"CAPE is a measure of how potentially explosive the atmosphere is, that is, how buoyant a parcel of air would be if you got it convecting, if you got it to punch through overlying air into the free troposphere," Romps said. "We hypothesized that the product of precipitation and CAPE would predict lightning."
The Chariot of Zeus — Project Gutenberg from the 1879 Stories from the Greek Tragedians by Alfred Church
Using US Weather Service data on precipitation, radiosonde measurements of CAPE and lightning-strike counts from the National Lightning Detection Network at the University of Albany, State University of New York (UAlbany), they concluded that 77 percent of the variations in lightning strikes could be predicted from knowing just these two parameters.
"We were blown away by how incredibly well that worked to predict lightning strikes," he said.
They then looked at 11 different climate models that predict precipitation and CAPE through this century and are archived in the most recent Coupled Model Intercomparison Project (CMIP5). CMIP was established as a resource for climate modelers, providing a standard protocol for studying the output of coupled atmosphere-ocean general circulation models so that these models can be compared and validated.
The intensity of lightning flashes averaged over the year in the lower 48 states during 2011. Data from NLDN.
"With CMIP5, we now have for the first time the CAPE and precipitation data to calculate these time series," Romps said.
On average, the models predicted an 11 percent increase in CAPE in the US per degree Celsius rise in global average temperature by the end of the 21st century. Because the models predict little average precipitation increase nationwide over this period, the product of CAPE and precipitation gives about a 12 percent rise in cloud-to-ground lightning strikes per degree in the contiguous US, or a roughly 50 percent increase by 2100 if Earth sees the expected 4-degree Celsius increase (7 degrees Fahrenheit) in temperature. This assumes carbon dioxide emissions keep rising consistent with business as usual.
Exactly why CAPE increases as the climate warms is still an area of active research, Romps said, though it is clear that it has to do with the fundamental physics of water. Warm air typically contains more water vapor than cold air; in fact, the amount of water vapor that air can 'hold' increases exponentially with temperature. Since water vapor is the fuel for thunderstorms, lightning rates can depend very sensitively on temperature.
In the future, Romps plans to look at the distribution of lightning-strike increases around the U.S. and also explore what lightning data can tell climatologists about atmospheric convection.
Romps' co-authors are Jacob Seeley, also of the Department of Earth and Planetary Science at UC Berkeley, and David Vollaro and John Molinari of the Department of Atmospheric and Environmental Sciences at UAlbany.
The work was supported by the US Department of Energy's Office of Advanced Scientific Computing Research and Office of Biological and Environmental Research, and the National Science Foundation.
Related Information
- Projected increase in lightning strikes in the United States due to global warming (Science)
- David Romps' website
- National Lightning Detection Network
- World Wide Lightning Location Network
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