An investigation of clear air turbulence as a response to larger-scale dynamic processes

John Lawrence Keller, Purdue University

Abstract

The hypothesis that the dissipation of energy in the free atmosphere by Clear Air Turbulence (CAT) is a manifestation of the subgrid-scale response to grid-scale processes is examined. Based on this hypothesis a Specific CAT Risk (SCATR) index is defined which provides a relative risk of encountering CAT directly related to the vertically integrated energy dissipation rate. This index is applied to eight cases involving documented encounters with moderate to severe CAT. Diagnostic Richardson number and deformation energy tendency equations result from this formulation which govern the Richardson number and deformation energy for isentropic parcels. An isentropic forecast model, which uses archived rawinsonde data as input, and a trajectory model are used to calculate Richardson number and deformation energy budgets for nine parcel arrays passing through these CAT regions. The Richardson number and deformation energy values at the resulting 12-hour trajectory coordinates are also compared with the corresponding observed values. Qualitatively, there appears to be good correspondence between larger values of the index and significant CAT. The variations of the Richardson number as predicted from the diagnostic Richardson number tendency equation agree quite well with those calculated directly from the forecast model output. The deformation energy for the nine parcel arrays calculated using the SCATR index as an energy dissipation parameterization are closer to the observed values than those which assume inviscid flow.

Degree

Ph.D.

Advisors

Smith, Purdue University.

Subject Area

Atmosphere

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