Scientists at the Naval Research
Laboratory (NRL) report that measurements from their space-based
Polar Ozone and Aerosol Measurement instrument, (POAM III) indicate
that polar stratospheric clouds (PSCs) over Antarctica this winter
were unusually extensive compared to earlier years. The researchers
believe that this increase in the extent of PSCs over the winter
season set the stage for the near record size (in terms of area
coverage) of the Antarctic ozone hole this year reported by NASA
and NOAA. POAM scientists will present a mission overview and
results from the 1998 Antarctic ozone hole at the Fall 1998 American
Geophysical Union meeting in San Francisco, California, in
December.
PSCs form in the cold wintertime
polar stratosphere, high above the region in which more familiar
tropospheric clouds form. They are known to be a crucial ingredient
in the large chemical destruction of ozone which occurs in the
Antarctic ozone hole. These clouds typically begin to form over
Antarctica in the southern hemisphere late fall, and are observed
until the end of winter. In 1998 POAM III observed its first
PSC on May 22.
The POAM III measurements indicate
that the extent of stratospheric cloudiness was remarkable in
1998, say the scientists. POAM's orbit takes it over Antarctica
14 times each day, and from August 5 through September 6, every
POAM III measurement indicated a PSC. This is by far the longest
continuous stretch of 100% cloudiness ever observed by the POAM
instruments. During this period, the latitude of the POAM measurements
varied from 72°S on August 5 to 83°S on September 6.
Since this latitude range spans a large part of the Antarctic
continent, it could be inferred from these observations that,
during this entire period, the stratosphere over Antarctica was
essentially covered by PSCs. These very thin clouds are not readily
visible from the ground.
The POAM III measurements also
show a pronounced decrease in stratospheric water vapor coincident
with the development of the PSCs. This decrease in water vapor
is presumably the result of the drying out (or dehydration) of
the lower polar stratosphere by the settling out of ice
particles in PSCs. This phenomenon is thought to be important
in the formation of the Antarctic ozone hole.
The unusually large extent of
PSCs this year observed by POAM III is consistent with the recent
report by NOAA that temperatures at 20 km over Antarctica this
winter were among the lowest observed in the past twenty years.
Lower temperatures should lead to more PSCs. The increased extent
of PSCs has, in turn, presumably led to the record size of the
ozone hole reported by both NASA and NOAA from their Total Ozone
Mapping Spectrometer (TOMS) and Solar Back Scattered Ultraviolet
Instrument (SBUV) measurements. Also, consistent with NOAA
balloon-borne
ozone measurements obtained at the South Pole, the POAM III ozone
profile observations show ozone near the South Pole decreasing
at a very rapid rate during September (especially the latter
half of the month). The rate of ozone destruction at 20 km (about
65,000 feet) is similar to that observed in 1996, but larger
than that observed in 1994 and 1995.
Supported by the Office of Naval
Research and the DoD Space Test Program, POAM III is currently
the only operational satellite instrument providing continuous
coverage of the vertical distribution of ozone with good resolution
(1 km), and of PSCs in the polar stratosphere. As such, it complements
the measurements of the total ozone column abundance obtained
by the NASA/NOAA TOMS and SBUV instruments.
POAM III was launched on March
20,1998, and has been operational since late April. It is a follow-on
to the highly successful POAM II experiment, which provided unique,
simultaneous data on ozone depletion and PSCs from October 1993
through November 1996. POAM III measurements will provide valuable
information on the way the earth's ozone layer is responding
to the expected decrease in abundances of chlorine in the atmosphere,
as a result of restrictions in CFC emissions mandated in the
international Montreal Protocol, and to possible global temperature
changes.
Get NRL News:
About the U.S. Naval Research Laboratory
The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 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 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.