Equatorial Waves, Diurnal Tides and Small‐Scale Thermal Variability in the Tropical Lower Stratosphere From COSMIC‐2 Radio Occultation

Abstract

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.

Observation of Gravity Waves at the Tropical Tropopause Using Superpressure Balloons

“…Both maps highlight a clear contrast between quieter regions (Eastern Pacific Ocean, Eastern Atlantic Ocean) and more active regions (South America, Eastern Africa, and the Maritime Continent), in agreement with maps from Randel et al. (2021). The amplitude of geographic variability is different than that of Randel et al.…”

“…Both maps highlight a clear contrast between quieter regions (Eastern Pacific Ocean, Eastern Atlantic Ocean) and more active regions (South America, Eastern Africa, and the Maritime Continent), in agreement with maps from Randel et al (2021). The amplitude of geographic variability is different than that of Randel et al (2021), but this difference is certainly associated with their use of temperature disturbances to study gravity waves, while we have chosen momentum fluxes. Ern and Preusse (2012) also reported on  20 30% enhancements in gravity-wave momentum fluxes over convective regions as compared to non-convective ones.…”

“…The geographical bins encompass the whole    20 S 15 N latitude range sampled by the balloons, since the number of collected observations during the first Strateole-2 campaign did not enable us to clearly demonstrate inter-hemispheric differences. Both maps highlight a clear contrast between quieter regions (Eastern Pacific Ocean, Eastern Atlantic Ocean) and more active regions (South America, Eastern Africa, and the Maritime Continent), in agreement with maps from Randel et al (2021). The amplitude of geographic variability is different than that of Randel et al (2021), but this difference is certainly associated with their use of temperature disturbances to study gravity waves, while we have chosen momentum fluxes.…”

“…The amplitude of geographic variability is different than that of Randel et al. (2021), but this difference is certainly associated with their use of temperature disturbances to study gravity waves, while we have chosen momentum fluxes. Ern and Preusse (2012) also reported on % enhancements in gravity‐wave momentum fluxes over convective regions as compared to non‐convective ones.…”

The Stratospheric Diurnal Cycle in COSMIC GPS Radio Occultation Data: Scientific Applications

“…As a result, no thorough, continuous observational study of the tides in the global stratosphere was possible until COSMIC. More recently, the COSMIC‐2 mission obtained dense diurnal coverage, thus enabling the study of tides and other equatorial waves (Randel et al., 2021). COSMIC‐2 RO only covers the Tropics, however.…”