Transportation of Gaussian light beam in two-layer clouds by Monte Carlo simulation

Based on the radiative transfer theory, the backscattering characteristics of water clouds and ice-water two layers clouds irradiated by infinite narrow collimated light beam are studied by using the Monte Carlo method. The incident wavelength is 0.532 μupm, and the cloud particle shape is assumed to be of sphere or plate. The single scattering characteristics of the clouds are computed based on the Mie theory, and the scattering angle sampling is based on the Mie phase function. The photon step adjustment is considered when the step is large enough to cross the cloud layer. The variations of reflection functions of the water clouds and ice-water two layers clouds with the radial length r and zenith angle are given, and the interior light intensity distribution of clouds are given in two dimensions. From the computed results, we find that the reflection characteristics of the two layer clouds are greatly different from those of the pure water clouds. The reflection intensity of ice clouds covered with water clouds is bigger than that of ice clouds covered with water clouds. This reason is that the sizes of ice clouds are larger than those of the water clouds, so more photons will be scattered into the interior of the clouds.#br#The cloud layer is assumed to be linear and invariant, so the response to an infinitely narrow photon beam will be described by a Green's function of the clouds, and the response to the Gaussian beam can be computed from the convolution of the Green's function according to the profile of the Gaussian photon beam. The multiple scattering characteristics of the Gaussian photon beam are computed from the convolution of the impulse response, i.e., the response to an infinitely narrow photon beam, according to the profile of the Gaussian light photon beam. From the computed results, we find that the reflection function of clouds for Gaussian incidence has a great difference from that for the infinite narrow beam incidence. The reflected light intensity is inversely proportional to the size of the Gaussian beam at the location near r=0. So the laser spot must be considered when detecting the clouds by using of the lidar, and the method presented in this paper can give theoretical support.