• Introduction
• Lidar and SAGE II Measurement
• Discussion
• Conclusions
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3

Camagüey Lidar Station (CLS) has been in operation from late 1988, with several periods of interruption, but stills enough to provide measurements before, during and after Mt Pinatubo eruption. The authors describe the features of the integrated backscatter coefficient (IBC) measured by lidar and derived from satellite for the periods 1984 to 1990 and 1995 to 2000. During both periods, before and after the Pinatubo eruption lidar and satellite IBC is in the order of 10^-4 sr^-1. Satellite derived IBC for the two years before Pinatubo has a mean value of 1.138x10^-4. For the last two years of record after that eruption the IBC mean value is 0.976x10^-4, showing a lower content of aerosols for the actual conditions. This result is in agreement with recent reports both from lidar and satellite measurements that a new background condition have been reached for the stratospheric aerosol layer. This is the lowest IBC value measured over Camagüey. Several individual profiles from both periods are analyzed. Some particularities are discussed. Lidar derived IBC are compared with the one derived from space coincident Sage II measurements for both periods.

1. Introduction

Lidar is valuable tool for stratospheric aerosols studies. The main focus have been on measurement of the aerosols produced by volcanic eruptions, as in the recent case of the last century bigest eruption, Mt. Pinatubo (Russell et al., 1993; Antuña , 1996a; Barnes et al., 1997). Less information have been obtained from stratospheric aerosols background conditions.

In this presentation we analyze the stratospheric aerosols background conditions before and after Mt. Pinatubo eruption, as it have been measured at Camagüey Lidar Station (CLS). We combine such lidar aerosols measurements with the observations from the Stratospheric Aerosols and Gas Experiment (SAGE II) satellite.

Because of several different reasons the set of measurements at CLS from 1988 to the present show discontinuities. However the lidar record provide enough measurements to cover both volcanic and non-volcanic conditions. CLS lidar aerosol backscattering mesurements of Mount Pinatubo eruption have been reported before (Antuña et al., 1995; Antuña, 1996b; Estevan et al., 1998). Up to now the measurements during stratospheric background conditions have not. Here we make such report by first time.

2. Lidar and SAGE II Measurement

The CLS consists of a monofrequential lidar. It was installed in 1998. The main technical characteristics of the lidar, are shown in Table 1.

Lidar measurements and its processing at CLS have been described in detail before (CLS Report, 1992). We use mainly the Integrated Backscattering Coefficient (IBC) from 15 to 32 Km as well as several individual backscattering profile for individual comparison with SAGE II measurements. Lidar data for the pre-Pinatubo period cover the months of November and December 1988 and October 1989. The post-Pinatubo period shows cover the years 1992 to 1997.

Satellite data consist of vertical profiles of extintion at 0.525 m m, retrieved from the limb scanner SAGE II instrument (Mauldin et al., 1985; Chu et al., 1989). A set of SAGE II space coincident extinction measurements was selected. All the SAGE II measurement within ± 5° in latitude and ± 25° in longitude with respect to CLS location (21° 24' N, 77° 51' E) were considered as ‘space-coincident measurements’. Because of such criteria, the SAGE II measurements used for this study, are representative of an area extending approximately in a radius of 2,000 km around Camagüey. This criteria is based on the particularirites of the SAGE II sampling parameters. A detailed explanation of the reasons for selecting such a criteria have been reported recently (Antuña et al., 2000).

Figure 1. Daily IBC mean values of the space coincident SAGE II and lidar mensurements.

SAGE II data used for the present study cover the periods November 1984 to May 1991 and November 1993 to April 2000. The extinction profiles at 0.525 m m wich follow the coincidence criteria mentioned above were screened for several features. First the profiles, wich had at least one missing extinction value in the 15 - 32 Km were discarded. Secondly, a search was done for profiles containing cirrus clouds in lower levels, according to criteria obtained from lidar measurements at CLS (Estevan et al., 1998). Profiles showing the presence of cirrus clouds were also discarded.

A backscattering-to-extinction conversion coefficient of 40 sr was selected for converting SAGE II 0.525 m m extinction values to backscattering. This value is considered representative of stratospheric aerosols background conditions.

3. Discussion

Figure 1 shows the daily IBC mean values of the space coincident SAGE II and lidar mensurements. It covers the period before and after Mt. Pinatubo. Also lidar measurements taken during the Mt. Pinatubo decay period are included.

The pre-Pinatubo lidar IBC have a mean IBC value of 1.52x10^-4 sr^-1 and the post-Pinatubo IBC mean value is 2.71x10^-4 sr^-1. The apparent increase in the IBC is explained by the presence, during the years 1996 and 1997, of several IBC values higher than the previous ones reached during 1995. Reports from lidars at Mauna Loa (19.5°N, 155.6°W) and at Garmisch-Partenkirchen (47.5°N, 11.0°E) show the presence of aerosols layers in the stratosphere from unknow origin (GVN, 1996;1997). Also increased aerosol optical thickness was measured during that period at two sites: Seguin, Texas, and San Diego, California Those high IBC values were also reported at (Mims et al., 1996).

In general the lidar IBC mean values for both periods are in the order of magnitude reported from Mauna Loa (Barnes and Hofmann, 1997) and from Heffei (Hu, 1998), both tropical stations.

The SAGE II measurements for the same pre and post-Pinatubo periods covered by lidar show a mean IBC value of 1.44x10^-4 sr^-1 before the Pinatubo and 1.26x10^-4 sr^-1 after. There is a good agreement between the mean IBC values from lidar and SAGE II for the pre-Pinatubo period. For the second period the agreement is not so good. Comparatively, only few of the SAGE II measurements show the relative high IBC values, measured by lidar. A possible explanation is related to the SAGE II sampling features. In the tropical region the grid associated with the space sampling is around 24° in longitude by around 5° in latitude. And the time gap between consecutives SAGE II sampling of the same region of the earth is around six weeks. It means that aerosol clouds within such space dimensions and within that time frame could or could not be measured by chance. Ground based lidars sample always the same point of the earth with a commom time frame of around one week under background conditions. That is one of the features which makes lidar an satellites valuable complementary instruments.

