PSC Statistics and Climatological Analysis over 9 Years Measurements at McMurdo, Antarctica.

P. Massoli1, A. Adriani2, F.Cairo1, G. Di Donfrancesco3,M.Moriconi1, M.Snels1,C.Scarchilli1

(1) CNR- Institute for Atmospheric Science and Climate (ISAC), CNR, Rome, Italy(2) CNR- Institute for Physics of the Interplanetary Space (IFSI), CNR, Rome, Italy

(3) E.N.E.A. GEM-CLIM,C.R. CASACCIA, Rome , Italy

contact: p.massoli@isac.cnr.it

THE LIDAR AND ITS OPERATION SINCE 1993

All year around measurements of polar stratospheric clouds (PSC) have been taken since 1993 at McMurdo station, Antarctica (78S,167E) by a backscatter lidar system (Nd-Yag laser emitting a polarized light at 532 nm) installed in 1991 (Adriani A. et al, 1992). McMurdo is a primary site of NDSC (Network for Detection of Stratospheric Change) and it is often characterized by local temperature perturbations due to its particular position downwind to the Trans-Antarctic range. For that reason, a numerical code application has been performed in order to discriminate between synoptic PSC (generated by extended field of low temperature) and mesoscale PSC (locally induced by leewaves). Different PSC types have been also individuated for each group, and the seasonal evolution as been performed as well. In addition, the analysis of stratospheric physical parameters could help in linking the recognized cloud types to different climatic situations with the aim to establish either the conditions in which PSC types occur or to better assess the rule of mesoscale phenomena in their formation.

PSC measurements taken from May 15th until September 30th (the period during which they usually appear above McMurdo) show that about the 80% or even more of the profiles obtained contain a PSC

PSC start to appear in late May and can be only occasionally detected in the first days of October. Maximum occurrence is observed in mid winter, with peaks in July of 70% (25-30 km). Note the decreasing of the maximum occurrence altitude because of the lowering of TMIN and the

downward transport of H2OV and

HNO3V due to PSCs formation and

sedimentation.

Average PSC occurrence for the Antarctic winters 1993-2001.

(grid of 1 km pressure altitude and 0.5 month time scale)

STATISTICAL OBSERVATION OF PSCs

Zp = 6.5 ln (980/P) Pressure altitude used to account for the downward motion of air in the polar vortex

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The primary information from the Lidar are the Backscattering and Depolarization Ratios of the particles forming a cloud (errors estimations are 5% and 10-15% respectively). The separation of mesoscale and synoptic PSC (almost always present in the lidar profile) has been obtained applying an algorithm filter with a grid of 1.7 km (the typical thickness of stratospheric clouds forming after temperature perturbations). For each profile, a background of synoptic PSC has been obtained , while the peaks between the raw signal and this background can be interpreted as mesoscale PSC superimposed to the synoptic.

Mesoscale temperature perturbation (18-23km) with respect to the synoptic field generating stratified PSC layers in the lidar profile (see above)

ICE

Ia ENHANCED

NAT

1) NAT and mixed phases are prevalent in synoptic conditions (mostly with RT<1.3 and D<20%). 2) Liquid clouds (RT>1.5 and

D<1.44%), observed all winter long in 1993 and 1994 (volcanic aerosol from Mt. Pinatubo still in the polar vortex), only until June from 1995 onwards.The absence of others PSC types support the idea that synoptic conditions of extended low temperature fields mainly generate solid PSC (NAT or others), which probably in the Antarctic stratosphere represent a kind of background for the whole winter.

1) Liquid clouds (RT>1.5, D<1.44%)2) Solid particles mainly Ia enhanced with RT >1.6 and D>20%, and other solid

particles at lower D and RT (note the

very few PSC in the area between liquid and solid, at D=10%).3) PSC II (with RT >8 and D> 20%)

All these PSC grow in thin layers and could be generated by leewaves events associated by high cooling rates.

SYNOPTIC PSC 1993-2001

MESOSCALE PSC 1993-2001

1-1/RT used to constrain RT values in the range (0,1) for a better comparison with Aer. Depolarization. The scale is the frequency of the number of points falling in each pixel with respect to the total number (636 for synoptic, 167 for mesoscale)

SYNOPTIC

PSC with very low RT and D up to 20% start to appear in mid May and grow quickly, forming the synoptic background on which during the season others PSC with higher values (D=60 % in July-August) appear. Liquid clouds are very frequent in 1993-1994 for all the period, while by 1995 they only appear from 15 May until the end of June.

