P3.13
Global composite of volcanic ash “split window” geostationary satellite images
Frederick R. Mosher, Embry-Riddle Aeronautical University, Daytona Beach, FL
Volcanic ash is exceptionally hazardous for jet air craft in flight. Flying through ash can quickly damage the jet engines, causing surging, flame out and immediate thrust loss. Pilots can inadvertently penetrate volcanic ash clouds because airborne weather radar will not reflect off the small ash particles, and the visual appearance of an ash cloud may look very similar to an ordinary meteorological cloud. To aid pilots in avoiding ash cloud areas, the International Airways Volcano Watch (IAVW) agreements have developed under the guidance of the International Civil Aviation Organization (ICAO). Central to the IAVW are the Volcanic Ash Advisory Centers (VAACs) which have the responsibility of continually monitoring active volcanoes for potential ash clouds. The globe has been divided into areas of responsibilities, which have been assigned to 9 VAACs. The VAACs provide ash information to the Meteorological Watch Offices, which write official warnings (SIGMETs), which are used by airspace control centers to route aircraft away from the hazardous areas. The VAAC's use a variety of satellite image processing techniques which enhance the signature of ash clouds, while diminishing the signature of ordinary meteorological clouds. Generally these derived images of ash are not widely available outside of the VAAC and Watch Offices. On several of the VAAC web sites (such as Washington VAAC web site http://www.ssd.noaa.gov/VAAC/), some of the derived images are made available for the localized areas around frequently active volcanoes. However they do not make wide area products available that would cover the drifting ash cloud from a major eruption. Pilots and air traffic controllers generally like to have satellite image products that show the flight hazards, as well as the official SIGMET products. The flight crews like to verify the existence of the hazard as well as judge how close to the hazard area it is prudent to fly. The present effort has been to develop global composites of geostationary ash products that can be made available to pilots, dispatchers, etc. to supplement and help explain the official volcanic ash hazard products coming from the VAACs and Watch Offices. The original derived ash satellite product was the so called “split-window” or “reverse absorption” technique (Petra, 1989) which used the temperature difference between the 11 and 12 micron imager channels on the polar orbiting satellites. This technique takes advantage of an absorption band of silicates which influences the 12 micron image, but not the 11 micron image. The disadvantage of this technique is that there is also an absorption band for water that influences the 12 micron image, so the technique does not work well for new volcanic clouds which contain large amounts of steam. The technique also is of limited use for low ash clouds in a humid atmosphere. However, for jet aircraft operations, this technique offers a clear signature of high, dry ash that is readily identifiable by operational aircraft personal. Other ash detection techniques have been developed that take advantage of the 3.9 micron channels on the geostationary satellites. However, the 3.9 micron channel is influenced by solar reflection, so the use of these multi-channel ash techniques generally requires specialized user training. The 11 micron channel (the “window” channel) has been available on all the geostationary satellites for many years. The 12 micron channel has been available on the GOES satellites until recently. GOES-12 (current GOES-east) through GOES-15 do not have a 12 micron channel. The new European Meteosat Second Generation (MSG), Chinese FY2C, and Japanese MTSAT geostationary satellites all have a 12 micron image channel, as well as the current GOES-west (GOES-11). These operational satellites make it possible to generate global mosaics of the split window ash product, with the exception of a thin slice around 70W because of the lack of the 12 micron channel on GOES-east (GOES-12). The processing takes the difference between the 12 micron and 11 micron temperatures. The resultant temperature difference is stretched into the entire image brightness range so that ash looks white, no difference looks gray, and ice clouds look black. The differencing is done in the original satellite projections and then mapped into a 4 km rectilinear (equal latitude, longitude) 4 km grid. The various satellite grids are then combined into a global 4 km grid. The boundaries between satellites are fixed with no overlap between satellite coverage. The global composite is generated every 30 minutes. Gif images are generated at various locations and resolutions and posted to the Embry-Riddle web site at http://wx.erau.edu/erau_sat/. The attached image shows a sample of the global ash composite showing the area of coverage. The United States split window coverage is generated from the GOES-west satellite. While the western half of the US is covered every 30 minutes, the eastern half of the US is covered only every 3 hours during the full disk scans of the GOES-west. In addition to being able to detect high, dry volcanic ash, the split window images can also detect dust raised by dust storms and large dust devils. In the attached image note the white areas in the North African Sahara desert showing dust clouds.
Poster Session 3, Turbulence, Volcanic Ash, and Instrumentation Posters
Wednesday, 23 January 2008, 2:30 PM-4:00 PM, Exhibit Hall B
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Extended Abstract (936K)