Torrential rainfall a common climate feature in the future

Changes in the climate are expected to result in heavier rainfall. Calculations with climate models show that a heavy downpour will give 20-30 per cent more rain at the turn of the next century.

Thunder showers and torrential rainfall in the summer are part of what, in technical jargon, is called extreme or intensive short-term precipitation.

“Intensive short-term precipitation is the type of rain that causes most problems in towns and cities in connection with runoff and flooding, since it produces large quantities of rain in a short period of time,” says Jonas Olsson, who is a researcher in hydrology at the Swedish Meteorological and Hydrological Institute, SMHI.

Heavier precipitation more common

Skyfall
Intensive short-term precipitation causes most problems in towns and cities in connection with runoff and flooding, since it produces large quantities of rain in a short period of time.

One of the anticipated effects of climate change is more intensive short-term precipitation, or in other words precipitation that last less than an hour. This is because a warmer atmosphere may contain more water vapour, which creates the preconditions necessary for heavier precipitation.

In a study of extreme short-term precipitation, research staff have compared precipitation volumes over the 30-year period 1981-2010 with projections for precipitation at the end of the current century.

The study shows that the 10-year rain, or in other words the quantity of rain that falls on average once every ten years, will be 10 per cent greater in 2050, and 25 per cent greater by the end of the century, according to today’s climate models .
“At the end of the next century, it looks as though we will be likely to experience downpours of 25 mm of rain in an hour as often as we today have 20 mm of rain in an hour,” explains Jonas Olsson.

Precipitation is difficult to describe

Precipitation is very difficult to describe in models. Therefore, researchers have also taken a look backwards to see how well the climate models describe a period that has already occurred, and which can thus be compared with real values.

“The models manage to reflect historical variations fairly well, which is positive in that it increases the reliability of the results,” says Jonas Olsson.

The study entitled “Extreme short-term precipitation in climate projections for Sweden” consists of three parts: a historical extreme value analysis, a future extreme value analysis and a review of current research. You can read the entire report in SMHI’s Klimatologi No. 6, 2013 (in Swedish).