Ulla Wandinger, Albert Ansmann, and Claus Weitkamp
The nitrogen Raman depolarization ratio is measured with a lidar. The measurements show how a lidar profile of cloud parameters is affected by multiple scattering.
There is a strong need for direct measurements of multiple-scattering effects with lidars. Multiple scattering can significantly affect both the determination of the cloud optical depth from lidar return signals and the discrimination between the liquid and the solid phases of cloud layers by the use of polarization-lidar data. Multiple-scattering lidar experiments are needed for the verification of assumptions and results of model calculations, which, for the sake of simplicity, are usually performed for homogeneous clouds with well-defined particle shapes and scattering characteristics. Real clouds, however, are typically highly variable in particle number density and size distribution and, in the case of ice clouds, also in crystal shape. Microphysical properties vary with height. Scattering characteristics of the differently shaped particles are not well known.
A few investigations have been made to determine multiple-scattering effects from experiments, for example, from aerosol (elastic-backscatter) depolarization measurements,1-3 multiple-field-of-view measurements,4,5 and from a combination of both.6 The aerosol depolarization technique suffers from the fact that both multiple scattering and single scattering from nonspherical particles cause a depolarization of the elastically backscattered light, so that the method cannot be applied to measure the multiple-scattering effect in mixed-phase and ice clouds. The multiple-
U. Wandinger and A. Ansmann are with the Institut für Troposphärenforschung, Permoserstrasse 15, D-04318 Leipzig, Germany. C. Weitkamp is with GKSS-Forschungszentrum Geest-hacht, Postfach 1160, D-21494 Geesthacht, Germany.
Received 23 August 1993; revised manuscript received 12 April 1994.
field-of-view technique, on the other hand, has been used only for short measurement ranges.
In this contribution, we briefly report on a new method for the detection of multiple-scattering effects. A polarization Raman lidar is developed that permits the determination of both the elastic and the Raman depolarization ratios of nitrogen, i.e., the ratio of the components of radiation that are Raman scattered by nitrogen molecules (vibration-rotation branch) with electric-field vectors perpendicular and parallel to the electric-field vector of the transmitted, linearly polarized laser light. While the elastic depolarization ratio in clouds is influenced by single and multiple particle scattering, the Raman depolarization ratio is determined by one Raman-scattering process and additional elastic-scattering processes from particles.
The measured parallel and cross-polarized Raman-signal components can be written as
respectively. The signals contain singly scattered Raman light (║PλR
s and ┴PλRs) and radiation that
underwent elastic scattering from particles before or after the Raman-scattering process (║PλR
ms and ┴PλR
ms). Signals are detected at the Raman wavelength λR.
Analogous to the well-known elastic depolarization ratio, the linear Raman depolarization ratio may be defined as
Because the depolarization ratio that is due to Raman backscattering by the vibration-rotation branch of
is known from theory to be constant, any deviation of the measured ratio δλR from 0.094 results from multiple scattering.
The lidar system that we used has been described in detail previously7 and has been used to study cirrus clouds extensively.8,9 It is based on a XeCl excimer laser that transmits light pulses at the wavelength of 308 nm. Laser beam divergence and receiver field of view are 0.1 and 0.4 mrad, respectively. The lidar points exactly to the zenith. For linear polarization of the transmitted beam the laser light passes a dielectrically coated polarizer.10 The extinction of the perpendicular component is 10-3. The receiving optics10 allows one to measure five backscatter signals simultaneously: the two polarization components of the elastically scattered light at 308 nm, the two polarization components of the light Raman scattered from nitrogen at 332 nm, and the water-vapor signal at 347 nm. For the determination of the elastic and the Raman depolarization ratios, the respective transmission ratios of the measurement channels must be known. They are determined in a straightforward procedure from a measurement without the laser polarizer, i.e., with unpolarized laser light.
The first measurements of the Raman depolarization-ratio height profile were taken in November 1991.11 In Fig. 1 the measurement of a cirrostratus cloud made in Geesthacht on 9 November 1992 is
shown. The particle backscatter coefficient is determined from the ratio of the elastic to the inelastic lidar signal.8 The optical depth is calculated from the particle extinction profile, which is determined from the Raman signal profile.12 A measurement section of 7 min is shown.
The backscatter profile indicates the cirrus layer between 5 and 8.5 km. Below the cloud the molecular values of the depolarization ratios, 0.02 for Ray-leigh and 0.1 for Raman scattering, are obtained. At the cloud base the elastic depolarization ratio assumes typical values for suspended and thus randomly oriented crystals. It decreases to values near 0.1 as the backscatter coefficient increases at approximately 6 km. These smaller depolarization values are probably caused by specular reflection of falling and predominantely horizontally oriented ice crystals. Specular reflection leads to the peaks in the backscatter profile and also to the observed low values of approximately 3 sr of the extinction-to-backscatter ratio, which is not shown here. The increasing Raman depolarization ratio indicates an increasing influence of multiple scattering with height. Elastic and Raman depolarization ratios are correlated above 6 km; this is assumed to be caused by multiple scattering.
The differences in increase of the two depolarization ratios above 6 km can be explained as follows: Multiple-scattering calculations13 show that no more than three elastic forward-scattering processes close to the 0° direction before or after the Raman or elastic backscatter processes (close to 180°) contribute significantly to the backscatter signals. Such forward-
Fig. 1. (a) Particle backscatter coefficient (solid curve) and optical depth (dotted curve) and (b) elastic (dashed curve) and Raman depolarization ratio (solid curve) measured in a cirrostratus layer with a polarization Raman lidar. For the particle backscatter coefficient and the elastic depolarization ratio, the signal-smoothing length is 120 m; in the case of the Raman depolarization ratio it is 600 m. Calculation step width is 60 m. The error bars indicate the statistical error from signal noise.
scattering processes do not lead to an increase in the Raman and elastic depolarization ratios if the ice crystals are regularly shaped, randomly oriented hexagonal plates and columns.14 However, one can expect that irregularly shaped crystals do cause a depolarization of the forward-scattered light. Scattering phase matrices for such particles are not yet available. If we assume that irregularly shaped ice crystals were present above 6 km in the cirrus cloud shown in Fig. 1, the different but correlated increases of the elastic and Raman depolarization ratios are caused by the different shapes of the scattering phase functions for particles and molecules in the backward direction. While Raman scattering is an almost isotropic process, the phase function for larger particles is peaked at 180°. Thus, in the case of the Raman depolarization ratio, more forward-scattered photons contribute to the signals than in the case of the elastic depolarization ratio because, for elastic scattering, the backscattering efficiency decreases rapidly with increasing forward-scattering angles.
The Raman depolarization ratio thus proves to be an additional tool for the observation of the scattering properties of clouds. It indicates whether or not multiple scattering must be taken into account in the interpretation of elastic depolarization-ratio data. The elastic depolarization ratio is not influenced by multiple scattering if the additionally measured Raman depolarization ratio is range independent. With the new technique the forward-scattering behavior of ice crystals can be investigated, and layers with complex crystals may be detected in this way.
A more general result of our experiments is that in most measurements made in high-altitude ice and water clouds a range-independent Raman depolarization ratio was obtained. This finding supports the essential assumptions of model calculations that are used to determine the multiple-scattering effects on the optical properties derived from lidar measurements of clouds. The limitation to small-angle forward- and backward-scattering processes in the models appears to be sufficient. Wide-angle forward scattering (> 10°) would always lead to a significant depolarization effect. A paper is in preparation in which the theoretical and experimental findings are discussed in more detail.
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