Application of MODIS: The Eruption of Mt. Cleveland

Mount Cleveland is a 5,675-foot-tall (1,730 meters) volcano on the west side of Chuginadak Island in the middle of the Aleutian Islands, near Alaska. On February 19, 2001, the Cleveland Volcano erupted explosively, sending a thick plume of smoke and ash high into the atmosphere that was easily visible to overpassing satellites.

In visible wavelengths, the ash cloud is hard to distinguish from other clouds in the area, and even the now-covered Aleutian Islands. However, the MODIS sensor is also sensitive to infrared wavelengths, and these provide a few better ways of seeing the volcanoês ash. At mid-infrared wavelengths [3.9µm, left (cooler and darker pixels are blue, warmer and brighter pixels are red)], MODIS detects both emitted heat and scattered light. The small cluster of red pixels to the lower left of the ash plume is the heat of the magma inside the volcano. The yellow and red pixels in the curving plume itself are from infrared light that was scattered by the ash particles. Clouds and snow are cold, and they do not scatter or reflect light in this wavelength, so they appear dark blue.

MODIS also collects data in the thermal infrared wavelengths. The image to the right shows the eruption at 11µm. Radiation at this wavelength is emitted from the clouds sea, snow, and ashãthe warmer the surface, the more radiation it emits. Because this image has been inverted dark pixels (like the volcano itself) are dark, and cold surfaces, like the high-altitude ash and high clouds, are bright. Again, it is hard to distinguish ash from clouds, but the snow-covered mountains of the Aleutians are a bit warmer (and therefore darker) than the clouds and ash bumping up against the stratosphere.

The best view of the ash is created by subtracting the brightness ofone thermal infrared wavelength (12µm) from another (11µm), as shown to the left. The 11µm and 12µm images vary slightly, because the ash radiates at a different intensity than the water vapor in clouds, even though they are both the same temperature. The difference is magnified by subtracting one channel from the other, which results in an image where the ash plume stands out clearly.