Polar statospheric clouds (PSC) and stratospheric aerosol can be observed by ground-based UV-visible spectrometry by looking at the variation of the color of the sky during twilight. A radiative transfer model shows that reddenings are caused by high altitude (22-28 km) thin layers of scatterers, while low altitude (12-20 km) thick ones result in blueings. The color index method applied on 4 years of observations at Dumont d'Urville (67 deg S), from 1988 to 1991, shows that probably because the station is located at the edge of the vortex, dense PSC are uncommon. More unexpected is the existence of a systematic seasonal variation of the color of the twilight sky - bluer at spring - which reveals the formation of a dense scattering layer at or just above the tropopause at the end of the winter. Large scattering layers are reported above the station in 1991, first in August around 12-14 km, later in September at 22-24 km. They are attributed to volcanic aerosol from Mt Hudson and Mt Pinatubo respectively, which erupted in 1991. Inspection of the data shows that the lowest entered rapidly into the polar vortex but not the highest which remained outside, demonstrating that the vortex was isolated at 22-26 km

Goutail, F.

Pommereau, J. P.

Sarkissian, A.

English

NASA Technical Reports Server

N95- 11042
PSC AND VOLCANIC AEROSOL ROUTINE OBSERVATIONS IN ANTARCTICA
BY UV-VISIBLE GROUND-BASED SPECTROMETRY
A. Sarkissian, J.P. Pommereau and F. Goutail
Service d'Arronomie du CNRS, Verri_res-le-Buisson, 91371, France.
ABSTRACT
PSC and stratospheric aerosol can be observed by
ground-based UV-visible spectrometry by looking at the
variation of the color of the sky during twilight. A radiative
transfer model shows that reddenings are caused by high
altitude (22-28 km) thin layers of scatterers, while low altitude
(12-20 km) thick ones result in blueings. The color index
method applied on 4 years of observations at Dumont
d'Urville (67 ° S), from 1988 to 1991, shows that probably
because the station is located at the edge of the vortex, dense
PSC are uncommon. More unexpected is the existence of a
systematic seasonal variation of the color of the twilight sky
-bluer at spring- which reveals the formation of a dense
scattering layer at or just above the tropopause at the end of the
winter. Large scattering layers are reported above the station in
1991, first in August around 12-14 km, later in September at
22-24 km. They are attributed to volcanic aerosol from Mt
Hudson and Mt Pinatubo respectively, which erupted in 1991.
Inspection of the data shows that the lowest entered rapidly
into the polar vortex but not the highest which remained
outside, demonstrating that the vortex was isolated at 22-
26 km.
1. INTRODUCTION
Polar Stratospheric Clouds (PSCs) and volcanic
aerosol are involved in polar ozone depletion because of the
heterogeneous chemical reactions which take place on the
surface of their particles and therefore deserve to be
monitored. A method was proposed for PSC routine detection
from the ground by UV-visible spectrometry of the zenith sky
at twilight. Its validity was demonstrated from a single winter
observations in northern Scandinavia, to be operative under all
weather (Sarkissian et al., 1990). The method involves
monitoring the variation of a sky color index between 90 ° and
93 ° solar zenith angle (SZA) at twilight, which is observed to
stay constant in clear stratospheric condition, to shift to the red
in presence of high altitude (22-28 kin) geometrically thin
PSC, and toward the blue in presence of low ahitude (12-
20 km) thick ones. Four years of data are now available at
Dumont d'Urville (66°S, 140°E) in Antarctica, which allow the
investigation of the seasonal evolution of appearance of PSCs
above the station and in addition, in 1991, to trace the arrival
of volcanic aerosols injected into the stratosphere by the
eruption of Mt Pinatubo in Philippine Islands in June and Mt
Hudson in Chile in August.
2. INSTRUMENT
Spectral observations are performed with a broad-band
(300-600 nm) and low resolution (0.6 nm), diode array UV-
visible spectrometer SAOZ (Pommereau and Goutail, 1988).
Designed for measuring total ozone and NO2 twice a day at
twilight, the instrument makes observations of the sunlight
scattered at zenith by steps of 300 seconds during daytime
