Small-Scale Motions Observed by Aircraft in the Tropical Lower Stratosphere: Evidence for Mixing and its Relationship to Large-Scale Flows

Abstract

Measurments of temperature and ozone from instrumental aircraft in the tropical lower stratosphere show the presence of small-scale disturbances generated by 1) underlying cumulus convection and 2) Kelvin-Helmholtz instability. The disturbances associated with underlying convection have peak-to-peak vertical parcel excursions of ?300 m. Flying conditions were smooth, suggesting an ensemble of gravity waves and little or no turbulent mixing. It is nevertheless possible that these waves break at other altitudes, leading to turbulent mixing and net fluxes of vertically stratified tracers. Disturbances attributed to KH instability implied vertical parcel excursions of 300?400 m. The disturbances coincided with rough flying conditions, suggesting turbulent mixing. A linear stability analysis of the atmospheric basic state defined by high-resolution radiosondes shows fastest growing waves with horizontal wavelengths of 1.4?1.8 km, consistent with the aircraft observations. The strong shears responsible for the KH instability are due to large-scale waves propagating into a region of small intrinsic frequency. Radiosonde observations show that the zonal length scale of them waves is ?1000 km.