| Summary: | Four kinds of severe, mesoscale convective-line development have been identified. One type, back-building, involves the periodic appearance of a new cell upstream of the existing line. Ultimately this new cell merges with the line, as new cells continue to form in the upstream direction. The dynamic forces responsible for the formation of a back-building squall line are investigated. In an example of such a development, a cell to the southwest of a single cell is observed by an airborne Doppler radar. The pressure and buoyancy perturbation fields are retrieved using the motion field, synthesized from the airborne Doppler data, and a representative sounding as input in the momentum equations. The retrieved dynamic fields are examined in conjunction with the kinematic fields to better understand the back-building process of squall line formation. Positive vertical pressure gradients at low levels and positive buoyancy at mid- and upper levels contribute to storm development. Linear theory is applicable at low- and mid-storm levels to describe the pressure perturbation fields. A veering of the wind shear vector with height produces upward vertical pressure gradients to the south of an elongate updraft embedded in the observed cell. Once the updraft is initiated by the favorable vertical pressure gradients, the release of latent heat further accelerates air upwards.
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