Scientists Finally Track the Hidden Weather Systems Behind Most Tropical Rain

For the first time, researchers in Australia have identified and tracked the weather systems that, taken together, matter more to tropical rainfall than tropical cyclones do. The work offers a new way to study how tropical rain may change as the planet warms.

Outside the tropics, weather is organized around familiar features such as cold fronts and high- and low-pressure systems. In the tropics, scientists have lacked an equivalent framework for many of the systems that organize day-to-day clouds and rain. Earlier work either followed individual thunderstorms or looked at planet-scale atmospheric waves that affect many systems at once.

Theory had predicted a middle category called moisture modes, relatively slow-moving systems that organize tropical thunderstorms and rainfall over several days. Large-scale atmospheric gravity waves, which move clouds and rain over vast distances, are another piece. Neither has drawn much attention outside specialist circles. "Tropical cyclones are the divas of the tropics," said 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."

The team used satellite observations from 1984 to 2016 to find large areas of high, cold cloud that mark organized convection, the process by which warm, humid air rises and produces clouds and rain. High clouds are much colder than the surface and emit less radiation to space, which lets satellites spot them. The researchers treated each cloud area as an object and followed it through time, noting where it traveled, how long it lasted and how fast it moved.

Speed was the key to sorting them. Moisture modes generally moved slowly, gravity-wave systems moved much faster, and those in between were classed as mixed. "One of the novel parts of this work is defining a moisture mode as an object," said lead author Muhamad Reyhan Respati, a Ph.D. student at Monash. "It is no longer only part of a theory. It becomes a trackable object, like a tropical cyclone."

The results showed moisture modes are especially important near the northern and southern edges of the humid tropics, including regions near tropical Australia. In some areas they were linked to between 45% and 75% of extreme three-hourly rainfall. Co-author Martin Singh said the match with theory was better than expected. "Usually, when you try to apply a theory to the atmosphere, everything goes wrong," he said. "In this case, seeing it work so clearly in the real atmosphere was surprising."

The finding matters for Australia in particular, which stretches from the tropics into the extratropics. "In tropical Australia, most attention has focused on tropical cyclones," Respati said. "Our study shows that moisture modes are also extremely important to rainfall, so we need to start paying more attention to them."

The next question is what happens in a warmer world. "As the world warms up, the atmosphere can hold more moisture," Singh said. Moisture modes might get stronger, or more weather systems might become moisture modes. Understanding that, he said, would give researchers more confidence in projections of the Australian monsoon and of rainfall extremes.