Effect of periodic horizontal gradients on the retrieval of atmospheric profiles from occultation measurements

Spherical symmetry is generally assumed during the retrieval process of atmospheric profiles 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 find that only the waves, with horizontal wavelengths close to the horizontal distance mat rays travel in their final 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 profile 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 profiles, 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.