A multiple wavelength LIDAR for tropospheric aerosol and cloud measurement

Abstract

Clouds and aerosols play an important role on the earth's radiation budget. One of the optical properties of clouds and aerosols that are needed for radiative transfer calculations is the volume extinction coefficient. This can be retrieved from the LIDAR return signal through an inversion method. In the Philippines, the first LIDAR system was developed in 1992 at the Manila Observatory, Ateneo de Manila University. The system was initially designed to obtain the extinction coefficient and depolarization ratio of tropospheric clouds and aerosols at 532 nm wavelength. The depolarization ratio measurement provides a knowledge of the shape of the scatterers and effect of multiple scattering on the LIDAR return signal. To obtain a further understanding on the optical properties of tropospheric clouds and aerosols, especially on the effect of particle radius on the extinction coefficient, a multiple wavelength LIDAR system was developed using most of the components of the existing depolarization LIDAR. The multiple wavelength LIDAR measurement will have the capability of obtaining the wavelength dependence of the extinction and/or backscatter coefficients of different atmospheric particles. The wavelength dependence of the extinction coefficient is related to the microphysical properties of the particle such as the size distribution and refractive index. It can be used in principle to discriminate between different aerosol types, evaluate the particle size, and determine the size distribution of the particle. In this paper, data obtained from the multiple-wavelength LIDAR system and initial results of the extinction coefficients of clouds and aerosols, and its wavelength dependence (assuming the Angström relation) will be presented. The Angström coefficient δ can be obtained from the wavelength dependence. In the manner similar to Grant, et al., the difference in the values of the Angström coefficient between low-lying aerosols, low-altitude clouds and high-altitude clouds, provide a way of discriminating between clouds and aerosol particles.