Super Cloudy Stratosphere Foretells Biggest Ozone Hole Ever

11/2/1998 - 61-98r
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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.

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