Local Winds Play Key Role in Some Megafires
Drought and overgrown forests are often blamed for major wildfires, but new research shows that localized winds may play a larger role.
Although drought
and overgrown forests are often blamed for major fires in the western United
States, new research using unique NASA before-and-after data from a megafire site
indicates that highly localized winds sometimes play a much larger role -- creating
large, destructive fires even when regional winds are weak.The study was
led by the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
It focused on the 2014 King Fire, using data from airborne instruments managed
by NASA's Jet Propulsion Laboratory in Pasadena, California, with advanced
computer simulations from NCAR. The King Fire occurred in the Sierra Nevada
mountain range during California's severe multi-year drought and burned more
than 97,000 acres (39,000 hectares).The study team
found that winds -- both very localized winds related to topography and winds
created by the searing heat of the flames -- were the reason the fire suddenly
ran 15 miles (24 kilometers) up a steep canyon one afternoon. Winds like these,
sometimes only a few hundred yards (meters) across, often go undetected by
weather stations that may be several miles away. In fact, for several days
before the fire, nearby weather stations measured only weak winds."This brings
into question several widely held and largely unquestioned assumptions, such as
very large fires being caused by the accumulation of vegetation, persistent dry
conditions, or requiring extreme conditions," said NCAR scientist Janice
Coen, the lead author of the study. In the King Fire, she pointed out, "Small-scale
winds and winds generated by the fire had a much greater impact on this fire,
and potentially others like it, than any of the other factors."JPL scientist
Natasha Stavros, a coauthor on the study, said, "The
NASA airborne measurements were unique in that we observed the forest's
vertical structure before and after a fire. These observations let us better
identify the type of fuel -- grass, shrubs, or trees. That improved the model
simulations, particularly of how the fire spread in areas where previous fires
had burned or timber had been as harvested, and in areas where the burn
severity was greatest."Experimenting with a MegafireLarge and destructive
megafires are becoming more frequent in the western United States. Experts have
attributed this to a changing climate, which is causing hotter and sometimes
drier conditions, or to a century of fire suppression policies that have left
forests with more vegetation to fuel the flames than in the past. Scientists
cannot experiment with large and destructive wildfires, so they have fallen
back on examining statistical correlations to try to tease out the key factors
associated with megafires.The area
consumed by the King Fire, however, had been previously mapped by JPL's
Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and MODIS/ASTER
Airborne Simulator (MASTER) instruments in visible and thermal infrared
wavelengths, as well as by a
U.S. Forest Service lidar instrument, resulting in an extensive database about
the forest structure and vegetation types. In addition, the authors had access
to airborne thermal imagery collected during the fire. The detailed data gave them
a rare opportunity to recreate an actual wildfire within a sophisticated NCAR computer
model that combines weather prediction and fire behavior, testing the
importance of different factors.Simulations of
the King Fire under more extreme drought conditions did not change the ultimate
extent of the fire or greatly alter its expansion, and simulations with half of
the actual fuel load (as might exist in a less overgrown forest) unfolded in
about the same way as the real fire did.The scientists
concluded that the fire became stronger in the canyon because of the inclined
slopes. Drought conditions or increased vegetation did help the fire to
generate the strong updraft that drew flames up the canyon slope. These factors
had little impact while the fire was on flatter ground."This is
just one case, but it illustrates how the causes of a megafire have sometimes
been misunderstood," Coen said.The study, titled
"Deconstructing the King Megafire," was published in the journal Ecological
Applications. The research was funded by NASA.For more
information, see:https://www2.ucar.edu/atmosnews/just-published/133629/fanning-flames-megafires