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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.