Interpretation of gravity wave signatures in GPS radio occultations

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.

A gravity waves study close to the Andes mountains in Patagonia and Antarctica with GPS radio occultation observations

“…It can be seen that dominant wavelengths are found around 3 km, whereas the shape for amplitude is very similar to what has already been equivalently observed for potential energy by Baumgaertner and McDonald (2007). It must be stated that detected and real vertical wavelengths must be distinguished for non-vertical sounding trajectories like GPS RO, whereby the discrepancy depends on the angle defined by the LOS and the wave constant phase surfaces (e.g., Alexander et al, 2008). The deviation may be significant for mountain waves, as their phase surfaces generally differ from the local horizontal plane.…”

“…The circumstances under which the GPS RO technique is able to resolve gravity waves were extensively discussed by Alexander et al (2008). The horizontal resolution of each measurement is in the worst case in the order of a few hundred kilometers along the LOS of the satellite (Kursinski et al, 1997).…”

“…Source variability and critical level filtering are also important factors in the observed wave activity variability. The outcome of passive remote sensing satellite observations is also strongly dependent on the relative orientation of the line of sight (LOS) and the wavefronts to be detected (e.g., Preusse et al, 2002;Lange and Jacobi, 2003;Jiang et al, 2004;Alexander et al, 2008).…”

Orographic and convective gravity waves above the Alps and Andes mountains during GPS radio occultation events – a case study

“…It is accepted that w adequately highlights the main GW amplitudes and wavelengths belonging to high and medium intrinsic frequency regimes. As it is known (e.g., de la Torre and Alexander, 1995;P. Alexander et al, 2008), a distortion is introduced in the measured GW  H and  Z by atmospheric soundings performed in any other than in vertical and horizontal directions, as is the case during GPS RO events or during radiosoundings.…”

“…Alexander et al, 2008), a distortion is introduced in the measured GW  H and  Z by atmospheric soundings performed in any other than in vertical and horizontal directions, as is the case during GPS RO events or during radiosoundings. In the case of RO events, it is 210 known that a visibility condition imposed to the line of sight (LOS) described in P. Alexander et al (2008) must be satisfied.The distortion is more or less significant, depending on the elevation angle of the sounding path and the GW aspect ratio (de la Torre et al, 2017, hereafter respectively, are distinguished. In this case study we observe in Fig.…”