A team of researchers has found that evaporation from the land only modifies the frequency of summertime rainfall, and not its quantity.

“This is a major shift in our understanding of the coupling between the land surface and the atmosphere, and fundamental for our understanding of the prolongation of hydrological extremes like floods and droughts,” said Pierre Gentine, assistant professor of Applied Mathematics at The Fu Foundation School for Engineering and Applied Science at Columbia University.

They discovered that higher evaporation increases the probability of afternoon rainfall east of the Mississippi and in Mexico, while it has no influence on rainfall over the Western U.S. The cited the difference is due to the humidity present in the atmosphere. The researchers explained that the atmosphere over the western regions is so dry that no matter what the input of moisture via evaporation is from the surface, an added source of moisture will not trigger any rain.

This is because it will instantaneously dissipate into the atmosphere. On the contrary, the atmosphere over the eastern regions is sufficiently wet so that the added moisture from the surface evaporation will make it rain.

“If it starts getting really wet in the east,” noted Gentine, “then the surface will trigger more rain so it becomes even moister, and this sets up a vicious cycle for floods and droughts.” "Nature i.e. the land surface and the vegetation cannot control the rainfall process in the west but it can in the east and in the south. This is really important in our understanding of the persistence of floods and droughts,” Gentine added.

The study has been published in the current online edition of Nature Geoscience.

Keywords: climate change