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CLIMATE | Rainfall inequality

Globe gets half its annual rainfall in just 12 days


Caroline Floyd
Meteorologist

Monday, November 19, 2018, 3:49 PM - Rain can seem fickle. Between floods and droughts, we all have a pretty good understanding of how too much rain, or too little, can have dramatic impacts on our lives. As it turns out, though, the overall pattern of rainfall is less arbitrary than it looks.

A new study by researchers in Europe and the U.S. worked out a way to measure the unevenness of precipitation -- specifically, how long it takes for spots around the world to rack up half of their annual average rainfall. The average? A lot less time than you'd probably guess.

HOW THE STUDY WORKS

The researchers looked at daily rainfall data from 185 stations around the world, all with in 50º of the equator (which captures most of Canada's population regions), running from 1999 to 2014. The 50º north or south of the equator range allowed them to use satellite data to verify the data received, as well fill in some of the blanks.

They then used 36 climate models to run simulations on the data, both for the 1999 to 2014 period, and to make estimates for a potential future period -- 2085 to 2100.


Total rainfall, month to month, from 2010 to 2016. This image shows the seasonal fluctuations of the Intertropical Convergence Zone (the dark band in the tropics), as well as other seasonal shifts in rainfall. Image courtesy NASA Earth Observatory

WHAT THEY FOUND -- PRESENT DAY

Though the specific days of the year vary from spot to spot, the same pattern emerged for stations worldwide; half of the total precipitation a site receives in a year falls over the course of just 12 days. 75 per cent of the total annual rainfall at one spot falls on the wettest 30 days of the year.

That's a stark quantification of something we might recognize intuitively, but not think about too often; those few truly rainy days per year count for a lot more of our annual rainfall than persistent drizzly spells. It's also worth considering that this pattern holds for the whole globe, be it in a desert climate or a rain forest. That means that a place like Vancouver, where the average is about 1150 mm of rainfall per year, sees about 575 mm on its rainiest 12 days. Contrast that with a spot like Chittagong, Bangladesh, where the average annual rainfall is closer to 2800 mm; that's more than 100 mm per day on those 12 wettest days of the year.

(Related: 'Stalling' weather may mean heavier rain, heat waves)


This image, from the study paper, shows different measures of the unevenness of precipitation observed at stations. (a) Days per year for half of precipitation, and the fraction of annual precipitation falling on the wettest day each season: (b) December, January, and February (DJF) and (c) June, July, and August (JJA). (d) All‐day percentile for half of precipitation and (e) fraction of precipitation occurring beyond the 95th all‐day percentile. White indicates no data. Note that stations poleward of 50° are included.

WHAT THAT SAYS ABOUT THE FUTURE

Some interesting results emerged from the team's future projections. Adjusting model conditions to account for the expected rise in atmospheric carbon dioxide (CO2) by the end of the century, the team found that the unevenness of rainfall increases as the temperature increases. It's a generally accepted idea that a warmer climate would mean more rainfall, thanks to an increase in evaporation rates and how much water a warmer atmosphere can hold. 

But this study found that more rainfall doesn't correspond to more rainy days. Specifically, they found that the heavy rain on the rainiest days gets heavier in a warmer climate -- that the events we think of as 'extreme' increase, while the overall number of rainy days does not.

(See also: What regional rainfall tells us about climate change)



WET DAYS GET WETTER

While we might think of a warming climate as an overall shift at both ends of the scale, it doesn't look like the same is true for rainfall. And while 'more rain in a warmer climate' might sound like it's a good thing for drought-stricken regions, it turns out that's not the case, either; this study demonstrates that it's just more rain compressed into fewer days. That's a pattern that suggests more flooding, and the same -- or more intense -- periods of drought.

The team's ultimate conclusion? "Rather than assuming more rain in general, society needs to take measures to deal with little change most of the time and a handful of events with much more rain."

Sources: AGU | Science

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