Forcing Mechanisms of Internal Rear-Flank Downdraft Momentum Surges in the 18 May 2010 Dumas, Texas Supercell
Perturbation pressure retrievals of EnKF analyses reveal that the low-level RFD outflow structure, including the development of an internal RFD surge and an associated gust front along its leading edge, is primarily determined through perturbation pressure gradient forcing. Backwards trajectory analyses reveal that the majority of air parcels within the low-level RFD surge originate near the surface to the north of the low-level mesocyclone, then gradually accelerate by means of a favorable horizontal perturbation pressure gradient force as they wrap cyclonically around the low-level mesocyclone. Vertical excursions of air parcels within the RFD surge are primarily due to vertical perturbation pressure gradient forces along the periphery of the low-level mesocyclone, while buoyancy forcing provides “background” downward forcing throughout the RFD. A persistent trough of low perturbation pressure extending southward from the low-level mesocyclone is found to be responsible for the development of the internal RFD surge and associated gust front. For the occluded low-level mesocyclone in the Dumas supercell, this trough is displaced rearward from the primary RFD gust front, resulting in an abrupt transition from favorable to adverse horizontal perturbation pressure gradient forcing as air parcels cross the trough axis. This transition results in rapid deceleration of air parcels within the broad RFD and the formation of a local maximum in horizontal velocity with a region of strong convergence along its leading edge.