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

Evaporation ducts 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 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 evaporation duct 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 evaporation duct 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 variations of evaporation duct characteristics over ~60 km in range, but higher-order functional distributions of evaporation duct characteristics with range do occur albeit less frequently. Furthermore, 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 mid-latitude dataset exhibits more complex range distributions during frontal events; while the equatorial dataset shows the most complex range distributions near sunrise.