Pressure and Buoyancy Fields Derived from Doppler Radar Data in a Tornadic Thunderstorm

Carl E. Hane National Severe Storm Laboratory, Norman, OK 73069

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Peter S. Ray National Severe Storm Laboratory, Norman, OK 73069

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Abstract

A method for retrieval of pressure and buoyancy distributions in deep convection is applied to Doppler radar data collected at two analysis times during the tornadic Del City (Oklahoma) thunderstorm of 20 May 1977. Change of a previous version of the technique, necessitated by application to real data, include procedures for handling irregularly-bounded volumes and missing data and new assumptions to include reflectivity data and turbulent effects in the equations. Internal consistency cheeks on the quality of retrieved pressure fields imply that the input data are generally of good quality and point out times and heights within the storm at which greater confidence can be placed in the derived fields.

In the pretornadic stage the pressure distribution includes at each level a high–low couplet across the updraft with the maximum pressure gradient generally oriented along the environmental shear vector at that altitude. These results are in agreement with predictions of linear theory. Locations of vorticity maxima and areas of updraft development are also discussed in relation to pressure distributions. The buoyancy distribution includes a good correspondence between positive buoyancy and updraft areas. An analysis of the individual terms in the buoyancy equation reveals the importance of advective and vertical pressure gradient terms over water-related and turbulence terms.

In the tornadic stage the pressure field includes a pronounced minimum at low levels coincident with the mesocyclone. An analysis of the factors influencing the pressure distribution reveals that strong low-level vertical vorticity produces this minimum. Vorticity, vertical motion, and pressure relationships in the low-level mesocyclone region tend to agree quite well with results of recent fine-scale numerical simulations as well as with the observationally-based finding of others. The low-level buoyancy field, although noisier at this stage, tends to support the line of reasoning which stress the production of horizontal vorticity as a major factor in low-level mesocyclone development.

Abstract

A method for retrieval of pressure and buoyancy distributions in deep convection is applied to Doppler radar data collected at two analysis times during the tornadic Del City (Oklahoma) thunderstorm of 20 May 1977. Change of a previous version of the technique, necessitated by application to real data, include procedures for handling irregularly-bounded volumes and missing data and new assumptions to include reflectivity data and turbulent effects in the equations. Internal consistency cheeks on the quality of retrieved pressure fields imply that the input data are generally of good quality and point out times and heights within the storm at which greater confidence can be placed in the derived fields.

In the pretornadic stage the pressure distribution includes at each level a high–low couplet across the updraft with the maximum pressure gradient generally oriented along the environmental shear vector at that altitude. These results are in agreement with predictions of linear theory. Locations of vorticity maxima and areas of updraft development are also discussed in relation to pressure distributions. The buoyancy distribution includes a good correspondence between positive buoyancy and updraft areas. An analysis of the individual terms in the buoyancy equation reveals the importance of advective and vertical pressure gradient terms over water-related and turbulence terms.

In the tornadic stage the pressure field includes a pronounced minimum at low levels coincident with the mesocyclone. An analysis of the factors influencing the pressure distribution reveals that strong low-level vertical vorticity produces this minimum. Vorticity, vertical motion, and pressure relationships in the low-level mesocyclone region tend to agree quite well with results of recent fine-scale numerical simulations as well as with the observationally-based finding of others. The low-level buoyancy field, although noisier at this stage, tends to support the line of reasoning which stress the production of horizontal vorticity as a major factor in low-level mesocyclone development.

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