S. R. Pal

Polarization properties of lidar backscattering from clouds.

The polarization properties of the backscattering of a lidar pulse from atmospheric clouds have been measured. A linearly polarized signal from a ruby laser at 694.3 nm is transmitted, and the scattering is simultaneously measured on a three-channel receiver that has polarizers oriented parallel, perpendicular, and at 45 degrees to the transmitted polarization. Substantial depolarizations (up to 0.5) are observed, and from the spatial variation of this depolarization the contribution of multiple scattering can be deduced.A wide variation in polarization properties is observed in different cloud types, and the results indicate that polarization signatures could be useful for cloud characterization and classification.

Automated method for lidar determination of cloud-base height and vertical extent

An algorithm for the automated analysis of lidar cloud returns has been developed as part of the Experimental Cloud Lidar Pilot Study program. This automated method determines the cloud-base and cloud-top heights as well as the altitude of the maximum return signal. A large body of cloud data that were obtained at 532 and 1064 nm with a Nd:YAG lidar have been evaluated and it has been found that the algorithm can handle the wide range of complex cloud situations encountered. The need for a more careful definition of the cloud-base height and cloud-top height is described and discussed in relation to the existing measurements with rotating beam and laser ceilometers.

Measurements of gravity wave activity within and around the Arctic stratospheric vortex

Lidar measurements of gravity wave activity have been conducted at Eureka in the High Arctic since 1993. The waves are detected by the fluctuations they induce in temperature. It has been found that the amount of wave energy in the upper stratosphere is related to the position of the stratospheric polar vortex. In each of the four winters reported here, the wave activity was a maximum within the westerly jet at the edge of the vortex, a minimum inside the vortex near its centre and intermediate outside the vortex. The spectra of wave induced fluctuations show that it is at the longest resolved vertical wavelengths (8 to 15 km) that wave energy is being influenced by the background meteorological conditions. These findings are interpreted in terms of the Doppler shifting and critical level filtering that is imposed by the background wind profile.

Polarization anisotropy in lidar multiple scattering from atmospheric clouds.

Spatial variations in the polarization properties of multiple scattering have been observed in the lidar backscattering from atmospheric water droplet clouds. To detect these effects, the lidar receivers have been modified to incorporate spatial filters in the focal plane which block singly scattered radiation and transmit muliply backscattered radiation through sectors oriented at five azimuthal angles between 0 and 90° to the direction of the transmitted linear polarization. The parallel and perpendicular polarized components of the lidar multiple scattering have been measured as a function of pulse penetration depth for different cloud formations. The anisotropic distributions observed are found to resemble those previously recorded in our laboratory measurements on clouds of spherical scatterers. In this paper also, results of Mie scattering calculations are summarized which show that the observed polarization anisotropy originates directly from the polarization properties of the single scattering from spherical particles.

Multiple scattering in atmospheric clouds: lidar observations

The contribution of multiple scattering in lidar backscattering from clouds has been investigated using a ruby laser at 694 nm. The depolarization of an incident linearly polarized signal is measured with a multichannel lidar receiver. An analysis is presented whereby this information can be utilized to measure the multiple scattering in clouds in which single scattering retains the incident linear polarization. Experimental data are presented for cumulus clouds and for ground level fog, and the results are compared with some recent theoretical computations.

Ozone, column ClO, and PSC measurements made at the NDSC Eureka Observatory (80°N, 86°W) during the spring of 1997

During winter/spring 96/97 ozone levels over the Eureka NDSC observatory (80°N,86°W) were measured using a lidar, sondes, and a Brewer spectrophotometer. Column ClO measurements were also made using an FTIR system. Measurements show that lower stratospheric ozone mixing ratios decreased rapidly between mid-February and late-March though the ozone mixing ratio losses appear to have been less than for the 95/96 season. Elevated column amounts of ClO were found to be present over Eureka until late March.

Lidar observations of stratospheric ozone and aerosol above the Canadian high arctic during the 1994-95 winter

This letter reports on lidar observations of arctic stratospheric ozone and aerosol made from late December 1994 to mid-March 1995. These observations were conducted at Eureka (80°N,86.42°W) in the Canadian arctic. Based on NMC potential vorticity data and aerosol observations, the lower stratosphere over Eureka was seen to be clearly within the Polar Vortex for most of the observation period. The intravortex observations showed that in the stratosphere below the 500 K potential temperature level average ozone mixing ratios decreased on the order of 15% from early January to mid-February with an additional 15% decrease observed from mid February to mid-March. These trends are consistent with the Upper Atmospheric Research Satellite (UARS) Microwave Limb Sounder (MLS) measurements of ozone made during the same time periods.

Laser backscattering from dense water-droplet clouds

Laser backscattering from water-droplet clouds of known size distribution has been studied. A polarization analysis of the backscattered power as a function of attenuation coefficient, a has been made for receiver fields of view between 2.5 and 20 mrad. a values between 0 and 5 m−1 have been used. A linearly polarized argon laser was operated at 514 nm and a considerable amount of depolarization arising from multiple scattering was observed. The polarization information shows that there is no change in either the direction of the polarization or in the ellipticity of the backscattered light. Thus the linear depolarization ratio δ (the ratio of backscattered signals polarized perpendicular and parallel to the incident beam polarization direction) provides a direct and quantitative indicator of the amount of depolarization due to multiple scatter. Using polarization information, the contribution of multiple scattering to the cloud reflectance has been measured. The polarization characteristics of the pure multiply scattered component have been examined with the aid of aperture stops and receiver angles which block singly scattered radiation. This information is then used to analyze the reflected return from clouds and separate the contributions of single and multiple scattering. Comparison of the measured single scattering with theory shows good agreement. The contribution of multiple scattering to the total reflectance can be very large in dense clouds.

Determination of cloud microphysical properties by laser backscattering and extinction measurements

The extinction and backscattering of 514-nm laser radiation in polydisperse water droplet clouds have been studied in the laboratory. Three cloud size distributions with modal diameters of 0.02, 5, and 12 microm have been investigated. The relationships between the cloud optical parameters (attenuation coefficient sigma and volume backscattering coefficient beta(pi)) and the cloud water content C have been measured for each size distribution. It has been found that a linear relationship exists between sigma and C and between beta(pi) and C for cloud water content values up to 3 g/m3. The linear relationships obtained, however, have slopes which depend on the droplet size distribution. For a given water content both sigma and beta(pi) increase as the modal diameter decreases. The measured data are compared with existing theoretical analyses and discussed in terms of their application to lidar measurements of atmospheric clouds. It is concluded that the empirical information obtained can serve as a basis for quantitative lidar measurements.

Ozone and aerosol observed by Lidar in the Canadian Arctic during the winter of 1995/96

Lidar observations of stratospheric ozone made at Eureka (80.0oN,86.42oW) during the 95/96 winter show substantial declines in ozone mixing ra- tios. Reductions in ozone levels of up to 40 % between the 410 K and 580 K isentropic levels were observed be- tween mid-January and mid-March. The correlation of the ozone data with potential vorticity and concurrent lidar observations of stratospheric aerosol is consistent with the claim that significant chemical depletion did Occur.