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Western Wildfire Increases Hail and Heavy Rain in Central US

By October 23, 2022May 3rd, 2023aerosols, Extreme Weather, Fire, rainfall

Western Wildfires Increase Extremeness of Weather in the Central US by a Third

Climate change: Expect the unexpected. How can western wildfires increase the extremeness of weather in the Central US? Well, heat injected into the atmosphere enhances high pressure because heat rises and high pressure is rising air. This increase in high pressure increases westerly and southwesterly winds, which increase moisture transport into the Central us from the Pacific. So there is more moisture for thunderstorms to work with. this translates into more energy as well as more water because as moisture rises is condenses and gives of heat, that feeds back into the rising air column to increase even more lift, that increases the height of the thunderstorm allowing heavier precipitation and larger hail.

The aerosol connection is smoke. Smoke particles are aerosols, or little pieces of soot and burned organic material that are heavier than air, but because up winds and electrostatic effects they remain suspended in the atmosphere. These particles then can act as condensation nuclei in thunderstorms with more condensation nuclei creating greater condensation that feeds back into the thunderstorms convective heat engine. As more moisture condenses on more condensation nuclei, more heat is generated in the convection which of course increases the quantity and speed of rising air, feeding back into more moisture condensation, etc.

There is another feedback mechanism at work that increases these impacts of extreme weather. It used to be  that the traditional fall wildfire season didn’t really overlap with the traditional spring extreme weather season. But both wildfires and extreme weather are now exceeding their traditional seasons. Wildfire season is often starting in the spring now and extending later into early winter. In addition, extreme weather season is now seeing a spike in the fall, and even extending through the winter with a five-fold increase in winter tornadoes (Childs 2018).

The study reviewed rainfall and fire events from 2009 to 2018 where precipitation events were preceded by fire events by one of two days with fires greater than 20 square kilometers (7.7 square miles). Only storms with greater than 20 hail or heavy precipitation reports were considered, where storms occured 1, 2, 3 and 4 days consecutively.

Abstract, “We find that the western US wildfires notably increase the occurrences of heavy precipitation rates by 38% and significant severe hail (≥2 in.) by 34% in the central United States.Both heat and aerosols from wildfires play an important role. By enhancing surface high pressure and increasing westerly and southwesterly winds, wildfires in the western United States produce (1) stronger moisture and aerosol transport to the central United States and (2) larger wind shear and storm-relative helicity in the central United States.”

Zhang et al., Notable impact of wildfires in the western United States on weather hazards in the central United States, PNAS, October 17, 2022.