A new method for measuring the imaginary part of the atmospheric refractive index structure parameter in the urban surface layer

摘要

The atmospheric refractive index consists of both real and imaginary parts.The intensity of refractive index fluctuations is generally expressed as therefractive index structure parameter, with the real part reflecting thestrength of atmospheric turbulence and the imaginary part reflectingabsorption in the light path. A large aperture scintillometer (LAS) is oftenused to measure the structure parameter of the real part of the atmosphericrefractive index, from which the sensible and latent heat fluxes can furtherbe obtained, whereas the influence of the imaginary part is ignored orconsidered noise. In this theoretical analysis study, the relationshipbetween logarithmic light intensity variance and the atmospheric refractiveindex structure parameter (ARISP), as well as that between the logarithmiclight intensity structure function and the ARISP, is derived. Additionally,a simple expression for the imaginary part of the ARISP is obtained whichcan be conveniently used to determine the imaginary part of the ARISP fromLAS measurements. Moreover, these relationships provide a new method forestimating the outer scale of turbulence. Light propagation experiments wereperformed in the urban surface layer, from which the imaginary part of theARISP was calculated. The experimental results showed good agreement withthe presented theory. The results also suggest that the imaginary part ofthe ARISP exhibits a different diurnal variation from that of the real part.For the wavelength of light used (0.62 μm), the variation of the imaginarypart of the ARISP is related to both the turbulent transport process and thespatial distribution characteristics of aerosols.