The Combined Effect of Temperature and Humidity Fluctuations on Refractive Index

Marvin L. Wesely Argonne National Laboratory, Argonne, Ill. 60439

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Abstract

Depending on whether the radiation under consideration is acoustic, visible or microwave, either temperature or humidity fluctuations are ordinarily assumed to be an insignificant source of refractive index fluctuations. For applications in the atmospheric surface layer or in a free convection layer, the value of the local Bowen ratio β, which is the ratio of sensible to latent heat flux densities, can be used to determine when variations in both temperature T and water vapor pressure e are important considerations. When |β| < 0.3 for applications involving visible radiation, |β| < 0.6 for acoustic radiation, and |β|>0.32 for microwave radiation, both T and e fluctuations have at least a 10% effect on the amplitudes of refractive index fluctuations, provided T and e are highly correlated. If T and e are uncorrelated, this 10% level is |β| < 0.067, |β| < 0.13 and |β| > 1.45 for acoustic, visible and microwave radiation, respectively. With knowledge of β and the extent of the T and e correlation, refractive index “fluxes” and structure function coefficients can be calculated from (or inversely, can be used to calculate) the corresponding parameters for temperature and humidity.

Abstract

Depending on whether the radiation under consideration is acoustic, visible or microwave, either temperature or humidity fluctuations are ordinarily assumed to be an insignificant source of refractive index fluctuations. For applications in the atmospheric surface layer or in a free convection layer, the value of the local Bowen ratio β, which is the ratio of sensible to latent heat flux densities, can be used to determine when variations in both temperature T and water vapor pressure e are important considerations. When |β| < 0.3 for applications involving visible radiation, |β| < 0.6 for acoustic radiation, and |β|>0.32 for microwave radiation, both T and e fluctuations have at least a 10% effect on the amplitudes of refractive index fluctuations, provided T and e are highly correlated. If T and e are uncorrelated, this 10% level is |β| < 0.067, |β| < 0.13 and |β| > 1.45 for acoustic, visible and microwave radiation, respectively. With knowledge of β and the extent of the T and e correlation, refractive index “fluxes” and structure function coefficients can be calculated from (or inversely, can be used to calculate) the corresponding parameters for temperature and humidity.

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