AsianScientist (Mar. 30, 2015) – Increasing rainfall in certain parts of the tropics, colloquially described as the wet get wetter and warm get wetter, has long been a projection of climate change. Now observations published in Nature have shown that an increase in large thunderstorms is the primary reason for this phenomenon.
Joint research from the Monash University branch of the Australian Research Council Center of Excellence for Climate System Science (ARCCSS) and the National Aeronautics and Space Administration (NASA) found even though other types of rainfall has decreased in frequency and the total number of thunderstorms remained the same, the increase in big storms had elevated total rainfall.
“The observations showed the increase in rainfall is directly caused by the change in the character of thunderstorms in the tropics rather than a change in the total number of thunderstorms,” said lead author from ARCCSS Dr. Jackson Tan.
“What we are seeing is more big and organized storms and fewer small and disorganized storms.”
Thunderstorms play an important role in rainfall in the tropics. Despite organized deep convective storms only occurring five percent of the time in the world’s equatorial regions, they deliver almost 50 percent of all its rainfall.
The research has also contributed to answering the important question whether the increase in rainfall observed in the tropics was simply caused by the fact of a warmer atmosphere or whether the underlying circulation in that region had changed.
The changes to the deep convection discovered in the study suggested a dynamic change in the climate system was responsible for the change in rainfall.
“If this rainfall change was caused simply by a warmer atmosphere holding more moisture, we would have expected an increase in the average rainfall when each system, organized or disorganized, occurs,” said Tan
“Instead, the number of organized storms, which is largely controlled by the dynamics of the atmosphere, have increased in frequency, suggesting that the increase in rainfall is related to more than a simple warming of the atmosphere.”
Climate model results have long suggested that we would see increased precipitation in the tropics as a result of climate change. However, the exact nature of this change remained unclear.
The revelation that large thunderstorms appear to be the source of increased precipitation in the tropics explains why climate models may have difficulties in accurately representing the details of tropical rainfall.
The small-scale processes giving rise to thunderstorms make their direct simulation in climate models impossible given current computing power.
“This limitation, which is a well-known issue in global climate models, might well be a contributing factor to the precipitation errors and the bias towards light rain,” said another author from Monash University, Professor Christian Jakob.
“Given how important these large storms are to rainfall in the tropics, it is vital that there is a renewed effort to represent convective organisation in global climate models if we are to fully understand precipitation changes in the future.”
The article can be found at: Tan et al. (2015) Increases in Tropical Rainfall Driven by Changes in Frequency of Organized Deep Convection.
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Source: Monash University; Photo: NASA on The Commons/Flickr/CC.
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