until loss of sensitivity, that is about 94-95 °SZA, depending
of the cloud cover. Data analysis, conducted in real time,
allows to calculate by a differential method the molecular
absorptions by 03, NO2, 04 (collision induced absorption
bands of 02) and H20. The corresponding absorption features
are then removed, leaving at the end of the process a spectrum
whose characteristics (and therefore the color), depend on the
scattering properties of the atmosphere by molecules
(Rayleigh) and particles (Mie) only.
3. COLOR INDEX METHOD
In order to characterize the color of the sky at zenith, a
color index (CI) was defined as the ratio between fluxes
observed simultaneously at 550 and 350 nm, corrected for
molecular absorptions. Uncertainties in CI originate mostly
from the removal of absorption by ozone in the Chappuis
bands with a precision of the order of 1% (Goutail et al., same
proceedings). The CI, sensitive to scattering at all altitudes
including the troposphere, varies from one day to another,
from the blue during clear days to the red in cloudy
conditions, the largest CI (red) being observed in extreme
scattering conditions during fog or blizzard episodes. In
general, during twilight and whatever is its absolute value, the
CI is observed to stay almost constant between 90 ° and 93 °,
except sometimes in winter, when large variations can occur.
Because the troposphere is already in the darkness at twilight,
these large changes must originate in the stratosphere. They
are caused by PSCs. It is then possible to define a CI variatkm
index or ACI = CI(SZA) / CI(90°)), whose properties during
twilight will depend of the scattering properties of the
stratosphere only, providing the tropospheric cloud cover does
not vary too much during the one hour twilight period.
A simple scattering radiative transfer model was used
to simulate the color index variation (Sarkissian et al., 1990).
The atmosphere is divided into l km thick spherical shells in
which absorption ,and scattering are calculated. According to
the model, reddenings (ACI>I) resuh from high altitude (22-
28 km) and geometrically thin (1 5 kin) scatterers. The
amplitude at 92-03 ° SZA increases v,'ith the ahitude and optical
586
https://ntrs.nasa.gov/search.jsp?R=19950004629 2020-06-16T11:32:09+00:00Z
thicknessof the cloud and decreases with its geometrical
thickness. Blueings (ACI<I) are generated by low altitude
(12-20 km), geometrically thick (5-10 km) scatterers. The
blueing amplitude increases with the optical and geometrical
thicknesses of the layer.
4. FOUR YEARS OF OBSERVATIONS AT DUMONT
D'URVILLE IN ANTARCTICA
Data recorded from January 1988 until the end of 1991
at Dumont d'Urville are shown in figure 1: CI at 90 ° SZA
(upper panel), ACI between 90 and 92.5 ° SZA (middle panel)
and 50 hPa temperature measured by radio-sondes (lower
panel). CI and ACI reported twice a day at twilight are shown
together with 30 days running means. ACI data are missing in
summer during the polar day when the sun zenith angle does
not reach 92.5 ° at midnight and in August 1990 (missing
data). Temperature data are often missing in winter because of
balloon bursts at low temperature. The problem was partly
overcome after August 1990 by the use of an oil dipping
method for protecting the balloon.
Day to day and seasonal variations of CI and ACI
appear very similar from one year to another until August
1991, where the first volcanic aerosol arrived above the station
making the color indices to depart significantly from the
previous picture. The two periods will be discussed
separately.
Before volcanic eruptions
Daily fluctuations of CI at 90 ° SZA are generally of
about 20%. They result from weather changes. Large
reddenings occur from time to time during blizzard or snow
showers episodes, which result in large enhancements of
multiple scattering.
Daily fluctuations of ACI sensitive to stratospheric
scattering only as said above, are generally small (15%)
during the warm stratosphere season and before August 1991.
Spikes in ACI are not correlated with spikes in CI
demonstrating that the tropospheric contribution is efficiently
removed in the ACI. The relation between extreme AC1 and
5- , , - ,
i
PSCs is demonstrated by the plot shown in figure 2. Large
ACI departures occur at temperature below 193 K only, that is
the one of formation of PSCs. Year 1988 was particularly rich
