Abstract
The horizontal averaging of global positioning system radio occultation retrievals produces an amplitude attenuation and phase shift in any plane gravity wave, which may lead to significant discrepancies with respect to the original values. In addition, wavelengths cannot be straightforwardly inferred due to the observational characteristics. If the waves produce small departures from spherical symmetry in the background atmosphere and under the assumption that the refractivity kernel may be represented by a delta function, an analytical expression may be derived in order to find how 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 in variance calculation, which is found to be around 19%. A larger discrepancy was obtained when comparing an occultation variance with the one computed from a numerical simulation of that case. Wave amplitudes can be better resolved when the fronts are nearly horizontal or when the angle between the occultation line of sight and the horizontal component of the wave vector approaches π/2. Short horizontal scale waves have a high probability of becoming attenuated or of not being detected at all. We then find geometrical relations in terms of the relative orientation between waves and sounding, so as to appropriately interpret wavelengths extracted from the acquired data. Only inertio‐gravity waves, which exhibit nearly horizontal fronts, will show small differences between detected and original vertical wavelengths. Last, we analyze the retrieval effect on wave phase and find a shift between original and detected wave that generally is nonzero and approaches π/4 for the largest horizontal wavelengths.
Highlights
[2] Different sounding techniques have been employed in the last decades to explore lower- and middle-atmosphere physics and chemistry
In the present study we focus on one of the limb techniques presently providing a huge amount of atmospheric information, the so-called Global Positioning System (GPS) radio occultation (RO)
In de la Torre and Alexander [2005] it was emphasized that the capability of limb-sounding devices and, in particular, GPS RO experiments on board of low Earth orbit satellites, to detect typical mountain waves with horizontal wavelengths shorter than 150 km, depended on the horizontal angle between the line of sight (LOS) and the wave phase surfaces to be detected
Summary
[2] Different sounding techniques have been employed in the last decades to explore lower- and middle-atmosphere physics and chemistry. We use an equation relating the one-dimensional (1D) retrieved and two-dimensional (2D) original refractivities through a profile retrieval 2D mapping kernel [Ahmad and Tyler, 1998], and study the ability to detect the effects by GW assuming that they induce small departures from sphericity in the background atmosphere These considerations keep the problem of amplitude and phase analytically tractable. The perturbations may be detected if (E may be interpreted as the ratio between derived and original amplitudes, a kind of retrieval attenuation factor) R00/A = E > 0.1 Note that this condition is a function solely of the vertical to horizontal scales (given by the wavelengths) aspect ratio a 1⁄4 k lz ð10Þ m lx Figure 5. For the perturbation to background refractivity ratio (or apparent relative amplitude), which we call the response, we obtain: R
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