--David D. Herring (dherring@climate.gsfc.nasa.gov), Science System & Applications, Inc. (SSAI), EOS AM-1 Outreach Coordinator
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During the SCAR campaigns, the MODIS Airborne Simulator was flown aboard NASA's ER-2 aircraft (pictured above). |
Earth scientists at NASA's Goddard Space Flight Center recently demonstrated that the MODIS instrument, scheduled to launch in mid-1999 on NASA's Earth Observing System AM-1 satellite, will have unprecedented abilities to detect forest fires anywhere on the surface of our planet. In two scientific papers forthcoming, the scientists announce that MODIS will measure the intensities of fires and will therefore enable them to detect active fires and accurately estimate rates of combustion, as well as derive the amounts of emission productssuch as smoke, greenhouse gases, and aerosol particlesthey produce.
According to the papers' principal authors, Yoram Kaufman (GSFC) and Chris Justice (U. of Virginia), the new fire monitoring capability MODIS provides could greatly enhance our abilities to more-efficiently manage natural resources and more-effectively combat wild fires. Moreover, in concert with other space-based sensors including the MODIS scheduled to fly on EOS PM-1 in the year 2000, MODIS will enable scientists to assess the impacts fires may have on air quality near local communities, or even metropolitan areas.
Kaufman, EOS AM-1 project scientist, explains that due to MODIS' moderate (1 km) resolution, most fires will appear relatively small from its perspective in orbit. Consequently, it will only detect about 25 percent of the fires burning worldwide. Yet, because those are the larger, more-intense fires, they are responsible for 80-to-100 percent of the biomass burned.
"This is an important point, because until recently, scientists didn't fully understand the relationships between the radiative energy a fire emits, its rate of combustion, and the resulting emission products," Kaufman states. Based upon the results from the 1995 NASA-sponsored "Smoke, Clouds and Radiation" (SCAR) campaign in Brazil, the authors conclude that "a fire's radiative energy output is highly correlated (97 percent) with the generation of the burn scar and therefore with the consumption of biomass."
Fires generally burn at temperatures anywhere from 500 K to 1400 K (440°F to 2060°F). Fires toward the higher end of this range become so hot they actually burn down into the soilin addition to consuming plants, leaching the soil of its essential nutrients for sustaining life. Because MODIS is more sensitive to fires' temperatures than previous satellite remote sensors, scientists can better assess the damage they cause, as well as help firefighters prioritize which fires are in most urgent need of fighting.
"For individual wild fires," Kaufman continues, "the rate of combustion is an indicator of the strength and danger of a fire, and can be useful to firefighters in decisions on whether or not to fight the fire, as well as in developing a strategy for fire containment and suppression."
Scientists will try to use the MODIS data to determine the phase of a given firewhether it is flaming or smoldering (flaming combustion is the least polluting). Although the rate of combustion is higher during the flaming phase, more smoke, aerosol particles, and greenhouse gases are emitted during the smoldering phase due to the lower combustion efficiency. Therefore smoldering fires have the greatest influence on regional pollution and, potentially, on global climate change. "From late May to early July, there were more than 1,900 fires in Florida that burned more than 425 square miles of land in and around urban communities. Most of the fires are believed to have been caused by lightning strikes. An estimated 40,000 residents were evacuated from their homes, while citizens in neighboring counties with respiratory problems were urged to remain indoors due to health risks presented by the heavy smoke emissions."
In recent months, the public media have focused on a number of unusually large fires burning in various parts of the world. In October 1997, smoke from the widespread burning in Indonesia covered a large portion of the Asia-Pacific region and was linked to a high number of health problems and some deaths in that country. Since early May of this year, more than 1,000 fires have burned in Southern Mexico and Central America, producing a cloud of smoke so dense and widespread that visibility in parts of southern Texas was reduced to 2 km. Some flights in that region were cancelled due to poor visibility and 53 counties were placed on a health advisory.
"Most people are unaware how extensive fire is on our planet," observes Justice, lead scientist for the MODIS Land Discipline Group. "With increasing variability in weather events and climate, we're likely to see changes in fire frequency and extent and the associated disturbance to natural and managed ecosystems. For example, we have seen considerable interannual variability in fire occurrence in Brazil, Africa, and Florida, United States, over the last few years. The occurrence of large fires near populated areas in California in recent years has caused considerable concern for state and local authorities and highlighted the need for improved fire management and monitoring."
Co-author Darold Ward, of the U.S. Forestry Service, states that an estimated 6 petagrams (about 6.3 billion tons) of biomass are burned worldwide each year. He notes that about 80 percent of all biomass burning takes place in tropical countries. About 3-to-5 percent of the worldwide total--or 7 million acres--is burned annually here in the United States.
