Earth & Climate

Photo by: Marty Feely

Wildfire-induced thunderstorms provide natural laboratory for studying the effects of aerosols on clouds

Thunderstorms that form over wildfires, known as pyroCbs, help scientists understand how smoke affects clouds. Using satellites, we found evidence that smoke from a fire altered a storm’s internal mechanics to increase the cloud’s lifetime.

Every summer, wildfires burn thousands and thousands of acres across the Northern Hemisphere.  These fires generate a tremendous amount of heat and also release water vapor into the atmosphere; both are ingredients for thunderstorms to develop.  It is therefore no surprise that storms are frequently observed directly over wildfires, particularly when the atmosphere is ripe for thunderstorm development.  A relatively new term has been coined for storms of this nature: pyroCbs (http://glossary.ametsoc.org/wiki/Pyrocumulonimbus).

In addition to releasing heat and water vapor, fires also produce smoke (of course).  Smoke is made up of a variety of tiny particles called aerosols, the exact nature of which depends on what is burning.  A large portion of the aerosols tends to be soluble, meaning they readily dissolve in water, and therefore are effective at creating additional water droplets that make up clouds, which can alter the cloud’s behavior.

Using GOES-9 satellite data from 4-5 July 1998, we identified numerous pyroCbs over fires in British Columbia and the Yukon Territory.  Based on the amount of solar energy that is reflected from the storm tops, known as anvil clouds, we estimated the average size of the cloud’s ice crystals.  We found that the pyroCbs had significantly smaller ice crystals than the nearby “clean” thunderstorms, which are normal thunderstorms not occurring over fires and therefore not ingesting smoke. We then used frequent geostationary satellite images to track both the pyroCb anvils and the clean storm anvils.  The pyroCb anvils persisted hours longer than the regular anvils, including one lasting until the next morning, where it prevented solar heating over a region in northwest Canada.

This case study provides evidence that smoke from wildfires can produce thunderstorm anvils lasting longer than anvils from clean storms.  Longer-lasting anvil clouds may alter the net amount of energy entering or leaving the Earth system, so this effect is important to understand for longer-term climate studies.

Original Article

Funded by: NOAA

Written by: Dan Lindsey and Michael Fromm.

Link to journal article