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Numerical Simulation of Thunderstorm Outflow Dynamics. Part I: Outflow Sensitivity Experiments and Turbulence Dynamics

Kelvin K. DroegemeierSchool of Meteorology, University of Oklahoma, Norman, OK 73019

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Robert B. WilhelmsonDepartment of Atmospheric Sciences, University of Illinois, Urbana, IL 61801

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Abstract

In this first paper of a two-part series, a two-dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows. By focusing only on the outflow and using essentially inviscid equations and high spatial resolution, we are able to explicitly represent important physical processes such as turbulent mixing. To simplify interpretation of the results, the model atmosphere used in all experiments is calm and dry adiabatic. This approach allows us to establish basic characteristics of modeled outflows in simple physical settings, and provides a foundation for future studies using more realistic environments.

All simulated outflows are initialized by prescribing a (controlled) horizontal flux of cold air into the model domain through a lateral boundary. In a series of sensitivity tests, we examine three parameters of the cold air source region: 1) the vertical temperature deficit profile, 2) the magnitude of the temperature deficit, and 3) the cold-air depth. By holding two of these quantities fixed while varying the third, we establish relationships among outflow speed, depth, and internal temperature deficit by comparing model results with laboratory density current experiments, inviscid fluid theory, and observations of thunderstorm outflows.

Our simulations indicate that the internal outflow head circulation is governed primarily by the outflow's vertical temperature distribution, and that this circulation plays a key role in determining the gust front propagation speed and outflow head depth. A pressure jump precedes the onset of cold air at the surface, and is shown to be dynamically induced by the collision of the outflow and environmental air masses at the gust front. In addition, the surface pressure distribution behind the outflow head is a consequence not only of hydrostatic effects, but also horizontal rotation aloft associated with a breaking head wave.

Turbulent mixing within the modeled outflows is associated with breaking Kelvin-Helmholtz billows which form at the shear interface atop the cold-air pool. These billows are qualitatively and quantitatively similar to turbulent eddies found in laboratory density currents, and are found to be sensitive to the magnitude of the computational smoothing as well as the grid resolution. Time-dependent air parcel trajectories are utilized to elucidate the kinematic structure of the simulated flow fields.

Abstract

In this first paper of a two-part series, a two-dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows. By focusing only on the outflow and using essentially inviscid equations and high spatial resolution, we are able to explicitly represent important physical processes such as turbulent mixing. To simplify interpretation of the results, the model atmosphere used in all experiments is calm and dry adiabatic. This approach allows us to establish basic characteristics of modeled outflows in simple physical settings, and provides a foundation for future studies using more realistic environments.

All simulated outflows are initialized by prescribing a (controlled) horizontal flux of cold air into the model domain through a lateral boundary. In a series of sensitivity tests, we examine three parameters of the cold air source region: 1) the vertical temperature deficit profile, 2) the magnitude of the temperature deficit, and 3) the cold-air depth. By holding two of these quantities fixed while varying the third, we establish relationships among outflow speed, depth, and internal temperature deficit by comparing model results with laboratory density current experiments, inviscid fluid theory, and observations of thunderstorm outflows.

Our simulations indicate that the internal outflow head circulation is governed primarily by the outflow's vertical temperature distribution, and that this circulation plays a key role in determining the gust front propagation speed and outflow head depth. A pressure jump precedes the onset of cold air at the surface, and is shown to be dynamically induced by the collision of the outflow and environmental air masses at the gust front. In addition, the surface pressure distribution behind the outflow head is a consequence not only of hydrostatic effects, but also horizontal rotation aloft associated with a breaking head wave.

Turbulent mixing within the modeled outflows is associated with breaking Kelvin-Helmholtz billows which form at the shear interface atop the cold-air pool. These billows are qualitatively and quantitatively similar to turbulent eddies found in laboratory density currents, and are found to be sensitive to the magnitude of the computational smoothing as well as the grid resolution. Time-dependent air parcel trajectories are utilized to elucidate the kinematic structure of the simulated flow fields.

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