THUNDERSTORM-ENVIRONMENT INTERACTIONS DETERMINED FROM AIRCRAFT AND RADAR OBSERVATIONS

JAMES C. FANKHAUSER National Center for Atmospheric Research, 1 Boulder, Colo.

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Abstract

Aircraft measurements at low- and mid-cloud levels near two isolated and persistent Great Plains thunderstorms concurrently scanned by radar are analyzed to determine the kinematic properties of the near-cloud air flow, the moisture budget, and the dynamical interactions between the cloud and its surroundings. Time variation in velocity divergence, relative vorticity, and moisture flux convergence in the subcloud layer relate well to changes in storm development and translation. Aircraft winds and radar chaff trajectories substantiate the premise in some models that mature thunderstorms, moving more slowly than ambient winds, divert and distort mid-tropospheric air motion in a manner similar to solid obstacles in relative streaming flow. Observed ingestion of mid-tropospheric air motion tracers demonstrates that, at the same time, internal circulations aloft are not entirely insulated from the environment.

A graphical synthesis of three-dimensional air flow within and around a typical Great Plains cumulonimbus is presented, accommodating concepts in earlier models with the resolved circulation features. The involvement of cool, dry middle-level air in the internal circulation and its role in maintaining downdrafts are discussed. In considering factors influencing storm movement, it is concluded that, in addition to propagative mechanisms, hydrodynamical drag and deflection forces acting at cloud boundaries may play a significant role in determining a thunderstorm's preferred path with respect to the mean winds.

Abstract

Aircraft measurements at low- and mid-cloud levels near two isolated and persistent Great Plains thunderstorms concurrently scanned by radar are analyzed to determine the kinematic properties of the near-cloud air flow, the moisture budget, and the dynamical interactions between the cloud and its surroundings. Time variation in velocity divergence, relative vorticity, and moisture flux convergence in the subcloud layer relate well to changes in storm development and translation. Aircraft winds and radar chaff trajectories substantiate the premise in some models that mature thunderstorms, moving more slowly than ambient winds, divert and distort mid-tropospheric air motion in a manner similar to solid obstacles in relative streaming flow. Observed ingestion of mid-tropospheric air motion tracers demonstrates that, at the same time, internal circulations aloft are not entirely insulated from the environment.

A graphical synthesis of three-dimensional air flow within and around a typical Great Plains cumulonimbus is presented, accommodating concepts in earlier models with the resolved circulation features. The involvement of cool, dry middle-level air in the internal circulation and its role in maintaining downdrafts are discussed. In considering factors influencing storm movement, it is concluded that, in addition to propagative mechanisms, hydrodynamical drag and deflection forces acting at cloud boundaries may play a significant role in determining a thunderstorm's preferred path with respect to the mean winds.

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