OPTICAL REVIEW Vol. 7, No. 3 (2000) 235-240 TWO-Color Dual-Polarlzatlon Pulsed Blstatlc Lldar for Measurlng Water Cloud Droplet Size Nobuo SUGIMOTO National Institute for Environmental Studies. 16-2. Onogawa. Tsukuba lbarakl 305 0053 Japan (Received December 21, 1999; Accepted February 21, 2000) The concept of a pulsed bistatic lidar for measuring water cloud particle size is presented. The method uses a two-color laser and a receiver with a polarization analyzer located at a suitable scattering angle. The dependence of Mie scattering on scattering angle, wavelength, and polarization is used to derive water cloud droplet size. The measurement was simulated for the C1 and C2 clouds, and the technique for determining mode radius was studied. The result shows the lidar system with a two-wavelength laser (1064 nm and 532 nm) and a dual-polarization receiver fixed at a scattering angle of around 178 deg can be used to measure a cloud particle size (mode radius) of 4 to 12 pm. Evaluation of the effect of multiple scattering showed that the method can be applied not only for the measurement at the cloud base but also in the cloud where multiple scattering is not negligible . Key words: Iidar, cloud, Mie scattering, particle size distribution, glory l. Introduction A method for measuring cloud droplet size is required to study the effect of aerosols on cloud formation, which is currently the most uncertain factor in the radiative process of the atmosphere concerning climate change. Cloud particle size has recently been analyzed from satel- lite data, and the relationship between aerosol concentra- tion discussed.1) However, more direct quantitative obser- vations are required to validate satellite analyses and to understand the process of the aerosol-cloud interaction. Several lidar methods were reported for measuring cloud droplet size; these are lidars using multiple scatter- ing, such as the multiple field-of-view lidar,2) and the Ra- man lidar which measures liquid water Raman scatter- ing.3) In this paper, we describe another simple method using Mie scattering, basically single scattering of cloud particles. This method utilizes the dependence of Mie scattering on scattering angle , wavelength, and polariza- tion. We earlier4) described a lidar method termed "glory lidar" which utilizes a multicolor laser and two receivers 10cated at suitable scattering angles. The technique presented here is an improved method which utilizes the wavelength dependence of the ratio of the polariza- tion components of Mie scattering to derive particle size of the scatterer. With the use of polarization, the meas- urement system can be much simpler. 2. Lidar Method and Particle Size Measurement Figure 1(a) shows the lidar configuration we consi- dered in our previous paper.4) A multicolor laser beam is transmitted vertically, and the intensity of the scattering from clouds is received by two multicolor receivers at two different angles, 180 deg and another suitable angle. The wavelength dependence of the signal received by E-mail: nsugimot@nies .go. jp receiver 2 is normalized by that received by receiver 1, and the normalized spectrum is analyzed. In the new method, we use only one receiver at a suitable angle, as shown in Fig. 1(b). This is a kind of a bistatic lidar.5) It is, however, a pulsed lidar which measures range-resolved li- dar signals using a multi-color transmitter and a dual polarization receiver. The direction of polarization of the transmitted laser is 45 deg to the scattering plane, and the parallel and perpendicular components are received separately. Since the angular dependence of Mie scatter- ing is different for the polarization components,6,7) we can use the ratio of these components to extract the wavelength dependence. We calculated the angular dependence of the ratio of the polarization components of Mie scattering for a water cloud at three laser wavelengths (1064 nm, 532 nm and 355 nm). We assumed that the cloud droplets have a gamma size distribution written as n(r) C(rlr*) exp [-(a/y)(r/r*)y], (1) where r* is the mode radius and C is a normalization con- stant. We assumed two models of cloud known as the C1 cloud (a=6, y= 1) and the C2 cloud (a=8, y=3).8,9) The size distributions are shown in Fig. 2(a) for r~ =4, r* = 6, r* 8 and r* 12 (pm). The width of the size distribu- tion peak is broader in the C1 cloud than in the C2 cloud, as seen in Fig. 2(a). In the lidar backscattering signal, the intensity is approximately proportional to r2. Larger par- ticles consequently contribute more in the lidar signal. We indicate r2n(r) in Fig. 2(b) and define the peak as the effective particle radius. Effective radii for r*=4, r* = 6, r*=8 and r*=12 are 5.3, 8.0, 10.7 and 16.0 for the C1 cloud, and 4.3, 6.5, 8.6 and 12.9 for the C2 cloud. The ratio of the perpendicular polarization component, P., to the parallel component, P1' is shown in Fig. 3 and Fig. 4 for r*=4, r*=6, r*=8 and r*= 12. As seen, P* /Pl has a clear dependence on scattering angle and mode 235