This Earth image shows a combination of fire data taken globally by two different remote sensing satellites during two different time periods. Those regions with brighter pixels (Central America, Venezuala and Brazil, Central and Southern Africa, Madagascar, Southeast Asia, Indonesia, and Northern Australia) are a composite picture of where fires occurred during the time periods from April 1992 to May 1993, and from October 1994 to March 1995. Data from the Defense Meteorological Satellite Program's (DMSP) Operational Linescan System (OLS) and the National Oceanic and Atmospheric Administration's (NOAA) Advanced Very High Resolution Radiometer (AVHRR) were used to produce this experimental fire imagery. The AVHRR data were processed by the International Geosphere-Biosphere Program Data and Information System (IGBP-DIS). (Credit: Rob Simmon, EOS AM-1 Visualization Team)
"Approximately 2-to-5 million acres are burned by wild fires in the U.S. each year," Ward states, "while 5 million acres are burned as 'prescribed' fires, or fires that are beneficial in managing the ecosystems. Prescribed fires are deliberately planned and set by fire management officers for a variety of reasons."
Forest managers often set prescribed fires to prepare a particular site for tree planting, as well as to kill undergrowth to reduce the competition from unwanted vegetation. Prescribed fires are used extensively for wildlife habitat improvements. Prescribed fires are also set and managed to reduce the potential hazard of larger, uncontrolled wild fire outbreaks. These fires reduce dead vegetation, or consume "fuel," that has accumulated on the forest floor.
"In the western United States, forests have been well-protected from fire for the last 60-70 years," Ward explains. "These ecosystems that evolved in the presence of occasional 'natural' wild fires are now developing 'unnaturally' in competition with off-site species. Consequently, the forests have become unhealthy because of the large amount of dead and dying vegetation. This may lead to the spread of insects, disease, and to disastrous wildfires.
"So, we recognize that fires are an important part of the ecosystem and we're trying to bring them back into the ecosystem in a responsible way," Ward concludes. "This requires fairly aggressive prescribed burning by land managers. We think MODIS data will be useful in helping us monitor their smoke output in light of air quality regulations. Additionally, through remote sensing, we can do a better job of assessing the impacts of fires in restoring the fire-dependent ecosystems."
MODIS' advanced fire monitoring capabilities were first tested during the SCAR campaign held in Brazil in 1995. Jointly sponsored by NASA and the Brazilian government, SCAR was conducted by a team of government and university scientists from both nations. During the campaign, a MODIS Airborne Simulator was flown aboard NASA's ER-2 aircraft (a U-2 airplane modified for science use) at altitudes of up to 40,000 feet. The purpose of this simulator is to enable scientists to gather "MODIS-like" data so that they may refine their data processing techniques in anticipation of the launch of the real MODIS.
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Tropical biomass burning and smoke plume, Central Brazil, near the Xingu River, August 23, 1995. This image was created from multispectral MAS data with 50-meter nadir pixel resolution. |
According to Elaine Prins, co-author and research scientist with the NOAA/NESDIS Office of Research and Applications, MODIS is an improvement upon previous satellite sensors due to its higher "saturation thresholds" as well as its larger number of spectral channels to better characterize the atmosphere and fires. By analogy, the sun is so bright that it quickly "saturates" a human eye when looked at directly. Similarly, remote sensors can become saturated when viewing hot, bright fires, rendering it impossible for them to distinguish important characteristics about that fire, such as its rate of combustion.
"The primary fire detection channel on the AVHRR (Advanced Very High Resolution Radiometer) has a saturation threshold of approximately 320 K," Prins states. "MODIS has four channels that are sensitive to fires. Two are located in the infrared portion of the electromagnetic spectrum at 4 and 11 micrometers with thresholds at 500 K and 400 K, respectively, and can be used to monitor fires day and night. The other two are located at 1.6 and 2.1 micrometers for nighttime fire detection."
Coincident images of the Brazilian rain
forest, at 0.657 µm and 4.05 µm, respectively.
The visible light image shows only smoke, while the thermal
infrared image show details of the
fire beneath it. They were acquired by the MODIS Airborne
Simulator on August 23, 1995.
Once certain of its viewing pixels pass the "brightness temperature" test that identifies them as potential fires, the data are then computer processed to differentiate them from background pixels that are obviously not fire. Then, the 4-micrometer channel data are compared with data from MODIS' 11-micrometer channel to provide an additional perspective.
The 4- and 11-micrometer channels were chosen because they are "window" channels that enable remote sensors to "see" more clearly through the atmosphere than most other channels. At night, MODIS can use its 1.6- and 2.1-micrometer channels for detecting fires, when data in these channels are acquired. NASA is planning two field campaigns in 1999 to test the accuracy of the MODIS fire product for Brazil and Southern Africa, two major fire regions of the World.
According to the authors, MODIS data will also be applied in other useful and relevant ways. For instance, a fire- potential product is being developed to indicate what regions are susceptible to wild fire outbreaks. Another co-author plans to use MODIS data to monitor volcanoes and, based upon sudden temperature increases, help forecast when a given volcano is about to erupt.
In cooperation with MODIS science team members, as well as other agencies such as NOAA, the EOS AM-1 outreach team created a Global Fire Monitoring Web site. The URL is http://modarch.gsfc. nasa.gov/fire_atlas.
The purposes of this site are to provide an up-to-date view of fires around the globe on a "near-real-time" basis, and to provide an evolving view of the state of the science with respect to emissions of greenhouse gases, smoke, and particles from fires.