Energy Dissipation Within Intermittent Clear Air Turbulence Patches

Daniel Cadet Laboratoire de Météorologie Dynamique, École Polytechnique, 91120 Palaiseau, France

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Abstract

Mean dissipation rates of kinetic energy in the lower stratosphere are estimated from data obtained with an instrumented, superpressure, constant-level balloon launched to measure the vertical wind shear at 200 mb. The flight record shows that while the balloon was located above a mountainous region localized patches of turbulence were encountered.

The record is divided into 20 periods. The structure function is calculated over each period and, under the hypothesis of an inertial subrange, the mean dissipation is in turn estimated over each period. Our calculations indicate that the mean dissipation rate of kinetic energy can vary to a great extent over distances as short as a few tens of kilometers. Thus, when the shear layer breaks down, the mean dissipation rate can be 10 times higher than just before or after the event. It can also be 100 times higher within moderately intense clear air turbulence patches than in very weak intensity turbulence zones. These results show the sporadic nature of mixing processes in the lower stratosphere associated with shearing instabilities.

Abstract

Mean dissipation rates of kinetic energy in the lower stratosphere are estimated from data obtained with an instrumented, superpressure, constant-level balloon launched to measure the vertical wind shear at 200 mb. The flight record shows that while the balloon was located above a mountainous region localized patches of turbulence were encountered.

The record is divided into 20 periods. The structure function is calculated over each period and, under the hypothesis of an inertial subrange, the mean dissipation is in turn estimated over each period. Our calculations indicate that the mean dissipation rate of kinetic energy can vary to a great extent over distances as short as a few tens of kilometers. Thus, when the shear layer breaks down, the mean dissipation rate can be 10 times higher than just before or after the event. It can also be 100 times higher within moderately intense clear air turbulence patches than in very weak intensity turbulence zones. These results show the sporadic nature of mixing processes in the lower stratosphere associated with shearing instabilities.

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