The application of an evaporation duct model in operational and climatological assessments of propagation and the sensitivity of the model to meteorological measurements are examined. The unexpectedly frequent occurrence of evaporation duct heights greater than 40 m is related to thermally stable conditions in the atmospheric surface layer. The existence of stable conditions over the ocean is analyzed in terms of meteorological conditions and temperature measurement accuracies. Air‐sea temperature differences measured by transiting ships are found to be biased toward stable conditions. The evaporation duct model is shown to be sensitive to this bias and yields high duct heights that are not supported by propagation measurements. A modified duct height calculation is proposed and applied to a radiometeorological data set.
Abstract-Existing sea clutter models are in general agreement on the trends and magnitudes of sea reflectivity at low (1" -10") grazing angles. However, at extremely low grazing angles (< lo), models, theory, and measurements show considerable differences. The effects of the oceanic evaporation duct on grazing angle are investigated. Significant changes in grazing angle due to the evaporation duct are found and the illumination of the sea surface by the radar is shown to extend well beyond the standard (4 / 3 earth) radar horizon range. A series of sea clutter model comparisons shows that much of the difference between models can be attributed to evaporation duct effects.
Path loss formulas for sea surface backscatter are developed for horizontally stratified tropospheric ducting environments. First‐order Bragg backscatter results coupled with ray and waveguide formalism serve as the basis for the development. Two features of composite model formalism, namely shadowing and tilting of the surface normal in the plane of incidence, are included in the development. Though developed for hh polarization, conventional composite model modifications for vertical polarization and depolarization can be incorporated into the echo models. Comparisons are made with previously generated parabolic equation results and with empirical model results generated by Engineers Refractive Effects Prediction System. These comparisons are for a frequency of 9.6 GHz for the standard atmosphere and for 14‐ and 28‐m evaporation ducts. Results apply to wind speeds of 10, 20, 30, and 40 knots (≈ 5, 10, 15, and 20 m/s).
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