Characterizing Range-Dependent Variations of the Evaporation Duct: A Meteorological Perspective

Evaporation ducts (EDs) are an observed electromagnetic phenomenon caused by rapid decreases of humidity with altitude. Sensing technologies that operate at X-band frequencies (8–12 GHz) exhibit the extension of radar signals beyond the radar horizon and holes in coverage at high altitudes during evaporative ducting conditions. In addition, the evolution of the ED over range has been reported to cause further adverse effects on these technologies in some circumstances. However, in terms of the predictability of these effects, more research is still needed. This research explores ED variations over range and characterizes them based-on two numerical datasets from different latitudes, different hemispheres, and during different seasons as well as correlating their range distributions to distributions of other atmospheric variables. It is found, for both datasets, that most often a linear function accurately represents the variations of ED characteristics over ~60 km in range, but higher order functional distributions of ED characteristics with range do occur albeit less frequently. Furthermore, the range distributions of mean wind speed and mean specific humidity within the evaporation layer are strongly correlated to the range distributions of duct height; while range distributions of duct shape are mainly related to the specific humidity gradient in the evaporation layer. The midlatitude dataset exhibits more complex range distributions during frontal events, while the equatorial dataset shows the most complex range distributions near sunrise.