A new study based in part on NASA satellite data has shown that an increase in large, well-organized thunderstorms is behind increased rainfall in the wettest regions of the tropics.
Many scientists have long thought that in a warming world some regions are likely to see more rain because a warmer atmosphere is capable of holding more water vapor. The idea seemed to be supported by recent observations showing strong precipitation increases in the wettest tropical regions, sometimes referred to as a 'rich-get-richer' pattern.
Joint research from the Australian Research Council Centre of Excellence for Climate System Science (ARCCSS) and NASA published online in Nature today reveals that rainfall increases seen in places such as the western Pacific in recent decades are actually due to large storms - what the authors call "organized deep convection" - happening more frequently, rather than from individual storms producing more rain.
"The observations showed the increase in rainfall is directly caused by the change in the character of rain events in the tropics rather than a change in the total number of rain events," said lead author Jackson Tan, who conducted this research while at Australia's Monash University but now works at NASA's Wallops Flight Facility, Wallops Island, Virginia. "What we are seeing is more big and organized storms and fewer small and disorganized rain events."
The study helps chip away at one of the big questions facing climate change science: To what degree will a warmer world accelerate the water cycle and patterns of rainfall and drought? In particular, this study revolves around what scientists call organized deep convection - in short, large thunderstorms. These storms make up about five percent of the weather systems in the tropics but are responsible for about 50 percent of tropical rainfall.
While this study does not delve into what's causing the increase in large storms, it does reveal a tight correlation between this trend and increasing rainfall.
The new finding also raises an intriguing next question: Why are these weather systems organizing in the atmosphere more frequently? That's not yet known, Tan and Tselioudis said.
However, the finding does point the way to a new line of research on the topic, Tselioudis said. Scientists have been focusing on the transfer of heat and moisture in the atmosphere - the thermodynamics of the system. Tselioudis said this finding should direct attention to the dynamics of the atmosphere - the fundamental physics of circulation.
"For me, it's high time we push in this direction," Tselioudis said. "We've been looking at the water cycle as if it's mostly a thermodynamic issue. Just being wet doesn't rain make. There has to be more emphasis on the dynamics. If you look at the grand challenges of figuring out the water cycle - this is where the emphasis is moving to."