Observations of stratospheric gravity waves in the tropics: can GNSS-RO extend the SABER climatological record?

Abstract. The quasi-biennial oscillation (QBO) is the most important phenomenon in the tropical stratosphere. It is mainly driven by small-scale gravity waves. Still, the representation of QBO in models is challenging because small-scale gravity waves are not well resolved in the models and the majority of the parametrization schemes are limited to vertical propagation only of gravity waves. One solution to this is to use high-resolution satellite observations to understand the gravity wave (GW) forcing on the QBO. However, the results can vary from one observation to another due to the unique observational filter of each instrument. Here we investigate how these differences in the observational filters between SABER and GNSS-RO satellite measurements affect our ability to capture the interactions between GWs and the QBO. To test this, we sample temperatures from the high-resolution GEOS model as if they were observed by SABER and GNSS-RO and estimate synthetic GW potential energy (E_p) observations. We then systematically vary the viewing angle and the vertical and horizontal resolutions of the instruments to determine which aspects have the most significant effect on the observed GW E_p. This allows us to understand how the observational filter of each instrument influences the observation of GW-QBO interaction and if we can bring the two observations close enough to get nearly the same results. Our results reveal that vertical resolution is the most significant factor deriving the differences between the results of both instruments. By adjusting the vertical resolution of GNSS-RO temperatures to match that of SABER, we found that the GW E_p and vertical wavelength measurements from both instruments could be brought into very close agreement.

This study not only focuses on the importance of selecting appropriate observational methods for gravity wave research but also highlights the potential of GNSS-RO to extend the long-term studies of GW interaction with the QBO that has been carried out by SABER for more than 23 years, especially as SABER approaches the end of its operational lifespan. Our findings contribute to a more comprehensive understanding of GW observations in the tropics and provide a foundation for future applications using merged GNSS-RO observations.