Beginning with the Geostationary Operational Environmental Satellite (GOES)-M spacecraft (expected launch in July, 2001) through GOES-P (2007), a 12.0 mm “Split Window” InfraRed (SWIR) band (4 km resolution) on the Imager will be replaced by a 13.3 mm channel (8 km resolution). Brightness temperature differences between the SWIR and the IR window at 10.7 mm have been successfully used in tracking hazardous volcanic ash clouds since first becoming available on GOES-8 in 1994. SWIR data is relied upon heavily at the Washington Volcanic Ash Advisory Center (VAAC) for operational monitoring of a large, volcanically-active portion of the Caribbean, and Central and South America.

In order to assess the effect of this Imager channel reconfiguration on volcanic ash detection, GOES Imager or Sounder data from several prior eruptions were analyzed to see how volcanic ash coverage would look with and without the SWIR. The GOES Sounder has all IR bands currently on the GOES Imager, plus the 13.3 mm band, although at a lower resolution (10 km). The analysis was completed using software which generates principle component images that display unique information available from the input IR bands. The best principle component depiction of the ash cloud for each event was compared to a subjective best estimate of the actual ash cloud coverage based on: (1) analysis of all available GOES data, including visible images, (2) the evolution of the volcanic cloud from its eruption to image time, and (3) VAAC advisories.

Preliminary results indicate that the loss of the SWIR band will result in an increase of mis-identified (“false”) ash pixels which seems to vary diurnally. During the daytime, the increase in false pixel rate will be minimal (~1-2%) due to the positive contribution of reflectance in the 3.9 micrometer shortwave IR band. At night, the false pixel rate increases by a factor of ~ 4, likely due to the loss of shortwave reflectance. Most of the mis-identified ash is not contiguous with the true ash, however, indicating that a human analyst will be able to provide a reasonably good estimate of ash area. Inclusion of data from the 13.3 micrometer band appears to provide a slight positive impact, allowing for better discrimination of ash from thin cirrus clouds. Any gain achieved from the 13.3 mm IR data will be offset somewhat by its lower resolution (8 km), until GOES-O in 2005. In summary, we will still be able to observe and track significant volcanic ash clouds in the GOES-M through P era (2002-2007+), with some degradation that will be most significant at night.