More important than tropical cyclones: Scientists finally track the hidden weather systems behind most tropical rainfall

Monash University

For the first time, researchers in Australia have identified and tracked the weather systems that are collectively more important to rainfall across the tropics than tropical cyclones.

The work, published in the journal Weather and Climate Dynamics, has advanced a new way to investigate how tropical rain may change as the Earth warms up.

Weather outside the tropics is commonly understood through recognisable systems such as cold fronts, high-pressure systems and low-pressure systems. However, in the tropics, scientists have lacked an equivalent framework for identifying many of the systems responsible for organising day-to-day clouds and rain. Previous work has tended to either focus on following individual thunderstorms, or looking at planetary-scale atmospheric waves that influence many weather systems at once.

In recent decades, emerging theories have predicted the existence of moisture modes, relatively slow-moving systems which can organise tropical thunderstorms and rainfall over several days. Other work has focused on large-scale atmospheric gravity waves, influencing broad regions of rising and sinking air, and moving clouds and rainfall over vast distances.

Unlike tropical cyclones, however, these moisture modes and gravity waves have received little attention beyond specialist areas of atmospheric science.

“Tropical cyclones are the divas of the tropics,” said Professor Christian Jakob of Monash University.

“It’s fair that they receive a lot of attention because they have such major impacts. But less dramatic systems occur much more frequently, and collectively they are very important for rainfall.”

Seeking to finally categorise moisture modes and learn more about their behaviour, the researchers used more than three decades of satellite observations to identify moisture modes as trackable weather systems and examine their contribution to tropical rainfall. The results could help fill a longstanding gap in meteorology.

“One of the novel parts of this work is defining a moisture mode as an object,” said Muhamad Reyhan Respati, lead author of the paper and a PhD student at Monash University.
“It is no longer only part of a theory. It becomes a trackable object, like a tropical cyclone.”

Associate Professor Martin Singh of Monash University, a co-author of the work, said the observations aligned with the theoretical framework more closely than the researchers had expected.

“Usually, when you try to apply a theory to the atmosphere, everything goes wrong,” Martin said.

“In this case, seeing it work so clearly in the real atmosphere was surprising.”

The study used satellite observations collected between 1984 and 2016 to identify extensive areas of high, cold cloud associated with organised tropical convection - the process through which warm, humid air rises to produce clouds and rainfall.

High clouds are much colder than the Earth’s surface and emit less radiation to space, allowing the researchers to locate large areas of organised convection from satellite measurements.

They then treated those cloud areas as individual objects and followed them through time, examining where they travelled, how long they survived and how quickly they moved.

Speed was central to the classification. Moisture modes generally moved slowly, while gravity-wave systems travelled much faster. Systems between the two were classified as mixed.

The study demonstrated that moisture modes are particularly important near the northern and southern margins of the humid tropics, including regions near tropical Australia.

In some areas, they were associated with between 45 and 75 percent of extreme three-hourly rainfall.

Reyhan said the findings were particularly relevant to Australia because the continent extends from the tropics into the extratropics.

“In the extratropical part of Australia, we know a lot about the dynamics of the weather systems,” he said.

“In tropical Australia, most attention has focused on tropical cyclones. Our study shows that moisture modes are also extremely important to rainfall, so we need to start paying more attention to them.”

By turning a theoretical idea into weather systems that can be identified, tracked and compared, the researchers have created a new way to examine the atmosphere, and the rainfall it produces.

The next challenge is to determine how these newly recognised weather systems will behave in a warmer world.

“As the world warms up, the atmosphere can hold more moisture,” Martin said.

“Moisture modes might amplify, or perhaps more weather systems will become moisture modes. If we understand the processes on a deeper level, it gives us more confidence when we make projections about the Australian monsoon or precipitation extremes.”

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