Aircraft observations of gravity wave activity and turbulence in the tropical tropopause layer: prevalence, influence on cirrus and comparison with global-storm resolving models

The tropical tropopause layer (TTL) is a sea of vertical motions. Convectively-generated gravity waves create vertical winds on scales of a few to 1000s of kilometers as they propagate in a stable atmosphere. Turbulence from gravity wave breaking, radiatively-driven convection and Kelvin-Helmholtz instabilities stirs up the TTL on the kilometer scale. TTL cirrus, which moderate the water vapor concentration in the TTL and stratosphere, form in the cold phases of large-scale (> 100 km) wave activity. It has been proposed in several modelling studies that small-scale (< 100 km) vertical motions control the ice crystal number concentration (NI) and the dehydration efficiency of TTL cirrus. Here, we present the first observational evidence for this.

We use 20 Hz data from the National Aeronautics and Space Administration (NASA) Airborne Tropical TRopopause Experiment (ATTREX) campaign to quantify small-scale vertical wind variability in the TTL and examine its influence on TTL cirrus microphysics. We develop an algorithm to classify turbulence, and long wavelength (5 km < λ < 100 km) and short wavelength (λ < 5 km) gravity wave activity, during level flight legs of at least 100 km. The most commonly sampled conditions are: 1) a quiescent atmosphere with negligible small-scale vertical wind variability, 2) long wavelength gravity wave activity (LWGWA), and 3) LWGWA with turbulence. Turbulence rarely occurs in the absence of gravity wave activity. Cirrus with NI exceeding 10 per liter are rare in a quiescent atmosphere, but about 25 times more likely when there is gravity wave activity and 50 times more likely when there is also turbulence, confirming the results of the aforementioned modeling studies.

Our observational analysis shows that small-scale gravity waves strongly influence NI within TTL cirrus. Global storm-resolving models have recently been run with horizontal grid spacings between 1 and 10 km, sufficient to resolve some small-scale gravity wave activity. We use ATTREX observations to evaluate simulated small-scale (10-100 km) vertical wind power spectra from four global-storm resolving simulations from DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains (DYAMOND) that have horizontal grid spacings of 3–5 km. We find that all four models have too little resolved vertical wind at horizontal wavelengths less than 100 km, although the bias is much less pronounced in global SAM than in the other models. We expect that deficient small-scale gravity wave activity significantly limits the realism of simulated ice microphysics in these models.