• Chinese Journal of Lasers
  • Vol. 48, Issue 13, 1306002 (2021)
Renjiao Yu*, Zhenhua Li**, Jiancheng Lai, Chunyong Wang, and Zhixiang Wu
Author Affiliations
  • School of Science, Nanjing University of Science & Technology, Nanjing, Jiangsu 210094, China
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    DOI: 10.3788/CJL202148.1306002 Cite this Article Set citation alerts
    Renjiao Yu, Zhenhua Li, Jiancheng Lai, Chunyong Wang, Zhixiang Wu. Dynamic Propagation Characteristics of 1.55-μm Wavelength Laser in Rain and Fog Coexisting Weather[J]. Chinese Journal of Lasers, 2021, 48(13): 1306002 Copy Citation Text show less

    Abstract

    Objective The coexistence of rain and fog is a common atmospheric phenomenon in winter. When laser is transmitted in rain and fog weather, the attenuation is not only affected by rain but also by fog. Because of the small rainfall rate, fog attenuation is usually greater than rain. Globally, numerous studies have been conducted on the transmission characteristics of laser in rain and fog individually, but research on laser transmission characteristics in rain and fog coexisting weather is inadequate. To the best of our knowledge, the interaction between raindrops and fog droplets has not been considered. In this study, based on the mechanism of rain clearing fog, we improve the existing models and propose a prediction model of atmospheric attenuation in rain and fog coexisting weather, which dynamically shows the changes of atmospheric attenuation and transmittance with time in rain and fog coexisting weather. We believe that the findings of this study will have reference significance for the estimation and evaluation of atmospheric attenuation in wireless optical communication and related fields.

    Methods In rain and fog coexisting weather, the precipitation process has a significant effect on fog removal. As fog is removed by raindrops, the scale distribution of fog will change. In this article, we employ the general dynamic equation considering wet deposition to study the dynamic change of fog with the removal of raindrops. Then, we use the lognormal scale distribution model of raindrops and Gamma distribution model of radiation fog and advection fog to calculate the total attenuation of rain and fog after clearing. Further, we employ Lambert-Beer law to reckon the transmittance of laser after a certain distance. Finally, the numerical results are compared with the Monte-Carlo simulation results to verify the rationality of the proposed model to a certain extent.

    Results and Discussions The rainfall intensity positively correlates with the fog removal effect. Since the rainfall in rain and fog coexisting weather is small, the attenuation of fog gradually decreases with the removal of fog by rain (Fig. 1). When the rainfall rate is 1 mm/h, the transmittance of advection fog tends to be stable after 5 h of rainfall, whereas, the radiation of fog takes a longer time (Fig. 2). The water content of advection fog is higher than that of radiation fog, so the albedo of advection fog is higher than that of radiation fog. Owing to the obvious removal of advection fog by rain, the density of advection fog decreases, the proportion of raindrops per unit volume increases, the absorption increases, the scattering weakens, and the albedo of particle swarm increases (Tables 4 and 5). When the transmission distance is 1000 m and transmittance is less than 5%, the transmittance calculated using the Monte-Carlo method is larger than that calculated using the Lambert-Beer law, and vice versa in other cases.

    Conclusions In this article, we employed the lognormal scale distribution model of raindrops and Gamma distribution model of radiation fog and advection fog to study dynamic change of fog with raindrop removal using the general dynamic equation considering wet deposition. Based on the basic principle of atmospheric attenuation, the attenuation of laser propagation in the atmosphere changes with the fog scale distribution model. Because most of the radiation fog droplets are medium-sized aerosols, they are difficult to be wet removed by rainfall, so the transmittance increases slowly with time. The droplet size of the advection fog is large and quickly removed in the case of moderate and heavy rain, and then the transmittance is at a fixed value. Through Monte-Carlo simulation analysis of laser transmission in the atmosphere, in the case of small attenuation, the photon moving step is larger, the scattering times in a fixed distance range are less, the particles hardly reach the receiving plane after the collision, and the calculation results of the Monte-Carlo method are minute. With the increase in particle number density and transmission distance, the number of scattering increases, and the transmittance calculated using the Monte-Carlo method is slightly higher than that calculated using the Lambert-Beer law. Over time, owing to the removal of fog by rainfall, the droplet number density decreases, and the numerical difference between the two methods increases.

    Renjiao Yu, Zhenhua Li, Jiancheng Lai, Chunyong Wang, Zhixiang Wu. Dynamic Propagation Characteristics of 1.55-μm Wavelength Laser in Rain and Fog Coexisting Weather[J]. Chinese Journal of Lasers, 2021, 48(13): 1306002
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