Observations and ensemble Kalman filter analyses of multiple internal rear-flank downdraft momentum surges within the 18 May 2010, Dumas, Texas supercell

Analyses of data collected during a supercell occurring near Dumas, Texas on 18 May 2010 by the second Verification of the Origins of Rotation in Tornadoes Experiment are presented. The analysis period captures the development, intensification, and decay of a nontornadic low-level mesocyclone as well as four internal rear-flank downdraft (RFD) momentum surges. Observational analyses focus on a short-baseline, near-surface, dual-Doppler dataset collected by two Ka-band mobile Doppler radars, complemented by volumetric single-Doppler data collected with high temporal resolution by a phased array X-band mobile Doppler radar and in situ observations from a six-probe mobile mesonet. Radial velocity observations from two additional mobile Doppler radars, at X- and C-band, as well as data from the Amarillo, Texas WSR-88D are assimilated into an ensemble of numerical simulations of the Dumas supercell using a Kalman filter to provide a representative three-dimensional depiction of the wind and thermodynamic fields of the storm.

Observations reveal that the development of the final three observed internal RFD momentum surges followed the intensification of cyclonic azimuthal wind shear in the low-level mesocyclone to values greater than those farther aloft. Each surge developed on the western periphery of the low-level mesocyclone and wrapped cyclonically around it with time, suggesting that the RFD surges were driven by a downward-directed perturbation pressure gradient force. Kinematically, the highest wind speeds within the internal RFD momentum surges exceeded 40 m s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, with higher wind speeds and lower values of spectrum width and turbulence intensity observed within immature surges than those later in their lifecycle. This evolution, coupled with the very low values of turbulence intensity observed within surges compared to the expected values in thunderstorm outflows suggests that turbulence is suppressed in internal RFD surges and the maximum wind speeds are primarily determined by the mean component of the wind.

Three-dimensional perturbation pressure fields retrieved from the ensemble Kalman filter mean analyses of the Dumas supercell reveal that the location and gust front structure of the internal RFD momentum surges were primarily generated by variations in perturbation pressure gradient forcing. Backward trajectory and material circuit analyses show that air parcels within the RFD surges near the surface originated at low-levels within the forward-flank of the supercell and accelerated due to a horizontal perturbation pressure gradient force generated by the intensifying low-level mesocyclone as they wrapped cyclonically around the circulation before encountering an adverse perturbation pressure gradient force between the low-level mesocyclone and primary RFD gust front. The occlusion of the near-surface gust front structure and development of the occlusion downdraft displaces this adverse perturbation pressure gradient westward from the primary RFD gust front and results in development of the internal RFD gust front.