In the Figure 2 the monthly mean IBC values from SAGE II are shown. Both for El Chichon and Pinatubo the period selected cover from around two and half years after the eruption to aproximately

Figure 2. Monthly mean IBC values from SAGE II. Pre-Pinatubo (1984 - 1991) and post-Pinatubo (1993 - 2000).

seven years later. One clear feature is the presence of a seasonal cycle in the IBC, as it had been reported from global analysis of SAGE II data under background conditions (Thomason et al., 1997). Also exponential adjustment for both periods are shown. The decay rate is similar for the post-El Chichón and post-Pinatubo. The e-folding decay period was around 24 months. Additionaly in the case of the 1984 - 1991 period we can see the Nevado del Ruiz (September 11, 1985) eruption clearly superimposed over the decay trend after the El Chichón eruption.

The SAGE II IBC average for the two years before the Pinatubo has a value of 1.138x10^-4 sr^-1. In comparison the IBC average for the last two years in the record (May 1998 to April 2000) was 0.976x10^-4 sr^-1. The lower IBC value presently is also visible from figures 1 and 2. This result agree with reports that a lower background condition have been reached ultimately (Jager et al., 1998).

Figure 3. Coincident Lidar and SAGE II backscattering profiles.

The Figure 3 shows, on top, coincident lidar and SAGE II backscattering profiles for December 1^st 1988. It consists of one lidar profile and two consecutive SAGE II profiles measured at distances around 500 Km and 2000 Km. Despite the big differences in distance between the SAGE II profiles and the lidar site there are not so big differences. This feature could be attributed to the homogeneity of the stratospheric aerosols under background conditions. On the bottom coincident measurements for September 15^th 1995 are shown, in this case both SAGE II profiles are at around the same distance. No big differences are found.

4. Conclusions

The new background conditions in the stratospheric aerosol layer have been confirmed in the tropical area combining lidar and SAGE II measurements. The IBC mean value during the last two years of record shows the lowest record ever in that area for similar periods of time. Our results agree with former reports from lidar and SAGE II studies.

The decaying trend of the high aerosol content after El Chichón and Pinatubo studied for the period covering two and half and seven years after the eruptions show the same exponential trend. The e-folding decay for that period is around 24 months.

Individual extintion profiles comparisons showed little differece between measurements located near and far of the lidar. The homogeneity of the stratospheric aerosols under background conditions could be consider the reason for such feature.

5. References

Antuña J.C. and Sorochinski M. (1995). Mediciones de aerosoles estratosféricos en Camgüey, Cuba, durante 1992. Geofísica Internacional 34 (1), pp 143-145.

Antuña J.C., R. Estevan y I. Pomares. (1995) Lidar observations of stratospheric aerosols over Camagüey: 1992-1993. INSMET, Data Report CLS-01, 63pp.

Antuña J.C. (1996a). Lidar measurements of stratospheric aerosols from Mount Pinatubo at Camagüey, Cuba. Atmos. Env., 30 10/11, pp 1857-1860.

Antuña J.C. (1996b). Mount Pinatubo stratospheric aerosols decay during 1992 and 1993 as seen by Camagüey Lidar Station, Cuba. In NATO ASI series, Vol 42 "The effects of the Mt. Pinatubo eruption on atmosphere and climate". pp 3-9.

Antuña J.C., A. Robock, G. L. Stenchikov, L. Thomason, and J. Barnes (2000). Lidar Validation of SAGE II Aerosol Measurements After the 1991 Mount Pinatubo Eruption. Submitted to JGR.

Barnes J.E. and Hofmann D.J. (1997). Lidar measurements of stratospheric aerosol over Mauna Loa Observatory. Geophys. Res. Lett., 24, pp 1923-1926.

Chu W.P. and Mc Cormick M.P. (1989). SAGE II Inversion Algorithm. J. Geophys. Res., 94, 8339-8351.

Estevan R., Antuña J.C, Aroche R. Pomares I. and Cervantes S. (1998). Aerosols, cirrus clouds and temperature measurements with lidar in Camgüey, Cuba. 19^th ILRC, NASA/CP-1998-207671/PT1, 173-176.

GVN (1996) Bulletin of the Global Volcanism Network. Vol.22, 2.

GVN (1997) Bulletin of the Global Volcanism Network. Vol.23, 3.

Hu, H. (1998) Lidar measurements of stratospheric aerosol at Hefei, China during 1991-1997. 19^th ILRC, NASA/CP-1998-207671/PT1, 37-39.

Jager H. and Homburg F. (1998). A new aerosol background level in the stratosphere? Lidar observations of the period 1976 to 1997. 19^th ILRC, NASA/CP-1998-207671/PT1, pp 335-338.

Mauldin L.E., Azun N.H., McCormicl M.P., Guy J.H. and Vaughn (1985). Stratospheric Aerosol and Gas Experiment II instrument: a functional description. Optical Engineering, 24, pp 307-312.

Mims, F.M., III, Meinel, C., Roosen, R.G., Russell, R.T., Hawkins, G.P., and Easton, H., 1996, Stratospheric aerosol cloud of unknown origin: unpublished manuscript.

Thomason L.W., Kent G.S., Trepte C.R. and Poole L.R. (1997). A comparison of the stratospheric aerosol background periods of 1979 and 1989-1991. J. Geophys. Res., 102, pp 3611-3616.