Investigating equatorial waves with high-density ROMEX GNSS-RO observations

Remote sensing from satellites provides a powerful means of observing the Earth’s atmosphere with global coverage and high vertical resolution. Among these techniques, Global Navigation Satellite System radio occultation (GNSS-RO) offers a low-cost approach that delivers large volumes of high-quality atmospheric temperature profiles. While several tens of thousands of GNSS-RO observations are assimilated daily into numerical weather prediction systems from multiple satellite missions, only a subset of these are made available for community research. Here we explore the Radio Occultation Meteorology and Climate Experiment (ROMEX) dataset: a unique, community-available collection of high-density GNSS-RO observations, combining measurements from multiple satellite missions to provide approximately 30,000-40,000 profiles per day during September-November 2022, resulting in an unprecedented sampling density for scientific applications.

In this study, we investigate the ROMEX dataset to assess the additional insight enabled by such exceptionally dense spatial and temporal sampling, with a focus on fast-moving equatorial waves in the tropical atmosphere. The high sampling density of ROMEX is particularly suited to resolving planetary-scale equatorial wave modes with short periods, which are difficult to capture using conventional measurements such as those by radiosondes or sun-synchronous satellites. ROMEX’s strongest performance in the upper troposphere and lower stratosphere further provides access to a key region of equatorial wave activity. Using temperature perturbations derived from the GNSS-RO profiles, we separate symmetric and antisymmetric wave components and examine their distribution in frequency-wavenumber space to identify distinct equatorial wave modes and recover their characteristic horizontal structures. We show that multiple equatorial wave modes, including Kelvin waves, mixed Rossby-gravity waves, equatorial Rossby waves, and both eastward and westward inertia-gravity waves, can be exceptionally clearly identified and studied using ROMEX observations. Among these, Kelvin waves with periods of approximately 10-13 days are observed, with maximum amplitudes near 18 km. In addition to the large-scale planetary waves themselves, we also investigate their modulation of the small-scale gravity wave flux in the tropics, and vice versa, revealing new insights into wave-wave interaction and momentum driving reaching the mid stratosphere.

Our results demonstrate that dense GNSS-RO datasets such as ROMEX offer substantial potential for atmospheric science beyond their established role in numerical weather prediction. In particular, the unique coverage and vertical resolution of ROMEX open new opportunities to study tropical wave dynamics and their impact on the structure of the tropical and extratropical atmosphere.