MESOSCALE

Liquid and solid high depolarizing particles from 1995-2001start to appear in June, reaching a maximum after July 15th. PSC II concentrate in July-August, but can be also observed in September 1-15th (in agreement with nacreous clouds observations). Data from 1993 and 1994 show here a more random distribution than 1995-2001, with higher R and lower D: this leads to a strong lack of points observed yet in the region with D=10% (NAT region), supporting the idea of total separation between synoptic and mesoscale PSC in that area

STAGIONAL TREND OF SYNOPTIC AND MESOSCALE PSC

The seasonal evolution and “growth” of synoptic and mesoscale PSC is

shown. 1993 and 1994 datasets (red points) have been separately treated

to enhance the effects of volcanic aerosol (more liquid PSC in the

synoptic type during 1993 and 1994)

PSC TYPES OCCURRENCE

PSC types occurrence between May 15th and September 30th 1993-2001 is shown. They have beenobtained from the synoptic and the mesoscale classes and displayed with respect to the time of the year andaltitude. Synoptic PSC are concentrated between 12-22 km, while mesoscale appear higher (20-28 km): the

main synoptic types are represented by NAT and NAT rock between 20-30 km, and mixed phases up to 18 km.In mesoscale PSC, NAT and mixed types appear as well, while PSC II and Ia enhanced can only be seen in thatcase (July and August). Liquid clouds are also quite abundant, but in May-June. In addition, each year has beenprocessed separately in the same way (not shown) to retrieve the annual contribution to the whole dataset for all the PSC types and to better understand how different climatic situations influence them

PSC SELECTION CRITERIA

TYPE DEPOLARIZATION SCATTERINGLIQUID <1.25 % >1.25MIXED 1.65-10% whole rangeNAT >10% < 2ICE > 20% >10 Ia ENHANCED >10% >2.0-5.0NAT ROCK >30% < 1.25

PSC and mean temperature (80°S)

A correlation with the main stratospheric climatic diagnostics used to track the behaviour of the stratosphere is here shown, in order to associate our observations to the climatic situations. Data are from the Climatic Prediction Centre (CPC) of the National Center for Environmental Predictions (NCEP) of NOAA, and elaborated by the Atmospheric Chemistry and Dynamics Branch of Goddard Space Flight Center (GSFS) of NASA.

LEFT: Mean temperature sampled at 80°S and PSC 1993-2001 averaged on May 15th-September 30th is shown. The temperature shows lower values at higher altitudes, with the minimum shifted from July to August when going from 23 to 15 km (vortex subsidence). PSC occurrences in each period are similar in the vertical profile, but less extended in time at 23 km (no PSC in June and September). The maximum of PSC presence and the minimum of temperature are in phase at 23 and 19 km (15-31 July), but shifted at lower altitude (maximum for PSC on July and on August for temperature). Synoptic and mesoscale PSC distributions have values of 70-80 % and 20-30 % until 17 km, then mesoscale PSC increase abruptly up to 50% at 23 km, when the lowest temperatures are registered. This is possibly related to leewaves effects, very common in June and September when the observation of mesoscale clouds is higher than in mid winter.

RIGHT: The amplitude of zonal wavenumber one and two Fourier components of the geopotential height (hereafter W1 and W2) is an indicator of disturbed atmosphere related to the planetary wave propagation, and consequently to the stratospheric warmings which influence the strength and the duration of the polar vortex. A correlation between synoptic and mesoscale PSC occurrence and wave amplitude for the same levels is shown. Wave amplitudes are higher at 15 km (planetary waves propagate from the troposphere), with W1 always higher than W2: in addition, W1 shows a regular behaviour (not W2). Synoptic PSC are in phase with W1 up to 17 km, while at higher altitudes this correlation is good only after mid July. W1 and W2 amplitudes are constant at 23 and 19 km, then there’s a increase especially at 15 km in September when stratospheric warmings more frequently occur. A little correlation with mesoscale PSC (mostly influenced by gravity waves) only appears at 19 and 23 km until mid July. A more accurate annual analysis (and correlation with PV or zonal wind) and the study of particular cases is in progress .

REFERENCES

A.Adriani, T. Deshler, G.P.Gobbi, B.J.Johnson, G. Di Donfrancesco, Polar stratospheric clouds over McMurdo, Antarctica, during the 1991 spring: Lidar and particle counter measurements, Geophys.Res.Lett.,19,1755-1758,1992

A.Adachi, T.Shibata, Y.Iwasaka and M.Fujiwara, Calibration method for the lidar observed stratospheric depolarization ratio in presence of liquid aerosol particles, Applied Optics, 36, 6487-6595,2001

Acknowledgements

We gratefully thank the Italian National Program for Antarctic Research (PNRA) and the U.S National Science Foundation (NSF) for supporting this project research. Thanks are due to Raytheon Polar Service Company for providing excellent logistic and scientific support at McMurdo Station, and to the NCEP for allowing fast access to NASA data

Years with ozone amount under 100 DU (1994, 1998, 1999) are generally characterized by a large NAT presence: only in 1997 the highest NAT and NAT rock amount does not correspond to low ozone. Others factors can infact influence the ozone depletion as well: 1994 and 2000 were characterized by a large PSC area and extended low temperature field (T<185 K), while in 1999 the vortex was very strong, with one only warming at the end of September. The highest ozone was registered in 1996, corresponding to the lowest NAT and ICE amounts, and no PSC were observed after late August. After 1999 a regular ozone increasing can be observed, and this corresponds to a lowering of NAT and NAT rock amounts. No ozone data were unfortunatelly available in 1995, when ICE clouds were the main PSC type observed.

PSC occurrence and Ozone (TOMS, 60-90° S)

Ozone minima sampled the Total Ozone Mass

Spectrometer (mid September values) are here correlated with the total amounts of PSC

and PSC types

PSC and wave amplitude of geopotential height

LIDAR DETECTION LIMIT + CLASSES OVERLAP

RT MAX = 20

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