The Interpretation of Gravity Wave Signatures in GPS Radio Occultations

P. ALEXANDER; A. DE LA TORRE; P. LLAMEDO SORIA

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2008 vol. 113 p. 16117 - 16117

0148-0227

The horizontal averaging of global positioning system radio occultationretrievals produces an amplitude attenuation and phase shift in any plane gravity wave,which may lead to signi¯cant discrepancies with respect to the original values. Inaddition, wavelengths cannot be straightforwardly inferred due to the observationalcharacteristics. If the waves produce small departures from spherical symmetry in thebackground atmosphere and under the assumption that the refractivity kernel may berepresented by a delta function, an analytical expression may be derived in order to findhow the retrieved amplitudes become weakened (against the original ones). In particular,we study the range of waves that may be detected and the consequent reduction invariance calculation, which is found to be around 19 %. A larger discrepancy wasobtained when comparing an occultation variance with the one computed from anumerical simulation of that case. Wave amplitudes can be better resolved when thefronts are nearly horizontal or when the angle between the occultation line of sight andthe horizontal component of the wave vector approaches ¼=2. Short horizontal scalewaves have a high probability of becoming attenuated or of not being detected at all.We then ¯nd geometrical relations in terms of the relative orientation between wavesand sounding, so as to appropriately interpret wavelengths extracted from the acquireddata. Only inertio-gravity waves, which exhibit nearly horizontal fronts, will show smalldi®erences between detected and original vertical wavelengths. Lastly, we analyze theretrieval e®ect on wave phase and ¯nd a shift between original and detected wave thatgenerally is nonzero and approaches ¼=4 for the largest horizontal wavelengths.