Abstract. Spherical symmetry is generally assumed during the retrieval process of atmospheric prof'des from occultation measurements. The existence of periodic horizontal gradients, occurring on scales comparable to the distances traveled by the rays around their tangent point, is produced by gravity waves. These waves can introduce density perturbations of up to 1 or 2 % in amplitude and affect the retrieved parameters accordingly. We show the consequences of ignoring these gradients in the retrieved refractivity profiles when spherical symmetry is assumed. We f'md that only the waves, with horizontal wavelengths close to the horizontal distance that rays travel in their f'mal 6 or 8 km in the vertical before they are tangent in the atmosphere, will have an influence on the retrieved profiles by introducing a phase delay in the local prof'de of the retrieved refractivity. The horizontal wavelength of these waves corresponds to the minimum horizontal resolution associated with the retrieved profiles. We also find that smaller scale waves do not have any significant impact on the retrieved prof'des, as their contribution cancels out by averaging through the periodic perturbation, while waves with very long horizontal wavelengths are in good agreement with a local spherical symmetry assumption.

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The results of an open sea experiment devised to investigate the feasibility of detecting and monitoring oil slicks by sonar through their effect on the acoustic backscatter signal are presented. The analysis, carried out over an angular range 10° to 70° from the horizontal, reveals a reduction in backscatter from the slick. The reduction in the mean backscatter <σ> shows an angular dependence which reaches a maximum in the range 45° to 65°. A gradual decrease of <σ> as wind speed decreased and as the slick developed was also observed. Comparison of the acoustic data with Ku band radar reflection from an oil covered surface under similar wind speed conditions shows that differential radar backscatter from oil slicks may exceed that of sonar by a factor 5.