POAM III Provides Important New Altitude Observations on Volcanic Ash Plume

6/6/1999 - 27-99r
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The Naval Research Laboratory's (NRL's) Polar Ozone and Aerosol Measurement (POAM) III instrument, a satellite-borne sensor that measures vertical profiles of the atmosphere, monitored the evolving high-altitude plume from the recent eruption of the Shishaldin volcano in the Aleutian Islands (54.76° N, 196.0° E) on April 19.

POAM III detected significantly enhanced amounts of aerosols or particles in the atmosphere (presumably the result of volcanic ash), in narrow layers, starting on April 23 and continuing into early May. The April 23 observation, at 62.7°N, 197.1°E, detected volcanic aerosol as high as 17 km (55,750 ft). From the POAM III observations, NRL scientists have been able to infer that the plume was initially lofted to altitudes as high as 19 km (62,300 ft). The altitude and evolution of volcanic ash plumes are important because they are a well-known hazard to aviation.

Reports from the Alaska Volcano Observatory (AVO) show that the volcanic plume lofted to its highest reported altitude (initially reported to be "at least 45,000 feet" (13.7 km) within a few hours of the eruption, which occurred at 11:33 ADT (1933 UT). Imagery from the National Oceanic and Atmospheric Administration (NOAA) GOES weather satellite showed the plume spreading both north and south of the volcano. AVO reports, and wind patterns, indicate that at the lowest elevations the volcanic plume spread south, while the high-altitude portion of the plume spread north.

NRL scientists used tropospheric and stratospheric wind fields to evaluate the trajectory or path of the volcanic plume at various altitudes. These showed a very close coincidence between the location of the plume and POAM observations on April 23, 25, and 27. In addition, trajectories traced back in time from the POAM observations, at the level of the aerosol layer and below, revealed that the plume only entered POAM's measurement path above about 14 km altitude (45,900 feet)--the plume at lower altitudes spread mostly eastward in the days following the eruption.

The close coincidence between the plume trajectories and the POAM volcanic cloud observations allow these observations to be used to infer the settling rate of the volcanic material. By tracking the decline in altitude of the top, middle, and bottom of the aerosol layer, NRL scientists were able to estimate that the volcanic plume was settling at a rate of approximately 0.5 km (1,640 ft) per day. This measure suggests that the plume may have initially reached heights of approximately 19 km (62,311 ft). This information provides a more concrete estimate of the plume altitude than the AVO report.

POAM III, in operation for 13 months, is the successor to POAM II, which monitored polar stratospheric chemistry and cloud occurrence between 1993 and 1996. The POAM program at NRL has provided unique and valuable insights into the ozone hole phenomenon, possible Arctic ozone destruction, and a variety of polar atmospheric processes. The POAM III instrument, which is onboard the French SPOT 4 satellite, is supported jointly by the Office of Naval Research, the Department of Defense Space Test Program, NASA, and the French Space Agency (CNES).

For more information on Poam III, please refer to http://wvms.nrl.navy.mil/POAM/poam.html, or for information on AVO reports, refer to www.avo.alaska.edu.

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