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

Abstract

Abstract. 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 and the dehydration efficiency of TTL cirrus, but this has yet to be confirmed with observations. High-rate vertical winds measured by aircraft are a valuable and underutilized tool for constraining small-scale TTL vertical wind variability, examining its impacts on TTL cirrus, and evaluating atmospheric models.  We use 20 Hz data from five National Aeronautics and Space Administration (NASA) campaigns to quantify small-scale vertical wind variability in the TTL, and to see how it varies with ice water content, distance from deep convective cores, and height in the TTL.   We find that 1 Hz vertical winds are well represented by a normal distribution with a standard deviation of 0.2–0.4 m s-1.  We find that turbulence is enhanced within cirrus with high ice water content (IWC) (g m-3), which also have high ice crystal number concentrations. Consistent with a previous study, we find that turbulence is enhanced over the tropical West Pacific and within 500 km of convection, and is most common in the lower TTL (14–15.5 km) closer to deep convection, and in the upper TTL (15.5–17 km) further from deep convection. An algorithm to classify turbulence, and long wavelength (5 km < λ < 100 km) and short wavelength (λ <  5 km) gravity waves during level flight legs is applied to data from the Airborne Tropical TRopopause Experiment (ATTREX).  The most commonly sampled conditions are a quiescent atmosphere with negligible small-scale vertical wind variability, and long wavelength gravity wave activity with or without turbulence.  Turbulence rarely occurs in the absence of gravity wave activity. High-IWC cirrus are rare in a quiescent atmosphere, but 20 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. Spectral analysis of vertical wind observed in level flight legs over the tropical West Pacific TTL is compared with simulations by four global storm-resolving models with horizontal grid spacings of 3–5 km, sampled over the same region.  All four models have too little resolved vertical wind at horizontal wavelengths less than 100 km, but this bias is much less pronounced in global SAM than the other models.