Convective Meteorology (Mesoscale Dynamics)

Forcing mechanisms of internal rear-flank downdraft momentum surges in the 18 May 2010 Dumas, Texas supercell

Dr. Patrick Skinner
NOAA Affiliate

27 February 2015, 2:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The origin and forcing of internal rear-flank downdraft (RFD) momentum surges observed by the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2) within a supercell occurring near Dumas, Texas on 18 May 2010 are assessed through ensemble Kalman filter (EnKF) retrievals of the atmospheric state. EnKF retrievals are produced every 2-minutes following assimilation of mobile Doppler velocity data collected by the Doppler on Wheels and Shared Mobile Atmospheric Research and Teaching radars, as well as radial velocity and reflectivity data from the KAMA WSR-88D.

Pressure retrievals of EnKF analyses reveal that the low-level RFD outflow structure is primarily determined through dynamic perturbation pressure gradient forcing. A trough of low perturbation pressure within the broad-scale RFD pendant to the occluded low-level mesocyclone forces an abrupt deceleration in air parcels crossing the trough axis. This deceleration results in the development of a localized maximum in horizontal velocity with strong convergence along its leading edge indicative of an internal RFD momentum surge. 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 and accelerate into the trough axis due to favorable horizontal pressure gradient forcing. A secondary RFD surge source region originating in the environmental flow and descending in a region of downward perturbation pressure gradient acceleration was not found to penetrate below 500 m above ground level.

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