Equatorial waves, diurnal tides and small-scale thermal variability in the tropical lower stratosphere from COSMIC-2 radio occultation

A new constellation of radio occultation satellites called COSMIC-2 (Constellation Observing System for Meteorology, Ionosphere, and Climate-2) is providing unprecedented dense measurements of the tropical atmosphere, with on average more than 4,000 high quality observations per day over 40°N–S. We use these data to evaluate large- and small-scale thermal variability in the tropical lower stratosphere during October 2019 – April 2020. Space-time spectral analysis of 6-hourly gridded COSMIC-2 data reveals a rich spectrum of traveling planetary-scale waves, including Kelvin waves, mixed Rossby-gravity waves and inertia gravity waves, in addition to propagating diurnal tides. These coherent modes show enhanced amplitudes from the tropical tropopause through the lower stratosphere (∼17–25 km). Characteristics of small-scale temperature variances, calculated as deviations from the gridded fields, reveal systematic spatial patterns including time average maxima over Africa and South America overlying frequent deep convection. Small-scale variances also exhibit transient maxima in the equatorial lower stratosphere tied to local variations in static stability, associated with large-scale Kelvin waves. The new COSMIC-2 observations provide novel details on the rich spectrum of large- and small-scale waves near and above the tropical tropopause.