Dynamics in the Anvil Outflow of Tropical Convection
Lederman, Jerusha Isabel
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This thesis presents results of an experiment to investigate the dynamics of air motions within the outflow of tropical deep convection in the upper troposphere above Darwin, Australia. The research involved analyzing in situ measurements within the anvil outflow from the Egrett aircraft and also coincident laser remote sensing measurements of cloud structure from a King Air aircraft, flying directly below the Egrett. The data included the only in situ turbulence measurements that have been obtained with a resolution of 4 meters inside of an anvil outflow. The research shows that gravity waves and coherent structures exist in the anvil outflow. High frequency gravity waves were identified from the 90ᵒ phase lag between temperature and vertical wind oscillations. The gravity waves had wavelengths of 2 – 5 km and were observed as isolated waves with less than two oscillations, or, as extended waves with several oscillations. In some instances, correlations between temperature and vertical wind oscillations had zero phase lag between them and this was consistent with Rayleigh-Bénard convective rolls. Turbulence in the outflow was observed to be intense within patches separated by more quiescent flow. This turbulence had some characteristics that were unique in comparison to previous turbulence measurements at similar altitudes in the jet stream or midlatitude cirrus clouds. The ratio of the power spectral density of vertical wind fluctuations to the horizontal wind fluctuations was greater in the anvil outflow in comparison to the measurements in turbulence generated by shear in mid latitude jet stream and cirrus clouds. The relative magnitude of the vertical wind fluctuation spectrum decreases at all wavelengths with increasing distance away from the core region of the storm. This constitutes the first evidence that is consistent with theoretical predictions of “stratified turbulence.” Measurements in flights above and below the outflow base indicate that momentum flux is directed both upward and downward, away from the cloud base. This suggests the combination of shear and undulations in the cloud base are a mechanism for generating the observed high frequency gravity waves. This mechanism has not been previously identified from measurements or theoretical modelling and thus adds to our knowledge of processes that transport constituents and momentum within the tropical upper troposphere.