Abstract. The evaporation duct is a downward refracting layer that results from the rapid decrease in humidity with respect to altitude occurring in the atmospheric surface layer above bodies of water. The evaporation duct affects radar detection ranges at frequencies of approximately 1 GHz and above. Models based on Monin-Obukhov similarity theory are usually used to calculate evaporation duct refractivity profiles from bulk measurements of air temperature, humidity, wind speed, and the sea surface temperature. Modeling results by Pappert et al. [1992] indicated that the falloff of radar sea echo as a function of range was an increasing function of the evaporation duct height. On the basis of those results, the authors proposed inferring the evaporation duct height by a slope fit to modeled clutter power, a nonlinear least squares inversion procedure. Data for testing the inversion procedure were obtained using the S band Space Range Radar at Wallops Island, Virginia. Evaporation duct heights were inferred from the radar data on the basis of the assumption of a range-independent evaporation duct height and sea clutter radar cross section (o-ø). Validation data consist of buoy and boat in situ bulk measurements. The result of comparing the radar-inferred evaporation duct heights and those calculated from bulk measurements indicates that the radar-inferred duct heights are strongly correlated with those from the in situ measurements, but there is some uncertainty as to whether they are biased or unbiased. That uncertainty arises from the assumed dependence of o -ø on the grazing angle ½. That ½ dependence is currently a matter of debate in the open literature, with the lower and upper ends of modeling results being o.O • ½0 and o -ø • ½4, respectively. We show results for both dependencies and note that the o -ø • ½0 provides the best agreement with our measurements. It should be noted that inferring the evaporation duct height from radar sea echo is a problem that stresses the modeling of low-grazing-angle backscatter. IntroductionEvaporation ducts result from the rapid change in humidity and temperature that occurs within the surface layer of the atmosphere over bodies of water. The resulting vertical profiles of temperature and humidity change the profile of the index of refraction n and the derivative quantities of radio refractivity N and modified refractivity M so as to form a duct that affects electromagnetic (EM) propagation at frequencies of 1 GHz and above. Evaporation duct refractivity profiles are most often calculated using Monin-Obukhov similarity-theory-based models of Jeske [1971, 1973] surements of the sea surface temperature Ts, air temperature Ta, relative humidity RH, and wind speed U at a reference height above the sea surface. These are commonly referred to as "bulk measurements." The Jeske profile under neutral conditions (T a = Ts) is uniquely defined by the evaporation duct height 3, which is the height at which the refractivity profile changes from downward to upward refracting. The J...

A case study of beyond‐the‐horizon propagation previously examined in terms of an earth‐detached duct produced by an elevated layer at 2000 ft is reexamined in terms of ducting produced by evaporation ducts (duct heights ≤100 ft). The case study applies to a frequency of 3087.7 MHz, a transmitter altitude of 68 ft and a receiver altitude of 3000 ft. It is concluded that relatively shallow surface ducts of the type examined can significantly affect signal level beyond the horizon at elevations which are an order of magnitude or more greater than the duct height and could conceivably account in large measure for the disparities previously reported.

No abstract

No abstract