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Half the world's annual precipitation drops for only 12 days, reveals a new study



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THE PAINTING

THE PAINTING: Analysis of precipitation measured at time stations around the world between 1999 and 2014 found that the median time is needed for half a year …
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Credit: © UCAR. Image: Simmi Sinha.

Currently, half of the world's measured precipitation drops for a year only in 12 days, according to a new analysis of data collected at time stations around the world.

By the end of the century, climate models project that this uncertain distribution of rain and snow is likely to become even more distorted, with half of annual precipitation falling in 11 days.

These results are published in Geophysical research, a journal of the American Geophysical Union.

Previous studies have shown that we can expect an increase in extreme weather events and a smaller increase in average annual precipitation in the future as the climate warms up, but researchers continue to explore the relationship between these two trends.

"This study shows how these two parts fit together," said Angeline Pendergrass, a researcher at the National Center for Atmospheric Research (NCAR) and lead author of the new study. "What we found is that the expected increases are going on when it's already moist – the most dangling days become clearer."

Findings that indicate that floods and damage associated with it can also increase, have implications for water management, urban planners and emergency responders. The results of the survey are also a concern for agriculture, which is more productive when precipitation evenly spreads during the growing season.

The research was supported by the US Department of Energy and the National Science Foundation, sponsored by NCAR.

What it means to be extreme

Scientists studying extreme precipitation – and how such events can change in the future – use different metrics to define what qualifies as "extreme". Pendergrass noted that in some cases the definitions were so broad that extreme precipitation events actually included the bulk of all precipitation.

In these cases, "extreme precipitation" and "average precipitation" became essentially the same thing, which made it difficult for scientists to understand from existing studies that it would change independently as the climate warms up.

The other research team is also struggling with this problem. For example, a recent document attempted to quantify the inequality of precipitation by adjusting the Gini coefficient, a statistical tool that is often used to quantify income inequality, and instead look at the distribution of precipitation.

Pendergrass wanted to find something even simpler and more intuitive, which would be easily understood by both the public and other scientists. In the end, she decided to measure the number of days the half-yearly fall should fall. The results were surprising.

"I guess the number will be bigger – maybe a month," she said. "But when we looked at the mid or midpoint, from all the available observation stations, the number was only 12 days."

For analysis, Pendergrass worked with Reto Knutti, Institute of Atmospheric and Climate Sciences in Zurich, Switzerland. They used data from 185 earth stations for 16 years from 1999 to 2014, the period when measurements can be validated based on data from satellite for measurement of cost precipitation (TRMM). While stations were widespread on a global scale, most were in North America, Eurasia, and Australia.

To be hopeful, scientists used simulations from the world's 36 leading climate models that had data for daytime precipitation. They then specified what air conditioning model projections would translate into individual observation stations in the last 16 years of this century.

They found that total annual precipitation at observation points slightly increased in the model, but additional precipitation did not fall evenly. Instead, half the extra rain and snow fell for just six days.

This contributed to the total rainfall falling unevenly as today, with half a year falling in only 11 days to 2100, compared to 12 in the current climate.

"While climate models mostly design only a slight increase in rainfall in general, we believe this rise is overcrowded as events with much more rain and could therefore result in negative impacts, including flooding," said Pendergrass. "We need to take this into account when we think about how to prepare for the future."

The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under the auspices of the National Science Foundation. Any opinion, findings, and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.

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About the article

Title: Uneven nature of daytime precipitation and its change

Authors: Angeline G. Pendergrass and Reto Knutti

Journal: Geophysical research, DOI: 10.1029 / 2018GL080298

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of published news published on EurekAlert! contributing institutions or to use any information through the EurekAlert system.

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