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Half of the world's annual rainfall falls in just 12 days, new studies find

An analysis of precipitation measured at weather stations worldwide between 1999 and 2014 showed that the median time it took…

An analysis of precipitation measured at weather stations worldwide between 1999 and 2014 showed that the median time it took for half a year’s precipitation this fall was only 12 days. Quarterly rainfall fell in just six days and three quarters fell 27 days. Credit: © UCAR. Simmi Sinha.

Currently, half of the world’s measured precipitation falls in a year in just 12 days, according to a new analysis of data collected at weather stations worldwide.

By the end of the century, climate models show that this oblique distribution of rain and snow is likely to be even more skewed, with half of the annual rainfall falling within 11 days.

These results are published in Geophysical Research Letter a journal from the American Geophysical Union.

Previous studies have shown that we can expect both an increase in extreme weather events and a smaller increase in the average annual rainfall in the future when the climate is warming, but researchers are still investigating the relationship between these two trends.

“This study shows how these two pieces fit together,” said Angeline Pendergrass, a researcher at the National Center for Atmospheric Research (NCAR), and leading author of the new study. “What we found is that the expected increases happen when it’s already the wetest – the rainy days get raining.”

The results, which indicate that floods and the damage associated with it can also increase, have consequences for water managers, urban planners and emergency recipients. Research results are also an issue for agriculture, which is more productive when the rainforest is spread more evenly during the growing season.

The research was supported by the US Department of Energy and the National Science Foundation, which is the NCAR sponsor. [19659005] What it means to be extreme

Researchers studying extreme precipitation – and how such events can change in the future – have used a variety of metric values ​​to define what is “extreme”. Pendergrass noted that in some cases the definitions were so wide that extreme precipitation events actually covered most of all precipitation.

In these cases, “extreme precipitation” and “average precipitation” were essentially the same, which made it difficult for researchers to understand from existing studies how the two would change independently when the climate warms.

Other research groups have also been affected by this problem. For example, a recently published paper attempted to quantify the irregularity of precipitation by adjusting the Gini coefficient, a statistical tool commonly used to quantify income shortages, instead of looking at the distribution of precipitation.

Pendergrass would find something even simpler and more intuitive, easily understood by both the public and other researchers. In the end, she chose to quantify the number of days it would take for half a year’s precipitation to fall. The results surprised her.

“I’d guess the number would be bigger – maybe a month,” she said. “But when we looked at the median or the midpoint, from all available observation stations, the number was only 12 days.”

For the analysis, Pendergrass worked with Reto Knutti of the Institute of Atmospheric and Climate Science in Zurich, Switzerland. They used data from 185 ground stations during the 16 years from 1999 to 2014, a period when measurements could be validated against data from the Tropical Rainfall Measuring Mission (TRMM) satellite measurement. While the stations spread globally, the majority were in North America, Eurasia and Australia.

To look forward, the researchers used simulations from 36 of the world’s leading climate models that had data for daily rainfall. Then they decided what the climate model projections for the past 16 years of this century would translate into for the individual observation stations.

They found that the total annual rainfall at the observation stations increased somewhat in model runs, but the further downturn did not fall evenly. Instead, half of the extra rain and snow fell in just six days.

This meant that the overall precipitation also fell more uneven than it does today, with half of one year’s total rainfall falling in just 11 days by 2100, compared to 12 in the current climate.

“Although climate models generally only give a slight increase in the rain in general, we find that the rise will come as a handful of events with much more rain and can therefore lead to more negative effects including floods, Sade Pendergrass.” We must take Consider this when we consider how we prepare for the future. “

The University of Atmospheric Research Manages the National Center for Atmospheric Research under the sponsorship of the National Science Foundation. Any opinions, results and conclusions or recommendations expressed in this material do not reflect necessarily the National Science Foundation’s views.

Explore further:
What causes more extreme precipitation in the northeast?

More information:
Angeline G. Pendergrass et al. The uneven nature of the daily rainfall and its change, Geophysical Research Letters (2018). DOI: 10,1029 / 2018GL080298

Journal references:
Geophysical research letter

Provided by:
National Center for Atmospheric Research

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