Recent Advances in Gravity Wave Analysis from Long Term Global GPS Radio Occultation Observations

A. de la Torre, P. Alexander, P. Llamedo, T. Schmidt, and J. Wickert

Abstract A previous global analysis of wave potential energy using Global Positioning System (GPS) radio occultation (RO) temperature profiles revealed a considerable intense gravity wave activity (WA) at middle latitudes near to the Andes Range. WRF (Weather Research and Forecasting) and MM5 (Mesoscale Model 5) mesoscale model results for two selected cases in the vicinity of RO lines of tangent points, confirm an intense activity near to the mountains. A wavelet analysis led us to identify principal modes with two main horizontal wavelengths, clearly corresponding to mountain waves. Different hodograph analyses evidence that inertio gravity waves (IGWs) are due to mountain forcing and not to geostrophic adjustment at jet levels, as could be expected too. One of the simulations does not show intense WA in the vicinity of the tangent points, even though that the GPS-RO temperature profile detects it. We conclude that the GPS-RO technique is not by itself reliable enough to quantify and locate accurately WA of single events, but it should be considered as a useful tool to detect the global distribution of WA.

1 Introduction

As it is well known, gravity waves (GWs) play an important role in the momentum and energy budget of the lower and middle atmosphere. One form of momentum deposition is wave breaking, yielding body forces exerted on the synoptic circulation (Lindzen 1990). This drag effect has to be included, via parameterizations, for an accurate general circulation model (GCM). The other two main sources of GWs are

(i) adjustment of the mean flow after departures from geostrophic equilibrium and

(ii) upwards forcing during deep convection events. Any realistic parameterization of GW drag needs a sufficient coverage of observational evidence on the global scale (Fritts and Alexander 2003).

Departamento de Fisica, FCEN, Universidad de Buenos Aires, Argentina e-mail: [email protected]

The GPS-RO (Global Positioning System Radio Occultation) technique allows to localize and identify single large amplitude GW events, detected from tomographic global maps of GW activity (WA) (de la Torre et al. 2004). The GPS-RO technique provides important advantages in comparison with regular radiosoundings: Global coverage under all weather conditions, sub-Kelvin temperature (T) accuracy, high vertical resolution, and long-term stability (see e.g., Kirchengast 2004, and references therein). Nevertheless, several limitations to the specific observation of GWs could be pointed out: (i) the capability of GPS-RO (as any other limb sounding devices) observations to detect only GWs with horizontal wavelengths longer than 150 km (horizontal resolution), (ii) the distinction between detected (apparent) and real wavelengths depending on the angle defined by the line of sight (LOS) between satellites and the wave phase surfaces (de la Torre and Alexander 1995), (iii) the wavelength refraction due to the background wind (de la Torre and Alexander 1995; de la Torre et al. 2006a,b), and (iv) the spherical symmetry assumption used to retrieve the T profiles even though the GW field is essentially three-dimensional. Moreover, the horizontal averaging during a GPS-RO event results in an amplitude attenuation and phase shift in any retrieved wave, which may lead to significant discrepancies with respect to the original values.

The main purposes of this work are to interpret, via mesoscale simulations, the capabilities of RO techniques to detect, localize, and properly quantify WA. Section 2 discusses the capability of the GPS-RO technique to detect GWs. In Sect. 3 we analyze GWs signatures in mesoscale numerical results. In Sect. 4 we present our conclusions.

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