Diurnal cycles of the refractive index structure function coefficient

Abstract

The refractive index structure function coefficient Cn2 is an atmospheric parameter needed to describe scintillation and small-scale phase fluctuations of electromagnetic radiation propagated in the atmospheric surface layer. Since systematic direct measurements of Cn for many climates and seasons are not available, an indirect method is developed in which Cn is calculated from the estimates of sensible and latent heat flux components of the surface energy budgets. This indirect method is primarily for heights less than 4 meters, because low intermittency and a near-unity value of the ratio of the eddy diffusivity of heat to that of momentum are assumed. Diurnal variations of Cn at several heights above land for six combinations of climates, seasons, and surface conditions are calculated from heat fluxes measured by different investigators at many locations for moderate to high wind velocities. These predictions of Cn agree well with some direct measurements of Cn when assumptions of nearly ideal weather and sites are met. The effect of water vapor on Cn is usually an increase of about 16/β%, where β is Bowen's ratio, and thus is usually negligible above dry land but can be significant above wet surfaces.