more moisture in the surrounding atmosphere. Combined, this produced wider and stronger thunderstorm updrafts.

Updrafts can keep developing hailstones suspended within a storm for longer, allowing them to grow before eventually falling.

The result can be explained through a process known as entrainment, where surrounding air becomes mixed into a thunderstorm's rising air.

When relatively dry air enters an updraft, it can dilute the rising air and reduce its buoyancy.

Wider updrafts within a more humid surrounding atmosphere reduce that effect, allowing more of the available atmospheric energy to be converted into strong vertical motion.

“The storms that produced damaging hail were able to do more with the energy available to them,” Dr Raupach says.

“Greater wind shear and humidity helped produce stronger, broader updrafts that could support the growth of larger hailstones and give them a better chance of surviving all the way to the ground.”

Improving tropical hail detection

Dr Raupach says the results could eventually help improve both satellite hail detection and forecasting, by incorporating the characteristics into future satellite hail-detection methods.

However, he also says substantial uncertainty remains.

“There were no direct surface hail observations available against which to test the simulated storms,” he says.

“And different representations of cloud microphysics did not always agree on whether damaging hail reached the ground.”

The study was also limited to tropical Australia, meaning the proportion of satellite detections producing damaging hail cannot yet be assumed to apply across the world's tropics.

“We still need to investigate whether the findings hold in other tropical regions and further examine the roles of storm structure and microphysical processes,” says Dr Raupach.