EnKF and Polarimetric Analyses of the 31 May 2013 El Reno, Oklahoma Supercell during Tornadogenesis

This study utilizes a dataset collected by the rapid-scan, X-band, RaXPol mobile Doppler radar of a violent tornado near El Reno, Oklahoma to examine the three-dimensional wind field associated with several low-level polarimetric signatures observed prior to, and during, tornadogenesis.

The three dimensional wind and thermodynamic state of the El Reno supercell has been retrieved through assimilation of radial velocity data from RaXPol and NOAA’s National Weather Testbed Phased-Array radar using an ensemble Kalman filter (EnKF) technique. EnKF analyses with 500 m horizontal grid spacing have been produced for each minute during a 20-minute period centered on tornadogenesis. These analyses allow the retrieved three-dimensional wind field to be compared with RaXPol polarimetric data in order to diagnose the kinematic features responsible for observed polarimetric signatures.

Specifically, it is found that development of a region of relatively low differential reflectivity (Z_DR) within the hook echo of the supercell prior to tornadogenesis coincides with development of an internal rear-flank downdraft (RFD) momentum surge. The secondary RFD gust front bounding the momentum surge demarcates the lower Z_DR observations within the broad RFD from higher Z_DR values within the surge associated with the ZDR arc wrapping cyclonically around the low-level mesocyclone. Additionally, it is found that an elongated downdraft that develops in the forward flank of the supercell following tornadogenesis is co-located with polarimetric signatures indicative of melting hail; including attenuation in reflectivity and differential reflectivity, relatively low values or co-polar correlation coefficient, and large values of specific differential phase.