A Study of Fog Characteristics using Free-Space Optical Wireless Links

Abstract

A technique for modeling the fog droplet size distributions using modified gamma distribution has been demonstrated by considering two separate radiation fog events recorded in Graz (Austria) and Prague (Czech Republic). The measurement of liquid water content (LWC) and the optical attenuations at visible wavelength are used to form equations to obtain the three parameters of the modified gamma distribution i.e., the slope (Λ), the intercept (N0 ) and the shape parameter (m). Calculated attenuation or LWC from the retrieved parameters are in excellent agreement with attenuation or LWC obtained from the measurement. Hence this method is useful in the study of fog microphysics and in modeling the fog attenuations for terrestrial FSO links in situations when our measurement data contains values of attenuations only, or liquid water content only or both at a particular location. For the two case studies, Graz and Prague, we obtained the DSD parameters Λ=3.547 ± 1.935, N0 =3.834 ± 2.239, m=6.135 ± 2.692 and Λ=5.882 ± 2.889, N0 =13.41 ± 3.875, m=5.288 ± 3.113, respectively. It is evident that the observed behavior of computed modified gamma distribution parameters for Graz and Prague is closely the same and is consistent with the previous literature for the radiation (continental) fog. Moreover, we observed the variation of the computed DSD parameters at the different stages of fog (formation, maturity and dissipation phases) indicating different microphysical processes at each stage.