in events compared to the following years. PSCs are present
generally from May until the end of September. This is
consistent with SAMS II observations above Antarctica
(McCormick et al., 1989). Because Dumont d'UrviJle is
located at the edge of the vortex, stratospheric temperature are
not very cold and dense PSCs giving rise to extreme ACI are
unfrequent compared to what was observed above northern
Scandinavia in January 1990 (Sarkissian et al., 1990).
Figure 3 shows the observed evolution of ACI during
twilight during the 3.5 year compared to model calculation
without volcanic aerosol in the stratosphere, but including
background aerosol. Except for few cases when large
reddenings and blueings are observed in presence of dense
PSCs, the model is consistent with the observations.
More unexpected, and which is responsible for most of
the dispersion in figures 2 and 3, are the systematic CI and
ACI seasonal variations by some 20%: the twilight sky is bluer
at spring than in autumn. To simulate this, the model requires
an enhancement of scattering (x = 0.1) between 8 and 12 km at
spring. Two possible interpretations are proposed:
a) the formation of cirrus clouds at or above the
tropopause because of the seasonal temperature minimum
between 8 and 12 km in August and September, delayed
compared to all other levels below and above (figure 4).
b) a large downward diabatic transport of the
background aerosols inside the vortex in winter and/or
sedimentation by PSC crystals condensed onto the particles,
as proposed by McCormick and Trepte, (1987) and
McCormick et al., (1989). However, this does not seems to
be enough. Relatively blue CI and ACI are still present at
spring after the vortex has moved away from the station, and
the model shows that even if all background stratospheric
aerosols were transported downward between 10 and 14 km,
the result will not exceed a blueing by 5 %. A larger optical
thickness is required.
ig ' ' " ' " " '" ' ' ' "
o_ i i i i ....... , . , , , , . i
-lo , • , , - , . . . .
i [ T (50 hPa) /_ . l[T (50 hPa) J/L _" T('50 hPa) ][ T (50 hPa) , m_,_ ]
_JttANDAV OOnONT $$ _JttANDAV DUM0_r 89 JI_IANOAY DUn0NT 90 JL_PANOA¥ OOMONT 91
Figure 1. Color index, CI, at 90 ° SZA (upper panel), color index variation, ACI, between 90 ° and 92.5 °
SZA (middle panel) and 50 hPa temperature observed at Dumont d'Urville from 1988 to
1991.
587
260 ..... , ................... '•"
Before eruptions
250
E
_. 240 _._L
m° 230 ;;
J _'"
220 • t_.
210 _ _"i.oo
190
0.0 1.0 2.0 3.0
ACI
Figure 2. Relation between ACI and 50 hPa temperature.
Large departures are observed to occur at
temperature below 193 K only. They are caused by
PSCs.
3oi 2.8 CI
_ 2.4
2.2
20 , , , * , , i ,
90 180 270
JULIAN DAY (1988-1990)
250 ' ' '
24O
, I
0 360
230
220
210
200
1-
190
180
0
_--o-- 18KMi , I , i I , i L i i
90 180 270 360
JULIAN DAY 1989
Figure 4. Three years average seasonal variation of the sky
color index at 90 ° SZA and monthly mean
temperature in the lower stratosphere above
Dumont d'Urville. The bluer twilight sky at Spring
than in Fall, could be partly caused by the
formation of cirrus at or above the tropopause.
2.0b, r .... I .... I ' ' ' • '
efore eruptions
_(o) Novolcanicaerosol.
1.5 . ......, :,. :-;:
.. : :-' ; , ,_..._,_.-'--'._a_:_-_
-:
1.0 _
• • " " % • " ° "_ T'_ *
' °0.5.
90.0 91.0 92.0 93.0
SZA (°)
Figure 3. Color index variation at twilight (ACI) observed
during 3.5 years, and calculated with a simple
scattering model
AfIer volcanic eruptions
The picture is slightly different in 1991 (figure 1),
where larger blueings occurred since around day 215 (early
August), followed later by unusual reddenings highly
correlated to the 50 hPa temperature variations, indicative of
the motion of the polar vortex (figure 5). Figure 6 similar to
figure 2, but for the end of 1991, shows that both blueings
and reddenings can occur at warm temperature. Therefore they
are not PSC. According to the model, blueings would result
from a dense absorbing layer in the lower stratosphere, and
reddenings at higher altitude, the last being observed outside
the vortex only.
Lidar measurements performed at Dumont d'Urville
during favourable weather periods (Godin et al., same
proceedings) show that two layers were present above the
station: one almost permanently around 10-14 km (optical
