Seasonal Variation of Coastal Refractivity Gradients in a Coastal Environment

Abstract

Variation of refractive index of air is an important factor of interest concerning many applications of propagation of radio waves in the lower atmosphere, especially at microwave frequencies under clear-air ducting conditions. The propagation path and intensity of terrestrial microwave communications and radar transmissions can be substantially affected by changes in the vertical refractivity profiles in the troposphere. Large-scale variation of refractive index with height, and the extent to which it changes with time, are useful parameters for assessing the propagation characteristics of the troposphere which may vary greatly, depending on the type of air mass (Gossard, 1977). Lateral and vertical temporal variations of refractive index over time scales of minutes to hours can be used with appropriate propagation methods such as the parabolic equation method to predict diurnal signal level variations, especially in modelling terrestrial link performance as a function of time. Variations in vertical refractivity gradients change the propagation conditions to non-standard conditions such as sub refraction, super refraction or ducting, each of which has serious implications with regard to signal enhancement, anomalous propagation as well as variations in angle of arrival at the receiver (Valtr and Pechac, 2005). Less negative (or less severe) refractivity gradients increase the height of potential obstacles as well as the earth’s bulge on a line-of-sight (LOS) path, thereby increasing possibility of obstruction of the link.