GUIDO DI DONFRANCESCO, Ente per le Nuove Tecnologie, L'Energia E L'Ambiente Casaccia, Dipartimento Ambiente 00060, Rome, Italy
ALBERTO ADRIANI and FRANCESCO CAIRO, Istituto Fisica dell'Atmosfera, Consiglio Nazionale delle Ricerche, Rome, Italy
Since 1993, an optical radar or lidar (light detection and ranging) has been operating at McMurdo Station (78°S 167°E) during winter and spring. The instrument, described in more detail by Adriani et al. (1992), uses a pulsed laser source with 150 millijoules per pulse at 532 nanometers. The system can monitor the presence of volcanic aerosol and clouds in the atmosphere above the station by measuring the light backscattered from the atmosphere. In fact, after system calibration, the received signal is compared with the one expected from an atmosphere not containing particles. In this way, a parameter called backscattering ratio R is calculated. When particles are not present, R gives the value of 1, and any value larger than 1 is related to the presence of particles [usually between 5 and 25 kilometers (km) altitude]. Detectable particles in the antarctic stratosphere are due to large volcanic eruptions (e.g., Mount Pinatubo, June 1991) and to polar stratospheric clouds (PSCs). The latter form during the polar winter when the temperature drops below 195-196 K, and playing an important role in the heterogeneous chemistry of the polar stratosphere, they are strictly linked with formation of the "ozone hole." The kind of particles found in a PSC depends on the local temperature at the time of the observation as well as on the previous thermal history of the air mass in which the cloud has formed (Gobbi and Adriani 1993; Adriani et al. 1995).
The emitted laser light is polarized, and the lidar is able to detect the depolarization of the backscattered light induced by PSCs (the depolarization value D can be defined as the ratio between the depolarized and polarized lidar backscattering). In fact, spherical particles, as liquid droplets, do not depolarize the backscattered light, whereas aspherical particles do induce depolarization. So it is possible to discern among different kinds of particles (crystalline, amorphous, liquid) from the state of polarization of the backscattered light.
Figure 1 shows R (backscattering ratio) and D (depolarization) versus altitude and time. The first 2 months (Julian days between 90 and 150) show the background volcanic aerosol. PSC formation (steeply increasing R values) is observed in the period from June to September. Some of these clouds show a presence of ice particles (high values of depolarization), depending not only on the temperature at the time of the observation but also on the temperatures previously experienced by the observed air masses. Such ice clouds are observed in particular between mid-July and mid-August when the stratospheric temperature reaches the lowest values. On those occasions, R values up to 20 and D values up to 50 percent are measured above 20 km.
Above 25 km, the received signal is proportional to the molecular density of the air, and it is possible to retrieve by lidar the temperature profiles as described by Hauchecorne and Chanin (1980). Using our system, which averages the profiles (performed every 3-4 days) along the daily period of measurement (0.5 hour) and filters vertically to approximately 6 km of altitude resolution, we obtain temperature profiles between 25 and 65 km of altitude with an average temperature statistical error of 0.2 K at 25 km, 1.5 K at 40 km, and 10 K at 55 km.
Currently, satellite and lidar remote sensing are considered the most suitable methods for observing middle atmosphere temperatures. Satellites offer better coverage but lower accuracy and vertical resolution with respect to remote sensing, which is, however, localized and can perform only nocturnal observations. In wintertime, however, temperature retrievals by satellites may be unreliable under highly disturbed conditions such as stratospheric warmings (Di Donfrancesco et al. 1996).
Figure 2 shows temperature behavior versus altitude and time (day of the year) as measured by the lidar above McMurdo Station between 1 April and 1 October 1996. The stratosphere was very active during June and August, and strong perturbations (stratospheric warmings) appeared in the region between 40 km and 60 km altitude.
This work has been supported by Programma Nazionale Ricerche in Antartide. We would like to thank National Science Foundation and Antarctic Support Associates from the United States of America for giving us the opportunity to operate the lidar at McMurdo Station in wintertime. Thanks are due to F. Cardillo for his software assistance.
Adriani, A., T. Deshler, G. Di Donfrancesco, and G.P. Gobbi. 1995. Polar stratospheric clouds and volcanic aerosol during the 1992 spring McMurdo, Antarctica: Lidar and particle counter comparative measurements. Journal of Geophysical Research . 100(12), 25877-25897.
Adriani, A., T. Deshler, G.P. Gobbi, B.J. Johnson, and G. Di Donfrancesco. 1992. Polar stratospheric clouds over McMurdo, Antarctica, during the 1991 spring: Lidar and particle counter measurements. Geophysical Research Letters 19(17), 1755-1758.
Di Donfrancesco, G., A. Adriani, G.P. Gobbi, and F. Congeduti. 1996. Lidar observations of stratospheric temperature above McMurdo Station, Antarctica. Journal of Atmospheric and Terrestrial Physics , 58(13), 1391-1399.
Gobbi, G.P., and A. Adriani. 1993. Mechanisms of formation of stratospheric clouds observed during the antarctic late winter of 1992. Geophysical Research Letters , 20(14), 1427-1430.
Hauchecorne, A., and M.L. Chanin. 1980. Density and temperature profiles obtained by lidar between 35 and 70 km. Geophysical Research Letters , 7, 565-568.