Surface Duct Height Distributions for Wallops Island, Virginia, 19851994

A surface duct is defined as a layer of air adjacent to the earth's surface, in which temperature and humidity gradients cause microwave energy originating within the layer to be sufficiently refracted so that it becomes trapped into propagating along the surface. This layer then acts as a waveguide for microwave propagation and results in propagation beyond the horizon. Failure to consider such conditions may result in erroneous radar meteorological measurements. These ducts can be located by examining refractivity profiles derived from atmospheric measurements. Since 1985, over 3900 profiles of microwave refractivity have been measured using an instrumented helicopter over the Atlantic Ocean off the coast of Wallops Island, Virginia. This helicopter data acquisition system provides higher-resolution measurements than those obtained from radiosondes. This paper presents the heights and associated frequency distributions of surface ducts as determined from these profiles. The year is divided into four quarters to examine seasonal variations, and surface duct statistics are determined for each quarter. These statistics are useful for characterizing the climatology of radar and communication ducting phenomena for this region. Such statistics have been used in microwave propagation prediction models and are useful in radar design. Surface ducts were most commonly observed and had the largest mean heights in the April–June and July–September quarters.