thickness 0.06) and the second more sporadic and variable
between 22 and 26 km (optical thickness 0.03). The first is
thought to result from the eruption of Mt Hudson in Chile in
August, and the second from the one of Mt Pinatubo in
Philippine in June.
Figure 7 shows the result of inclusion of these layers
into the model. The low altitude layer makes the twilight color
index to shift toward the blue while the high altitude layer
would shift the color index toward the red, in agreement with
the experimental data.
SAOZ daily observations give the details of the
evolution of the two layers• The low altitude Hudson cloud
arrived above Dumont d'Urville in early August, that is
rapidly after the eruption and penetrated into the vortex, which
is then not closed at altitudes around 10-14 km. The high
altitude Pinatubo layer was observed above the station at the
time of the first warming only (day 248-249, 4-5 September)
and did not penetrate into the polar vortex, which appears then
to be isolated at an altitude of 22-26 km.
588
5.CONCLUSION
Zenithsky UV-visible spectrometry at twilight, able to
derive the daily variation of stratospheric scatterers from a
color index, and lidar, able to measure their vertical
distributions but during clear days only, appear to be a
powerful combination for monitoring stratospheric aerosol and
PSC.
The four years of spectrometric data available now at
Dumont d'Urville shows that dense PSCs are rather
unfrequent above this station located at the edge of the vortex.
They demonstrate also the formation at the end of the winter
above Antarctica, of a scattering layer at 8-12 km in the lower
stratosphere, which causes the color of the twilight sky to be
bluer at spring than in Autumn.
They demonstrate also that volcanic aerosol which
arrived above the station in August-September 1991,
penetrated rapidly into the polar vortex at 10-14 km, but not at
22-26 km.
REFERENCES
McCormick M.P. and C.R. Trepte, Polar Stratospheric
Optical Depth Observed Between 1978 and 1985, J. Geophys.
Res.. 92. 4297-4306, 1987.
McCormick M.P., C.R. Trepte and M.C. Pitts, Persistence of
Polar Stratospheric Clouds in the Southern Polar Region, J.
Geophys. Res.. 94, 11241-11251, 1989.
Pommereau, J. P., and F. Goutail, 03 and NO2 ground-based
measurements by visible spectrometry during Arctic winter
and spring 1988, Geophys. Res. Lett., 15, 891-894, 1988.
Sarkissian A., J.P. Pommereau and F. Goutail, Identification
of Polar Stratospheric Clouds from the ground by visible
spectrometry, Geophys. Res. Lett., 18,779-782, 1990.
ACKNOWLEDGEMENTS
This work was supported by the Commission of the
European Communities (Grant STEP 0013M), the Programme
Atmosphere Moyenne of CNRS and TAAF (Terms Australes
et Antarctiques Franqaises). The SAOZ instrument is run at
Dumont d'Urville by the wintering staff of TAAF and at
Sodankyla by the Finish Meteorological Institute. They are all
gratefully acknowledged.
-2C
T (50 hPa)
_ -6o
_ r
_ -80
I:
JULIAN DAY gl
" • i • " i , • , • • , .... 30
Z_O
i5
IO
" _05
_O0 _30
Figure 5. Color index variation at twilight (ACI) and 50 hPa
temperature observed from June until November
1991. An average shift toward the blue is observed
after day 215 while large reddenings occurred later,
highly correlated with stratospheric warmings.
v
Q.
I--
260 r ....
250
240
230
220 -
210 -
,....200 .*:
I ";"_°..
190 ":
.... , ....
, .... i .... , .... i .... , ....
After eruptions
: ...... "
'--:_._. ." ..
t*.|.. ,.
• t. .
. ? .'..'...
• r- •
.... i .... i .... , ....
0.0 1.0 2.0 3•0
Z_CI
Figure 6. Relation between ACI and 50 hPa temperature after
August 1991• Large temperature independent
departures compared to the previous years average
are observed. Persistent blueings are caused by low
altitude Mt Hudson aerosol layer• Large reddenings
are caused by the high altitude Mt Pinatubo aerosol
layer•
2.01" .... ' .... ' .... _1
_After eruptions • . :"
1-_ (0) No volcanic aerosol
}_ (1) Hudson
.... (2) Pinatubo ,'t ":"°t
1.5 •
• .:. ..i
.."." ", ..'.'.'"o::,2:,"::_
0.5 . ..." .... .,: :
90.0 91,0 92.0 93.0
SZA (o)
Figure 7. Color index variation observed at twilight at the end
of 1991 and model calculations with aerosol
distribution observed by lidar.
589


PSC and volcanic aerosol routine observations in Antarctica by UV-visible ground-based spectrometry

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server