Improved detection of airborne volcanic ash using multispectral infrared satellite data

Abstract

A technique for improved detection of airborne volcanic ash has been developed that uses three infrared (IR) bands from meteorological satellites. The three IR bands are centered near 3.9, 10.7, and 12.0 μm wavelength. The technique is based on the sum of two brightness temperature differences (BTDs), scaled to maximize the brightness and contrast of volcanic ash in the output image. The physical effects attributed to the observed BTDs that help distinguish the volcanic ash from various meteorological cloud types are (1) differential absorption by volcanic ash or sulfur dioxide at 3.9 μm, 10.7 μm, and 12.0 μm and (2) strong solar reflectance by ash at 3.9 μm, which varies diurnally. On the basis of two examples using data from the Geostationary Operational Environmental Satellite (GOES) the three‐band IR technique is shown to provide better discrimination of volcanic ash from meteorological clouds than is possible using existing two‐band methods. This conclusion is supported by comparisons of brightness count profiles and estimation of false ash detection rate statistics. The best results from the three‐band IR technique are obtained during daylight hours over any surface, and at night when the ash cloud is over the ocean or other large body of water. The three‐band IR technique is one of the tools currently being employed operationally at the Washington Volcanic Ash Advisory Center.