Climate scientists are reporting that increasing rainfall in the world's warmest and wettest regions are being fueled by a recent surge in large, well-organized thunderstorms.

Top image: Change in precipitation by end of 21st century, inches of liquid water per year. The dark blue regions in the Pacific represent projected increases of up to 60-inches per year. (NOAA)

Data presented by NASA and the Australian Research Council Centre of Excellence for Climate System Science (ARCCSS) shows that increases in tropical precipitation, such as the western Pacific Ocean, are not due to storms producing more rain, but rather on account of large storms occurring more frequently.

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"What we are seeing is more big and organized storms, and fewer small and disorganized rain events," noted study lead author and NASA climate scientist Jackson Tan in a statement.

The top map above shows the change in the rate of tropical rainfall between 1998 and 2009; the second map shows how much of that change is due to intense storms. Shades of green depict areas where the rate has increased, while shades of orange show areas where less rain has fallen. (Image and caption credit: NASA Global Climate Change)

The study focuses on the phenomenon of "organized deep convection" — a fancy term for short and large thunderstorms. Though they comprise only 5% of tropical weather systems, they're responsible for about half of tropical rainfall.

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"This work changes our perception of why tropical precipitation is increasing," said co-author George Tselioudis, a researcher at the NASA Goddard Institute for Space Studies. "We thought it was because the warmer atmosphere holds more moisture and, therefore, when storms occur they rain more. But that doesn't seem to be the case. Instead, the warmer tropical atmosphere becomes better organized to produce large storms more frequently."

The study could not establish a causative effect, but it does reveal a strong correlation between this trend and increasing rainfall. The next challenge, say the scientists, is to figure out why these weather systems appear to be organizing in the atmosphere with greater frequency.

[ NASA Global Climate Change ]