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).
The U.S. Naval Research Laboratory is the Navy's full-spectrum corporate laboratory, conducting a broadly based multidisciplinary program of scientific research and advanced technological development. The Laboratory, with a total complement of nearly 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 85 years and continues to meet the complex technological challenges of today's world. For more information, visit the NRL homepage or join the conversation on Twitter, Facebook, and YouTube.
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