A hard rain’s a-gonna fall

Scientists expect intense bursts of rain to become more frequent in a warming world. Their prediction is founded on the link between vapor pressure and temperature, expressed in the Clausius–Clapeyron relation, which suggests that the maximum rate of precipitation during extreme events will increase by 7% per degree Celsius of warming. But historical records show a more complex connection, and global climate models struggle to simulate precipitation on time scales shorter than a day.

Now a team led by Andreas Prein from the National Center for Atmospheric Research in Boulder, Colorado, has developed a high-resolution climate model that matches observed hourly precipitation intensities in regions throughout the continental US. The model runs on a 4 km x 4 km grid to allow explicit descriptions of convection. The researchers found that extreme short-duration rainfall depends not only on temperature but also on the amount of available moisture in the atmosphere.

Running the model with the temperature increases projected for the end of the century revealed that extreme precipitation events should become more frequent over almost all of North America (see map above). And because of anticipated increases in the amount of atmospheric moisture, the researchers expect hourly precipitation rates to continue to rise even as the temperature climbs to 30 ^oC; in today’s climate, those rates no longer increase beyond 25 ^oC. By 2100, states along the Gulf of Mexico could receive far more torrential downpours, adding up to about 40% more rain during summer.

Despite the model’s success in matching historical data, Prein’s team warns that future shifts in extreme precipitation events depend heavily on local humidity and regional characteristics, which are themselves affected by a warming climate. (A. Prein et al., Nat. Climate Change, in press, doi: 10.1038/nclimate3168 .)