Forest Fires: Particulate Forest Fires: Particulate Effects on Global ClimatologyEffects on Global Climatology
Akua Asa-Awuku, Christos Fountoukis, & Robyn Williams
Summary
• I. Introduction– What are aerosols?– How are clouds formed? – What is the direct/indirect effect (IPCC) chart
• Tropospheric Effects– Semi-direct effect– Amazon data on cloud coverage– Amazon pictures and article
• Stratospheric Effects• Conclusions
Biomass Burning• Wildfires consume a million or
more square kilometers per year. • Fire is a tool used in agriculture to
clear croplands and help return nutrients to the soil.
• Annually, humans burn anywhere from 750,000 to 8.2 million square km of forest and grassland around the world.
• It is expected that a surface warming trend at high northern latitudes will lead to an increase in boreal fires and their effects, such a trend toward increased burning has already been detected (Stocks et al., 1998)
Forest Fires emit Aerosols
• suspension of a fine solid or liquid particles in gas. – Primary
– Secondary
J.T. Houghton: “The science of climate change”
• Clouds significantly reflect incoming Solar Radiation
Not Just Water Vapor
• Many areas of the globe with high water vapor concentrations do not form clouds
Why are clouds linked to aerosols?
• Clouds form in regions of the atmosphere where water vapor is supersaturated.
• Water vapor supersaturation is generated by cooling (primarily through expansion in updraft regions and radiative cooling).
• Cloud droplets form from pre-existing particles found in the atmosphere (aerosols). This process is known as activation.
• Aerosols that can become droplets are called cloud condensation nuclei (CCN).
CCN that activatesinto a cloud drop
Aerosol particlethat does not activate
Cloud
More polluted:Smaller drops
Less polluted:Larger drops
Changing Cloud Properties• Direct Effect• Indirect Effect• The radiative properties are highly complex (and poorly
understood).
The Indirect Effect
The crushed smaller particle ice, reflects more than the larger ice particles
IPCC, 2001
What is the Impact?
Forest Fire Effects
• Tropospheric
• Stratospheric Differences
“Semi-direct” effect
• Reduction in cloud cover
• Warming of the surface– In the morning: heating of the atmosphere, cooling
of the surface (increased static stability, supressed convection)
– In the afternoon: less cloud cover, more sunlight in the surface
Heavy smoke “chokes” clouds
Decreased cloud cover
NASA Measurements
• Total amount of light reflected through the top of the atmosphere
• Area covered by clouds and by smoke
• “Optical thickness” of smoke
• Measurements in meteorologically different regions
NASA Results• Much less solar energy reflected back up to the
space with the reduction in cloud cover• Smoke “chokes off” cloud formation rather than
being a reflector of sun light• Similar examples in Africa and Canada• Global cooling influence of aerosols smaller than
previously thought• The semi-direct effect amounts to a reduction in
global annual mean cloud cover and LWP of 0.2% and 0.3g/m2, respectively
Stratospheric Effect
• Long Lifetime and Distance
• Unknown Degree of Prevalence
• Fromm and Servranckx Study
Smoke Particulate Transport
• Tropopause– Vertical Transport
Inhibitor
– Well Mixed Tropopause vs. Very Stable Stratosphere
Smoke Particulate Transport
• Volcanoes
• “Nuclear Winter”
• Supercell Convection– Boreal Fire - Summer of 1998
Supercell Convection
• Highly organized thunderstorm
• Extremely strong rotating updraft
• Ability to produce severe weather
Case Study: Chisholm Fire
• 28/29 May 2001
• 160km North of Edmonton, Alberta
Pre-Convection
Early Stage of Convection
Fully Developed Convection
Post-Convection
Case Study: Chisholm Fire
• Dynamic Relationship Between Fire and Convection– Surface Heating
– Destabilization of Air
– Generation of wind and lightening
– Aggravation of existing fires
– Supercell Storms
– Intense Interfacial Gravity Waves
Case Study: Chisholm Fire
• Transport from planetary boundary layer to upper troposphere/lower stratosphere
• 1998 Substantial Statospheric Aerosol Increase
Conclusions: Semi-Direct Effect
• In highly polluted regions: increase of BC causes a reduction of LWP and cloud cover
• However, indirect effect causes the opposite• Therefore, all aerosols radiative effects should
be calculated simultaneously in order to determine the net increase/decrease of cloud cover and LWP
Conclusions: Stratospheric Effect
• Volcanic Haze Effect
• Material Distribution with a radiative, chemical, and cloud impact
Where Do We Go From Here?
• Unknown Extent Quantify
• Increased Urgency of Increased Fire Prevention Methods
Questions?
