Friday, November 24, 2017

"Rivers From The Skies" - The Nature of Mega Thunderstorms in a Climate Change Future

The new study published in the journal Nature Climate Change on Monday was sober and unrelentingly grim. It points to a future - thanks to ramping up climate change-  of mega thunderstorms that will spawn such monster deluges they've been compared to "rivers in the skies" by one of the study's authors.

Think of what happened to New Orleans with Katrina and more recently to Houston with Harvey. Now try to imagine either scenario but with 60-80 percent more precipitation.

The U.S. in recent years has experienced prolonged drenchings that have doused Nashville in 2010, West Virginia and Louisiana in 2016 and Houston this year. The disasters cost about $20 billion a year in damage. Now, imagine the costs ten times more with the much wilder, vaster storms - covering entire metropolitan areas of cities.

By the end of century if emissions aren’t curbed, these deluges will be much worse because they will get bigger, said Andreas Prein, a climate scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, who led the study.

Prein and colleagues used high-resolution computer simulations to see how global warming will likely change the large thunderstorms that are already daily summer events in North America. Previous studies projected more frequent and wetter storms, but this is the first research to show they likely will be more widespread, covering an entire city instead of just half of it.

We see increases that are beyond our expectationsfar beyond our expectations,” Prein said. “It looks everything that can go wrong does go wrong concerning flooding.”

With the size of the storm factored in, according to Prein,  the total amount of rain in the U.S. South is projected to jump 80 percent between now and the end of the century.  For Mexico, the increase in rainfall would be 70 percent and 60 percent in the U.S. Southwest. Canada and the rest of the U.S. should expect a 40 percent rain increase from current levels.

About half of those increases are from the storms being larger, Prein said. These types of storms include tropical storms, but most of the storms studied are average thunderstorms.

"You can really think about these storms as rivers that come from the skies,” Prein said. “The largest ones are several times the Mississippi River discharge.”

Where is all this latent moisture coming from?  We already know the answer from basic thermal physics.  We can use the well known   Clausius-Clapeyron equation   to  ascertain that for every 0.5 C increase in warming there corresponds a  roughly 3% increase in average atmospheric moisture content.  For a climate 1 to 1.5C warmer than several decades ago this translates to an enhanced atmospheric moisture content of up to 9 percent.   One hundred years hence, hitting the 6 C temperature threshold, we are looking at an average 36 percent increased moisture content world wide.  Hence the looming risk of "rivers that come from the skies."

The costs of insurance, namely flood insurance, will soar - even in the U.S. southwest (projected to have a 60 percent  increase in precip -including hail).  This assumes people will be able to even purchase it. The takeaway from all this is that the incredible "gully washers" we see today will be like small rain bursts compared to what is experienced in the future.  As I've told my great nieces, nephews, be sure you get ready for it because you are the ones who will have to deal with it.

To read a summary of Prein et al's  paper, go to:

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