Applications of light scattering and refraction by atmospheric gases.

Abstract

LIDAR, an acronym for LIght Detection And Ranging, is a system used for studying

the scattering of laser light incident on a parcel of air. This thesis investigates the

atmosphere above the Durban region using two atmospheric LIDARs, referred to, in

this study, as the "old LIDAR" and the "new LIDAR".

The old LIDAR was used in a campaign of observation from July to October 1997 in

a study of aerosol concentrations over Durban. This thesis will focus on, among other

things, the local aerosol profiles for low altitude (0 to 10 km) and high altitude (10 to

35 km). In particular, the focus will shift on any long persistence in this region (it was

found that the aerosol layer observed by M. Kuppen (1996) on June 1994 at 25 km

may have moved to the higher altitude of 28 km in October 1997. This may be

explained by stratospheric upwelling, carrying the layer to higher altitude. These

aerosols are known to influence the local climate). This investigation will give some

useful insight into the local atmospheric dynamics.

The new LIDAR system (Rayleigh-Mie LIDAR) has been used to measure

atmospheric temperatures from 20 to 60 km as well as aerosol extinction coefficients

from 15 to 40 km. Height profiles of temperature have been measured by assuming

that the LIDAR returns are solely due to Rayleigh scattering by molecular species and

that the atmosphere obeys the perfect gas law and is in hydrostatic equilibrium

(Hauchecorne and Chanin 1980).

Since its installation in April 1999, the new LIDAR has been used to monitor

stratospheric temperatures and aerosol concentrations from 10 to 40 km. In this study,

we discuss in chapter 7 the results of a validation campaign conducted during the

period of April 1999 to December 2000. Average monthly LIDAR temperatures are

computed from April 1999 to December 1999 and compared with radiosonde

temperatures obtained from the South African Weather Service (SAWS) at Durban.

The monthly LIDAR temperature profiles over two years (1999 and 2000) were also

computed and compared with the climatological model Cospar International

Reference Atmosphere (CIRA)-1986 and with the average monthly European Centre

for Medium Range Weather Forecast (ECMWF) temperatures . The results show that

there is good agreement between LIDAR and SAWS radiosonde temperatures in the

20 and 30 km altitude range. Between 20 and 40 km, the monthly LIDAR

temperatures agree closely with the CIRA-86 and ECMWF profiles. However, during

winter, in the altitude range 40 to 60 km, LIDAR temperatures are warmer than

CIRA-1986 and ECMWF temperatures, and they show large variability. These

variations could be due to relatively fast transient phenomena like gravity waves or

planetary waves propagating vertically in the stratosphere. As part of the validation

process, the aerosol extinction coefficients retrieved from the LIDAR data have also

been compared with the extinction coefficients measured by Stratospheric Aerosol

and Gas Experiment (SAGE) II close to the LIDAR location and on coincident days.

Appendix E of this thesis also investigates the concept of refraction by atmospheric

gases as applied to gas lenses. A simple spinning pipe gas lens (SPGL) has been used

as the objective lens of a camera to take pictures of the moon and sun spots. The

SPGL is a varifocal length lens which depends on the temperature of the pipe and the

angular velocity at which it spins. For our purpose a focal length of 8 m has been

used. The moon pictures are compared with a lunar map so as to identify the maria.